Contact angle is an essential parameter to characterize substrate wettability.The measurement of contact angle in experiment and simulation is a complex and time-consuming task.In this paper,an improved method of meas...Contact angle is an essential parameter to characterize substrate wettability.The measurement of contact angle in experiment and simulation is a complex and time-consuming task.In this paper,an improved method of measuring contact angle in multiphase lattice Boltzmann simulations is proposed,which can accurately obtain the real-time contact angle at a low temperature and larger density ratio.The three-phase contact point is determined by an extrapolation,and its position is not affected by the local deformation of flow field in the three-phase contact region.A series of simulations confirms that the present method has high accuracy and gird-independence.The contact angle keeps an excellent linear relationship with the chemical potential of the surface,so that it is very convenient to specify the wettability of a surface.The real-time contact angle measurement enables us to obtain the dynamic contact angle hysteresis on chemically heterogeneous surface,while the mechanical analyses can be effectively implemented at the moving contact line.展开更多
Traditional microstructure scale parameters have difficulty describing the structure and distribution of a roughmaterial’s surface morphology comprehensively and quantitatively. This study constructs hydrophilic and ...Traditional microstructure scale parameters have difficulty describing the structure and distribution of a roughmaterial’s surface morphology comprehensively and quantitatively. This study constructs hydrophilic and underwateroleophobic surfaces based on polyvinylidene fluoride (PVDF) using a chemical modification method, and the fractaldimension and multifractal spectrum are used to quantitatively characterize the microscopic morphology. A new contactangle prediction model for underwater oleophobic surfaces is established. The results show that the fractal dimension ofthe PVDF surface first increases and then decreases with the reaction time. The uniformity characterized by the multifractalspectrum was generally consistent with scanning electron microscope observations. The contact angle of water droplets onthe PVDF surface is negatively correlated with the fractal dimension, and oil droplets in water are positively correlated.When the fractal dimension is 2.0975, the new contact angle prediction model has higher prediction accuracy. Themaximum and minimum relative deviations of the contact angle between the theoretical and measured data are 18.20%and 0.72%, respectively. For water ring transportation, the larger the fractal dimension and spectral width of the materialsurface, the smaller the absolute value of the spectral difference, the stronger the hydrophilic and oleophobic properties, andthe better the water ring transportation stability.展开更多
There are five most widely used contact angle schemes in the pseudopotential lattice Boltzmann(LB)model for simulating the wetting phenomenon:The pseudopotential-based scheme(PB scheme),the improved virtualdensity sch...There are five most widely used contact angle schemes in the pseudopotential lattice Boltzmann(LB)model for simulating the wetting phenomenon:The pseudopotential-based scheme(PB scheme),the improved virtualdensity scheme(IVD scheme),the modified pseudopotential-based scheme with a ghost fluid layer constructed by using the fluid layer density above the wall(MPB-C scheme),the modified pseudopotential-based scheme with a ghost fluid layer constructed by using the weighted average density of surrounding fluid nodes(MPB-W scheme)and the geometric formulation scheme(GF scheme).But the numerical stability and accuracy of the schemes for wetting simulation remain unclear in the past.In this paper,the numerical stability and accuracy of these schemes are clarified for the first time,by applying the five widely used contact angle schemes to simulate a two-dimensional(2D)sessile droplet on wall and capillary imbibition in a 2D channel as the examples of static wetting and dynamic wetting simulations respectively.(i)It is shown that the simulated contact angles by the GF scheme are consistent at different density ratios for the same prescribed contact angle,but the simulated contact angles by the PB scheme,IVD scheme,MPB-C scheme and MPB-W scheme change with density ratios for the same fluid-solid interaction strength.The PB scheme is found to be the most unstable scheme for simulating static wetting at increased density ratios.(ii)Although the spurious velocity increases with the increased liquid/vapor density ratio for all the contact angle schemes,the magnitude of the spurious velocity in the PB scheme,IVD scheme and GF scheme are smaller than that in the MPB-C scheme and MPB-W scheme.(iii)The fluid density variation near the wall in the PB scheme is the most significant,and the variation can be diminished in the IVD scheme,MPB-C scheme andMPBWscheme.The variation totally disappeared in the GF scheme.(iv)For the simulation of capillary imbibition,the MPB-C scheme,MPB-Wscheme and GF scheme simulate the dynamics of the liquid-vapor interface well,with the GF scheme being the most accurate.The accuracy of the IVD scheme is low at a small contact angle(44 degrees)but gets high at a large contact angle(60 degrees).However,the PB scheme is the most inaccurate in simulating the dynamics of the liquid-vapor interface.As a whole,it is most suggested to apply the GF scheme to simulate static wetting or dynamic wetting,while it is the least suggested to use the PB scheme to simulate static wetting or dynamic wetting.展开更多
With the help of digital image processing technology, an automatic measurement method for the three-phase contact angles in the pore throats of the microfluidic model was established using the microfluidic water flood...With the help of digital image processing technology, an automatic measurement method for the three-phase contact angles in the pore throats of the microfluidic model was established using the microfluidic water flooding experiment videos as the data source. The results of the new method were verified through comparing with the manual measurement data.On this basis, the dynamic changes of the three-phase contact angles under flow conditions were clarified by the contact angles probability density curve and mean value change curve. The results show that, for water-wetting rocks, the mean value of the contact angles is acute angle during the early stage of the water flooding process, and it increases with the displacement time and becomes obtuse angle in the middle-late stage of displacement as the dominant force of oil phase gradually changes from viscous force to capillary force. The droplet flow in the remaining oil occurs in the central part of the pore throats, without three-phase contact angle. The contact angles for the porous flow and the columnar flow change slightly during the displacement and present as obtuse angles in view of mean values, which makes the remaining oil poorly movable and thus hard to be recovered. The mean value of the contact angle for the cluster flow tends to increase in the flooding process, which makes the remaining oil more difficult to be recovered. The contact angles for the membrane flow are mainly obtuse angles and reach the highest mean value in the late stage of displacement, which makes the remaining oil most difficult to be recovered. After displacement, the remaining oils under different flow regimes are just subjected to capillary force, with obtuse contact angles, and the wettability of the pore throat walls in the microfluidic model tends to be oil-wet under the action of crude oil.展开更多
In this study,interface shapes of horizontal oil–water two-phase flow are predicted by using Young-Laplace equation model and minimum energy model.Meanwhile,the interface shapes of horizontal oil–water twophase flow...In this study,interface shapes of horizontal oil–water two-phase flow are predicted by using Young-Laplace equation model and minimum energy model.Meanwhile,the interface shapes of horizontal oil–water twophase flow in a 20 mm inner diameter pipe are measured by a novel conductance parallel-wire array probe(CPAP).It is found that,for flow conditions with low water holdup,there is a large deviation between the model-predicted interface shape and the experimentally measured one.Since the variation of pipe wetting characteristics in the process of fluid flow can lead to the changes of the contact angle between the fluid and the pipe wall,the models mentioned above are modified by considering dynamic contact angle.The results indicate that the interface shapes predicted by the modified models present a good consistence with the ones measured by CPAP.展开更多
An approach of dealing with contact angle hysteresis in lattice Boltzmann method is introduced in detail.The approach is also used to investigate droplet behaviors on surfaces of chemical inhomogeneities or roughness(...An approach of dealing with contact angle hysteresis in lattice Boltzmann method is introduced in detail.The approach is also used to investigate droplet behaviors on surfaces of chemical inhomogeneities or roughness(non-ideal surfaces).Droplet slipping on surfaces under gravity or in shear flows,and droplet impacting on surfaces are numerically simulated.It is found that the present approach is suitable to model droplet motions on non-ideal surfaces and the contact angle hysteresis has an obvious effect on the motion of droplets.展开更多
Interactions between water and solid substrates are of fundamental importance to various processes in nature and industry.Electric control is widely used to modify interfacial water,where the influence of surface char...Interactions between water and solid substrates are of fundamental importance to various processes in nature and industry.Electric control is widely used to modify interfacial water,where the influence of surface charges is inevitable.Here we obtain positively and negatively charged surfaces using Li Ta O_(3) crystals and observe that a large net surface charge up to 0.1 C/m;can nominally change the contact angles of pure water droplets comparing to the same uncharged surface.However,even a small amount of surface charge can efficiently increase the water contact angle in the presence of aerosols.Our results indicate that such surface charges can hardly affect the structure of interfacial water molecular layers and the morphology of the macroscopic droplet,while adsorption of a small amount of organic contaminants from aerosols with the help of Coulomb attraction can notably decrease the wettability of solid surface.Our results not only provide a fundamental understanding of the interactions between charged surfaces and water,but also help to develop new techniques on electric control of wettability and microfluidics in real aerosol environments.展开更多
It is well known that surface roughness has a very important effect on superhydrophobicity.The Wenzel and Cassie-Baxtermodels,which correspond to the homogeneous and heterogeneous wetting respectively,are currently pr...It is well known that surface roughness has a very important effect on superhydrophobicity.The Wenzel and Cassie-Baxtermodels,which correspond to the homogeneous and heterogeneous wetting respectively,are currently primary instructions fordesigning superhydrophobic surfaces.However,the particular drop shape that a drop exhibits might depend on how it is formed.A water drop can occupy multiple equilibrium states,which relate to different local minimal energy.In some cases,both equilibriumstates can even co-exist on a same substrate.Thus the apparent contact angles may vary and have different values.Wediscuss how the Wenzel and Cassie-Baxter equations determine the homogeneous and heterogeneous wetting theoretically.Contact angle analysis on hierarchical surface structure and contact angle hysteresis has been put specific attention.In particular,we study the energy barrier of transition from Cassie-Baxter state to Wenzel state,based on existing achievement by previousresearchers,to determine the possibility of the transition and how it can be interpreted.It has been demonstrated that surfaceroughness and geometry will influence the energy required for a drop to get into equilibrium,no matter it is homogeneous orheterogeneous wetting.展开更多
It is known that contact lines keep relatively still on solids until static contact angles exceed an interval of hysteresis of static contact angle(HSCA), and contact angles keep changing as contact lines relatively s...It is known that contact lines keep relatively still on solids until static contact angles exceed an interval of hysteresis of static contact angle(HSCA), and contact angles keep changing as contact lines relatively slide on the solid.Here, the effects of HSCA and boundary slip were first distinguished on the micro-curvature force(MCF) on the seta.Hence, the total MCF is partitioned into static and dynamic MCFs correspondingly. The static MCF was found proportional to the HSCA and related with the asymmetry of the micro-meniscus near the seta. The dynamic MCF, exerting on the relatively sliding contact line, is aroused by the boundary slip. Based on the Blake–Haynes mechanism, the dynamic MCF was proved important for water walking insects with legs slower than the minimum wave speed 23 cm · s-1. As insects brush the water by laterally swinging legs backwards,setae on the front side of the leg are pulled and the ones on the back side are pushed to cooperatively propel bodies forward.If they pierce the water surface by vertically swinging legs downwards, setae on the upside of the legs are pulled, and the ones on the downside are pushed to cooperatively obtain a jumping force. Based on the dependency between the slip length and shear rate, the dynamic MCF was found correlated with the leg speed U, as F^C_1 U + C_2U^(2+ε), where C_1 and C_2 are determined by the dimple depth. Discrete points on this curve could give fitted relations as F^U~b(Suter et al., J. Exp. Biol. 200, 2523–2538, 1997). Finally,the axial torque on the inclined and partially submerged seta was found determined by the surface tension, contact angle,HSCA, seta width, and tilt angle. The torque direction coincides with the orientation of the spiral grooves of the seta,which encourages us to surmise it is a mechanical incentive for the formation of the spiral morphology of the setae of water striders.展开更多
Wetting phenomena are widespread in nature and industrial applications. In general, systems concerning wetting phenomena are typical multicomponent/multiphase complex fluid systems. Simulating the behavior of such sys...Wetting phenomena are widespread in nature and industrial applications. In general, systems concerning wetting phenomena are typical multicomponent/multiphase complex fluid systems. Simulating the behavior of such systems is important to both scientific research and practical applications. It is challenging due to the complexity of the phenomena and difficulties in choosing an appropriate numerical method. To provide some detailed guidelines for selecting a suitable multiphase lattice Boltzmann model, two kinds of lattice Boltzmann multiphase models, the modified S-C model and the H-C-Z model, are used in this paper to investigate the static contact angle on solid surfaces with different wettability combined with the geometric formulation(Ding, H. and Spelt, P.D. M. Wetting condition in diffuse interface simulations of contact line motion. Physical Review E, 75(4), 046708(2007)). The specific characteristics and computational performance of these two lattice Boltzmann method(LBM) multiphase models are analyzed including relationship between surface tension and the control parameters, the achievable range of the static contact angle, the maximum magnitude of the spurious currents(MMSC), and most importantly, the convergence rate of the two models on simulating the static contact angle. The results show that a wide range of static contact angles from wetting to non-wetting can be realized for both models. MMSC mainly depends on the surface tension. With the numerical parameters used in this work, the maximum magnitudes of the spurious currents of the two models are on the same order of magnitude. MMSC of the S-C model is universally larger than that of the H-C-Z model. The convergence rate of the S-C model is much faster than that of the H-C-Z model. The major foci in this work are the frequently-omitted important details in simulating wetting phenomena. Thus, the major findings in this work can provide suggestions for simulating wetting phenomena with LBM multiphase models along with the geometric formulation.展开更多
Dynamic wetting plays an important role in the physics of multiphase flow, and has a significant influence on many industrial and geotechnical applications. In this work, a modified smoothed particle hydrodynamics (SP...Dynamic wetting plays an important role in the physics of multiphase flow, and has a significant influence on many industrial and geotechnical applications. In this work, a modified smoothed particle hydrodynamics (SPH) model is employed to simulate surface tension, contact angle and dynamic wetting effects at meso-scale. The wetting and dewetting phenomena are simulated in a capillary tube, where the liquid particles are raised or withdrawn by a shifting substrate. The SPH model is modified by introducing a newly developed viscous force formulation at the liquid-solid interface to reproduce the rate-dependent behaviour of the moving contact line. Dynamic contact angle simulations with the interfacial viscous force are conducted to verify the effectiveness and accuracy of this new formulation. In addition, the influence of interfacial viscous forces with different magnitude on the contact angle dynamics is examined by empirical power-law correlations;the derived constants suggest that the dynamic contact angle changes monotonically with the interfacial viscous force. The simulation results are consistent with experimental observations and theoretical predictions, implying that the interfacial viscous force can be associated with the slip length of flow and the microscopic surface roughness. This work demonstrates that the modified SPH model can successfully account for the rate-dependent effects of a moving contact line, and can be used for realistic multiphase flow simulations under dynamic conditions.展开更多
Electrowetting,as a well-known approach to increasing droplet wettability on a solid surface by electrical bias,has broad applications.However,it is limited by contact angle saturation at large voltage.Although severa...Electrowetting,as a well-known approach to increasing droplet wettability on a solid surface by electrical bias,has broad applications.However,it is limited by contact angle saturation at large voltage.Although several debated hypotheses have been proposed to describe it,the physical origin of contact angle saturation still remains obscure.In this work,the physical factors responsible for the onset of contact angle saturation are explored,and the correlated theoretical models are established to characterize electrowetting behavior.Combination of the proper 3-phase system employed succeeds in dropping the saturating contact angle below 25?,and validates that the contact angle saturation is not a result of devicerelated imperfection.展开更多
Observation and measurement were conducted to investigate contact angle and its hysteresis on rough surface. The experimental results indicate that the increase in solid surface roughness enlarges advancing contact an...Observation and measurement were conducted to investigate contact angle and its hysteresis on rough surface. The experimental results indicate that the increase in solid surface roughness enlarges advancing contact angle and decreases receding contact angle, resulting in enhanced hysteresis. It was observed that when Young's contact angle θy ＜ 90°, as the roughness of solid surface increased the extent of the decrease in receding contact angle exceeded that of the increase in advancing contact angle. Based on the experimental observations, the concept of hysteresis tension was introduced to describe the contact angle hysteresis behavior on rough solid surface. The model provides a thoughtful understanding of the physical nature of contact angle hysteresis, in particular an instructive description of the influence of surface roughness on the hysteresis. The prediction of the model is found in quite good agreement with the experimental observation and measurement.展开更多
When characterizing flows in miniaturized channels, the determination of the dynamic contact angle is important. By measuring the dynamic contact angle, the flow properties of the flowing liquid and the effect of mate...When characterizing flows in miniaturized channels, the determination of the dynamic contact angle is important. By measuring the dynamic contact angle, the flow properties of the flowing liquid and the effect of material properties on the flow can be characterized. A machine vision based system to measure the contact angle of front or rear menisci of a moving liquid plug is described in this article. In this research, transparent flow channels fabricated on thermoplastic polymer and sealed with an adhesive tape are used. The transparency of the channels enables image based monitoring and measurement of flow variables, including the dynamic contact angle. It is shown that the dynamic angle can be measured from a liquid flow in a channel using the image based measurement system. An image processing algorithm has been developed in a MATLAB? environment. Im- ages are taken using a CCD camera and the channels are illuminated using a custom made ring light. Two fitting methods, a circle and two parabolas, are experimented and the results are compared in the measurement of the dynamic contact angles.展开更多
Contact angle hysteresis(CAH) is one of the significant physical phenomena in electrowetting on dielectric(EWOD).In this work, a theoretical model is proposed to characterize electrowetting evolution on substrates wit...Contact angle hysteresis(CAH) is one of the significant physical phenomena in electrowetting on dielectric(EWOD).In this work, a theoretical model is proposed to characterize electrowetting evolution on substrates with CAH, and the relationship among apparent contact angle, potential, and some other parameters is quantified. And this theory is also validated experimentally. The results indicate that our theory and equation based on energy balance succeed in describing the electrowetting response of potential with significant contact angle hysteresis. The CAH in EWOD, ranging from 0° to about 20° in electrowetting cycle, increases with the increase of voltage and climbs up to about 20° when voltage is increased to about 38 V, and then decreases to zero with the further increase of voltage.展开更多
This is the first one that applies the Zisman critical surface tension technique successfully to textile materials. It was accomplished by carefully determination of the contact angle of fabric. The deviation caused b...This is the first one that applies the Zisman critical surface tension technique successfully to textile materials. It was accomplished by carefully determination of the contact angle of fabric. The deviation caused by the porous structure of the fabric will be taken into account. To do so, a Jens equation is applied, and the measured contact angles can be corrected. The surface porosity was determined by measurement and approximate calculation, and the chemical composition of the surface was characterized by means of attenuated total reflection Fourier-transform infrared(FTIR/ATR).展开更多
In this work, contact angle measurements for soot samples collected from a kerosene lantern, wood-burning fireplace, and municipal bus engine exhaust lines are reported. Contact angles for both freshly collected soot ...In this work, contact angle measurements for soot samples collected from a kerosene lantern, wood-burning fireplace, and municipal bus engine exhaust lines are reported. Contact angles for both freshly collected soot and samples treated with various doses of O3 (g), HNO3 (g), and H2SO4 (g) are considered. Use of a literature method has allowed estimation of the enthalpy of immersion (Himm) for the soot samples based on contact angle observed. Contact angles for freshly collected soot were 65 - 110 deg. indicating its hydrophobic nature. Chemical processing of soot usually resulted in smaller contact angles and large increases in immersion enthalpy. However, the dose of ozone, nitric or sulfuric acid vapor required to achieve alteration of the soot surface appeared to be considerably larger than that expected to be experienced by authentic atmospheric samples during the soot particles lifetime. The most significant variability of soot contact angle was observed for the municipal bus exhaust samples, suggesting that combustion chemistry may significantly affect wetting behavior.展开更多
The formation mechanism of the contact angle and the sliding angle for a liquid drop on a solid surface plays an important role in producing hydrophobic surfaces. A new half soakage model is established in this paper ...The formation mechanism of the contact angle and the sliding angle for a liquid drop on a solid surface plays an important role in producing hydrophobic surfaces. A new half soakage model is established in this paper as a substitute for Wenzel (complete soakage) and Cassie (no soakage) models. The model is suited to many solid surfaces, whether they are hydrophilic or hydrophobic, or even superhydrophobic. Based on the half soakage model, we analyse two surfaces resembling lotus, i.e. taper-like surface and corona-like surface. Furthermore, this new model is used to establish a quantitative relationship between the sliding angle and the parameters of surface morphology.展开更多
We present a novel precise angle measurement scheme based on parallel multiplex laser feedback interferometry(PLFI), which outputs two parallel laser beams and thus their displacement difference reflects the angle var...We present a novel precise angle measurement scheme based on parallel multiplex laser feedback interferometry(PLFI), which outputs two parallel laser beams and thus their displacement difference reflects the angle variation of the target. Due to its ultrahigh sensitivity to the feedback light, PLFI realizes the direct non-contact measurement of noncooperative targets. Experimental results show that PLFI has an accuracy of 8 within a range of 1400. The yaw of a guide is also measured and the experimental results agree with those of the dual-frequency laser interferometer Agilent5529 A.展开更多
基金the Guangxi Bagui Scholar Teams for Innovation and Research Projectthe National Natural Science Foundation of China(Grant Nos.12272100,81860635,and 12062005)。
文摘Contact angle is an essential parameter to characterize substrate wettability.The measurement of contact angle in experiment and simulation is a complex and time-consuming task.In this paper,an improved method of measuring contact angle in multiphase lattice Boltzmann simulations is proposed,which can accurately obtain the real-time contact angle at a low temperature and larger density ratio.The three-phase contact point is determined by an extrapolation,and its position is not affected by the local deformation of flow field in the three-phase contact region.A series of simulations confirms that the present method has high accuracy and gird-independence.The contact angle keeps an excellent linear relationship with the chemical potential of the surface,so that it is very convenient to specify the wettability of a surface.The real-time contact angle measurement enables us to obtain the dynamic contact angle hysteresis on chemically heterogeneous surface,while the mechanical analyses can be effectively implemented at the moving contact line.
基金the Natural Science Basic Research Program of Shaanxi(Program No.2023-JC-YB-351)the Scientific Research Program Funded by the Shaanxi Provincial Education Department(Program No.20JS118)the Xi’an Shiyou University Graduate Innovation and Practice Ability Training Plan(YCS21212097,YCS21212092).
文摘Traditional microstructure scale parameters have difficulty describing the structure and distribution of a roughmaterial’s surface morphology comprehensively and quantitatively. This study constructs hydrophilic and underwateroleophobic surfaces based on polyvinylidene fluoride (PVDF) using a chemical modification method, and the fractaldimension and multifractal spectrum are used to quantitatively characterize the microscopic morphology. A new contactangle prediction model for underwater oleophobic surfaces is established. The results show that the fractal dimension ofthe PVDF surface first increases and then decreases with the reaction time. The uniformity characterized by the multifractalspectrum was generally consistent with scanning electron microscope observations. The contact angle of water droplets onthe PVDF surface is negatively correlated with the fractal dimension, and oil droplets in water are positively correlated.When the fractal dimension is 2.0975, the new contact angle prediction model has higher prediction accuracy. Themaximum and minimum relative deviations of the contact angle between the theoretical and measured data are 18.20%and 0.72%, respectively. For water ring transportation, the larger the fractal dimension and spectral width of the materialsurface, the smaller the absolute value of the spectral difference, the stronger the hydrophilic and oleophobic properties, andthe better the water ring transportation stability.
基金sponsored by the National Natural Science Foundation of China under Grant No.52206101Shanghai Sailing Program under Grant No.20YF1431200the Experiments for Space Exploration Program and the Qian Xuesen Laboratory,China Academy of Space Technology under Grant No.TKTSPY-2020-01-01.
文摘There are five most widely used contact angle schemes in the pseudopotential lattice Boltzmann(LB)model for simulating the wetting phenomenon:The pseudopotential-based scheme(PB scheme),the improved virtualdensity scheme(IVD scheme),the modified pseudopotential-based scheme with a ghost fluid layer constructed by using the fluid layer density above the wall(MPB-C scheme),the modified pseudopotential-based scheme with a ghost fluid layer constructed by using the weighted average density of surrounding fluid nodes(MPB-W scheme)and the geometric formulation scheme(GF scheme).But the numerical stability and accuracy of the schemes for wetting simulation remain unclear in the past.In this paper,the numerical stability and accuracy of these schemes are clarified for the first time,by applying the five widely used contact angle schemes to simulate a two-dimensional(2D)sessile droplet on wall and capillary imbibition in a 2D channel as the examples of static wetting and dynamic wetting simulations respectively.(i)It is shown that the simulated contact angles by the GF scheme are consistent at different density ratios for the same prescribed contact angle,but the simulated contact angles by the PB scheme,IVD scheme,MPB-C scheme and MPB-W scheme change with density ratios for the same fluid-solid interaction strength.The PB scheme is found to be the most unstable scheme for simulating static wetting at increased density ratios.(ii)Although the spurious velocity increases with the increased liquid/vapor density ratio for all the contact angle schemes,the magnitude of the spurious velocity in the PB scheme,IVD scheme and GF scheme are smaller than that in the MPB-C scheme and MPB-W scheme.(iii)The fluid density variation near the wall in the PB scheme is the most significant,and the variation can be diminished in the IVD scheme,MPB-C scheme andMPBWscheme.The variation totally disappeared in the GF scheme.(iv)For the simulation of capillary imbibition,the MPB-C scheme,MPB-Wscheme and GF scheme simulate the dynamics of the liquid-vapor interface well,with the GF scheme being the most accurate.The accuracy of the IVD scheme is low at a small contact angle(44 degrees)but gets high at a large contact angle(60 degrees).However,the PB scheme is the most inaccurate in simulating the dynamics of the liquid-vapor interface.As a whole,it is most suggested to apply the GF scheme to simulate static wetting or dynamic wetting,while it is the least suggested to use the PB scheme to simulate static wetting or dynamic wetting.
基金Supported by National Science and Technology Major Project of China (51674271)Major Technical Field Test of PetroChina (2019F-33)。
文摘With the help of digital image processing technology, an automatic measurement method for the three-phase contact angles in the pore throats of the microfluidic model was established using the microfluidic water flooding experiment videos as the data source. The results of the new method were verified through comparing with the manual measurement data.On this basis, the dynamic changes of the three-phase contact angles under flow conditions were clarified by the contact angles probability density curve and mean value change curve. The results show that, for water-wetting rocks, the mean value of the contact angles is acute angle during the early stage of the water flooding process, and it increases with the displacement time and becomes obtuse angle in the middle-late stage of displacement as the dominant force of oil phase gradually changes from viscous force to capillary force. The droplet flow in the remaining oil occurs in the central part of the pore throats, without three-phase contact angle. The contact angles for the porous flow and the columnar flow change slightly during the displacement and present as obtuse angles in view of mean values, which makes the remaining oil poorly movable and thus hard to be recovered. The mean value of the contact angle for the cluster flow tends to increase in the flooding process, which makes the remaining oil more difficult to be recovered. The contact angles for the membrane flow are mainly obtuse angles and reach the highest mean value in the late stage of displacement, which makes the remaining oil most difficult to be recovered. After displacement, the remaining oils under different flow regimes are just subjected to capillary force, with obtuse contact angles, and the wettability of the pore throat walls in the microfluidic model tends to be oil-wet under the action of crude oil.
基金supported by the National Natural Science Foundation of China(Grant Nos.41974139,41504104,11572220,51527805)Natural Science Foundation of Tianjin,China(19JCYBJC18400)。
文摘In this study,interface shapes of horizontal oil–water two-phase flow are predicted by using Young-Laplace equation model and minimum energy model.Meanwhile,the interface shapes of horizontal oil–water twophase flow in a 20 mm inner diameter pipe are measured by a novel conductance parallel-wire array probe(CPAP).It is found that,for flow conditions with low water holdup,there is a large deviation between the model-predicted interface shape and the experimentally measured one.Since the variation of pipe wetting characteristics in the process of fluid flow can lead to the changes of the contact angle between the fluid and the pipe wall,the models mentioned above are modified by considering dynamic contact angle.The results indicate that the interface shapes predicted by the modified models present a good consistence with the ones measured by CPAP.
文摘An approach of dealing with contact angle hysteresis in lattice Boltzmann method is introduced in detail.The approach is also used to investigate droplet behaviors on surfaces of chemical inhomogeneities or roughness(non-ideal surfaces).Droplet slipping on surfaces under gravity or in shear flows,and droplet impacting on surfaces are numerically simulated.It is found that the present approach is suitable to model droplet motions on non-ideal surfaces and the contact angle hysteresis has an obvious effect on the motion of droplets.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12025407,11934003,9185012011774328)+1 种基金the Key R&D Program of China(Grant No.2016YFA0300902)the Chinese Academy of Sciences。
文摘Interactions between water and solid substrates are of fundamental importance to various processes in nature and industry.Electric control is widely used to modify interfacial water,where the influence of surface charges is inevitable.Here we obtain positively and negatively charged surfaces using Li Ta O_(3) crystals and observe that a large net surface charge up to 0.1 C/m;can nominally change the contact angles of pure water droplets comparing to the same uncharged surface.However,even a small amount of surface charge can efficiently increase the water contact angle in the presence of aerosols.Our results indicate that such surface charges can hardly affect the structure of interfacial water molecular layers and the morphology of the macroscopic droplet,while adsorption of a small amount of organic contaminants from aerosols with the help of Coulomb attraction can notably decrease the wettability of solid surface.Our results not only provide a fundamental understanding of the interactions between charged surfaces and water,but also help to develop new techniques on electric control of wettability and microfluidics in real aerosol environments.
文摘It is well known that surface roughness has a very important effect on superhydrophobicity.The Wenzel and Cassie-Baxtermodels,which correspond to the homogeneous and heterogeneous wetting respectively,are currently primary instructions fordesigning superhydrophobic surfaces.However,the particular drop shape that a drop exhibits might depend on how it is formed.A water drop can occupy multiple equilibrium states,which relate to different local minimal energy.In some cases,both equilibriumstates can even co-exist on a same substrate.Thus the apparent contact angles may vary and have different values.Wediscuss how the Wenzel and Cassie-Baxter equations determine the homogeneous and heterogeneous wetting theoretically.Contact angle analysis on hierarchical surface structure and contact angle hysteresis has been put specific attention.In particular,we study the energy barrier of transition from Cassie-Baxter state to Wenzel state,based on existing achievement by previousresearchers,to determine the possibility of the transition and how it can be interpreted.It has been demonstrated that surfaceroughness and geometry will influence the energy required for a drop to get into equilibrium,no matter it is homogeneous orheterogeneous wetting.
文摘It is known that contact lines keep relatively still on solids until static contact angles exceed an interval of hysteresis of static contact angle(HSCA), and contact angles keep changing as contact lines relatively slide on the solid.Here, the effects of HSCA and boundary slip were first distinguished on the micro-curvature force(MCF) on the seta.Hence, the total MCF is partitioned into static and dynamic MCFs correspondingly. The static MCF was found proportional to the HSCA and related with the asymmetry of the micro-meniscus near the seta. The dynamic MCF, exerting on the relatively sliding contact line, is aroused by the boundary slip. Based on the Blake–Haynes mechanism, the dynamic MCF was proved important for water walking insects with legs slower than the minimum wave speed 23 cm · s-1. As insects brush the water by laterally swinging legs backwards,setae on the front side of the leg are pulled and the ones on the back side are pushed to cooperatively propel bodies forward.If they pierce the water surface by vertically swinging legs downwards, setae on the upside of the legs are pulled, and the ones on the downside are pushed to cooperatively obtain a jumping force. Based on the dependency between the slip length and shear rate, the dynamic MCF was found correlated with the leg speed U, as F^C_1 U + C_2U^(2+ε), where C_1 and C_2 are determined by the dimple depth. Discrete points on this curve could give fitted relations as F^U~b(Suter et al., J. Exp. Biol. 200, 2523–2538, 1997). Finally,the axial torque on the inclined and partially submerged seta was found determined by the surface tension, contact angle,HSCA, seta width, and tilt angle. The torque direction coincides with the orientation of the spiral grooves of the seta,which encourages us to surmise it is a mechanical incentive for the formation of the spiral morphology of the setae of water striders.
基金Project supported by the National Natural Science Foundation of China(Nos.50874071 and51704191)the Shanghai Leading Academic Discipline Project(No.S30106)+1 种基金the Key Program of Science and Technology Commission of Shanghai Municipality(No.12160500200)the PetroChina Innovation Foundation(No.2017D-5007-0209)
文摘Wetting phenomena are widespread in nature and industrial applications. In general, systems concerning wetting phenomena are typical multicomponent/multiphase complex fluid systems. Simulating the behavior of such systems is important to both scientific research and practical applications. It is challenging due to the complexity of the phenomena and difficulties in choosing an appropriate numerical method. To provide some detailed guidelines for selecting a suitable multiphase lattice Boltzmann model, two kinds of lattice Boltzmann multiphase models, the modified S-C model and the H-C-Z model, are used in this paper to investigate the static contact angle on solid surfaces with different wettability combined with the geometric formulation(Ding, H. and Spelt, P.D. M. Wetting condition in diffuse interface simulations of contact line motion. Physical Review E, 75(4), 046708(2007)). The specific characteristics and computational performance of these two lattice Boltzmann method(LBM) multiphase models are analyzed including relationship between surface tension and the control parameters, the achievable range of the static contact angle, the maximum magnitude of the spurious currents(MMSC), and most importantly, the convergence rate of the two models on simulating the static contact angle. The results show that a wide range of static contact angles from wetting to non-wetting can be realized for both models. MMSC mainly depends on the surface tension. With the numerical parameters used in this work, the maximum magnitudes of the spurious currents of the two models are on the same order of magnitude. MMSC of the S-C model is universally larger than that of the H-C-Z model. The convergence rate of the S-C model is much faster than that of the H-C-Z model. The major foci in this work are the frequently-omitted important details in simulating wetting phenomena. Thus, the major findings in this work can provide suggestions for simulating wetting phenomena with LBM multiphase models along with the geometric formulation.
文摘Dynamic wetting plays an important role in the physics of multiphase flow, and has a significant influence on many industrial and geotechnical applications. In this work, a modified smoothed particle hydrodynamics (SPH) model is employed to simulate surface tension, contact angle and dynamic wetting effects at meso-scale. The wetting and dewetting phenomena are simulated in a capillary tube, where the liquid particles are raised or withdrawn by a shifting substrate. The SPH model is modified by introducing a newly developed viscous force formulation at the liquid-solid interface to reproduce the rate-dependent behaviour of the moving contact line. Dynamic contact angle simulations with the interfacial viscous force are conducted to verify the effectiveness and accuracy of this new formulation. In addition, the influence of interfacial viscous forces with different magnitude on the contact angle dynamics is examined by empirical power-law correlations;the derived constants suggest that the dynamic contact angle changes monotonically with the interfacial viscous force. The simulation results are consistent with experimental observations and theoretical predictions, implying that the interfacial viscous force can be associated with the slip length of flow and the microscopic surface roughness. This work demonstrates that the modified SPH model can successfully account for the rate-dependent effects of a moving contact line, and can be used for realistic multiphase flow simulations under dynamic conditions.
基金Project supported by the Fund from the Research Center of Optical Communications Engineering&Technology,Jiangsu Province,China(Grant No.ZSF0402)
文摘Electrowetting,as a well-known approach to increasing droplet wettability on a solid surface by electrical bias,has broad applications.However,it is limited by contact angle saturation at large voltage.Although several debated hypotheses have been proposed to describe it,the physical origin of contact angle saturation still remains obscure.In this work,the physical factors responsible for the onset of contact angle saturation are explored,and the correlated theoretical models are established to characterize electrowetting behavior.Combination of the proper 3-phase system employed succeeds in dropping the saturating contact angle below 25?,and validates that the contact angle saturation is not a result of devicerelated imperfection.
文摘Observation and measurement were conducted to investigate contact angle and its hysteresis on rough surface. The experimental results indicate that the increase in solid surface roughness enlarges advancing contact angle and decreases receding contact angle, resulting in enhanced hysteresis. It was observed that when Young's contact angle θy ＜ 90°, as the roughness of solid surface increased the extent of the decrease in receding contact angle exceeded that of the increase in advancing contact angle. Based on the experimental observations, the concept of hysteresis tension was introduced to describe the contact angle hysteresis behavior on rough solid surface. The model provides a thoughtful understanding of the physical nature of contact angle hysteresis, in particular an instructive description of the influence of surface roughness on the hysteresis. The prediction of the model is found in quite good agreement with the experimental observation and measurement.
基金This research was done as part of TEKES-funded PanFlow project and as part of a project OPTIMI funded by the Academy of Finland (grant number 117587) in Micro- and Nanosystems Research Group, Tampere University of Technology, Finland.
文摘When characterizing flows in miniaturized channels, the determination of the dynamic contact angle is important. By measuring the dynamic contact angle, the flow properties of the flowing liquid and the effect of material properties on the flow can be characterized. A machine vision based system to measure the contact angle of front or rear menisci of a moving liquid plug is described in this article. In this research, transparent flow channels fabricated on thermoplastic polymer and sealed with an adhesive tape are used. The transparency of the channels enables image based monitoring and measurement of flow variables, including the dynamic contact angle. It is shown that the dynamic angle can be measured from a liquid flow in a channel using the image based measurement system. An image processing algorithm has been developed in a MATLAB? environment. Im- ages are taken using a CCD camera and the channels are illuminated using a custom made ring light. Two fitting methods, a circle and two parabolas, are experimented and the results are compared in the measurement of the dynamic contact angles.
基金supported by the Natural Science Foundation of Jiangsu Province,China(Grant No.BK2011752)
文摘Contact angle hysteresis(CAH) is one of the significant physical phenomena in electrowetting on dielectric(EWOD).In this work, a theoretical model is proposed to characterize electrowetting evolution on substrates with CAH, and the relationship among apparent contact angle, potential, and some other parameters is quantified. And this theory is also validated experimentally. The results indicate that our theory and equation based on energy balance succeed in describing the electrowetting response of potential with significant contact angle hysteresis. The CAH in EWOD, ranging from 0° to about 20° in electrowetting cycle, increases with the increase of voltage and climbs up to about 20° when voltage is increased to about 38 V, and then decreases to zero with the further increase of voltage.
文摘This is the first one that applies the Zisman critical surface tension technique successfully to textile materials. It was accomplished by carefully determination of the contact angle of fabric. The deviation caused by the porous structure of the fabric will be taken into account. To do so, a Jens equation is applied, and the measured contact angles can be corrected. The surface porosity was determined by measurement and approximate calculation, and the chemical composition of the surface was characterized by means of attenuated total reflection Fourier-transform infrared(FTIR/ATR).
文摘In this work, contact angle measurements for soot samples collected from a kerosene lantern, wood-burning fireplace, and municipal bus engine exhaust lines are reported. Contact angles for both freshly collected soot and samples treated with various doses of O3 (g), HNO3 (g), and H2SO4 (g) are considered. Use of a literature method has allowed estimation of the enthalpy of immersion (Himm) for the soot samples based on contact angle observed. Contact angles for freshly collected soot were 65 - 110 deg. indicating its hydrophobic nature. Chemical processing of soot usually resulted in smaller contact angles and large increases in immersion enthalpy. However, the dose of ozone, nitric or sulfuric acid vapor required to achieve alteration of the soot surface appeared to be considerably larger than that expected to be experienced by authentic atmospheric samples during the soot particles lifetime. The most significant variability of soot contact angle was observed for the municipal bus exhaust samples, suggesting that combustion chemistry may significantly affect wetting behavior.
基金Project supported by the National Basic Research Program of China (Grant No. 2006CB302900)the National Natural Science Foundation of China (Grant No. 50872129)
文摘The formation mechanism of the contact angle and the sliding angle for a liquid drop on a solid surface plays an important role in producing hydrophobic surfaces. A new half soakage model is established in this paper as a substitute for Wenzel (complete soakage) and Cassie (no soakage) models. The model is suited to many solid surfaces, whether they are hydrophilic or hydrophobic, or even superhydrophobic. Based on the half soakage model, we analyse two surfaces resembling lotus, i.e. taper-like surface and corona-like surface. Furthermore, this new model is used to establish a quantitative relationship between the sliding angle and the parameters of surface morphology.
基金supported by the National Natural Science Foundation of China(Grant No.61036016)
文摘We present a novel precise angle measurement scheme based on parallel multiplex laser feedback interferometry(PLFI), which outputs two parallel laser beams and thus their displacement difference reflects the angle variation of the target. Due to its ultrahigh sensitivity to the feedback light, PLFI realizes the direct non-contact measurement of noncooperative targets. Experimental results show that PLFI has an accuracy of 8 within a range of 1400. The yaw of a guide is also measured and the experimental results agree with those of the dual-frequency laser interferometer Agilent5529 A.
