Twelve samples with periodic array square pillars microstructure were prepared on the silicon wafer by plasma etching techniques, on which space b of the square pillars increased from 5 to 60 μm. In order to study th...Twelve samples with periodic array square pillars microstructure were prepared on the silicon wafer by plasma etching techniques, on which space b of the square pillars increased from 5 to 60 μm. In order to study the effect ofb on the wettability of the rough surface, the effects of apparent contact angle (CA) and sliding angle (a) of the droplet on the rough surface were measured with the contact angle meter. The results show that the experimental values of CA well agree with the classical wetting theory and a decreases with the increase of b. Two drop shapes exist on the samples' surface, corresponding to the Cassie state and the Wenzel state respectively. The contact state in which a drop would settle depends typically on the size of b. On the role of gravitation, the irreversible transition of a drop from Cassie state to Wenzel state should occur at a certain space of the square pillars. Since the transition has implications on the application of super-hydrophobic rough surfaces, theoretically, the prediction of wetting state transition on square pillar array micro-structured surfaces provides an intuitionistic guidance for the design of steady superhydrophobic surfaces.展开更多
A numerical investigation of the unsteady motion of a deformed drop released freely in another quiescent liquid contaminated by surfactant is presented in this paper. The finite difference method was used to solve num...A numerical investigation of the unsteady motion of a deformed drop released freely in another quiescent liquid contaminated by surfactant is presented in this paper. The finite difference method was used to solve numerically the coupled time-dependent Navier-Stokes and convective-diffusion equations in a body-fitted orthogonal coordinate system. Numerical simulation was conducted on the experimental cases, in which MIBK drops with the size ranging from 1.24 mm to 1.97 mm rose and accelerated freely in pure water and in dilute sodium dodecyl sulphate (SDS) aqueous solution. The applicability of the numerical scheme was validated by the agreement between the simulation results and the experimental data. Both the numerical and experimental results showed that the velocitytime profile exhibited a maximum rising velocity for drops in SDS solutions, which was close to the terminal velocity in pure water, before it dropped down to a steady-state value. The effect of the sorption kinetics of surfactant on the accelerating motion was also evaluated. It is also suggested that introduction of virtual mass force into the formulation improved obviously the precision of numerical simulation of transient drop motion.展开更多
The accurate prediction of the droplet size distribution(DSD)in liquid–liquid turbulent dispersions is of fundamental importance in many industrial applications and it requires suitable kernels in the population bala...The accurate prediction of the droplet size distribution(DSD)in liquid–liquid turbulent dispersions is of fundamental importance in many industrial applications and it requires suitable kernels in the population balance model.When a surfactant is included in liquid–liquid dispersions,the droplet breakup behavior will change as an effect of the reduction of the interfacial tension.Moreover,also the dynamic interfacial tension may be different with respect to the static,due to the fact that the surfactant may be easily desorbed from the droplet surface,generating additional disruptive stresses.In this work,the performance of five breakup kernels from the literature is assessed,to investigate their ability to predict the time evolution of the DSD and of the mean Sauter diameter,when different surfactants are employed.Simulations are performed with the Quadrature Method of Moments for the solution of the population balance model coupled with the two-fluid model implemented in the compressible Two Phase Euler Foam solver of the open-source computational fluid dynamics(CFD)code Open FOAM v.2.2.x.The time evolution of the mean Sauter diameter predicted by these kernels is validated against experimental data for six test cases referring to a stirred tank with different types of surfactants(Tween 20 and PVA 88%)at different concentrations operating under different stirrer rates.Our results show that for the dispersion containing Tween 20 additional stress is generated,the multifractal breakup kernel properly predicts the DSD evolution,whereas two other kernels predict too fast breakup of droplets covered by adsorbed PVA.Kernels derived originally for bubbles completely fail.展开更多
A droplet undergoes spreading,rebounding or splashing when it impacts solid boundary,which is a typical phenomenon of free surface flow that exists widely in modern industry.Smoothed particle hydrodynamics(SPH)method ...A droplet undergoes spreading,rebounding or splashing when it impacts solid boundary,which is a typical phenomenon of free surface flow that exists widely in modern industry.Smoothed particle hydrodynamics(SPH)method is applied to numerically study the dynamical behaviors of the droplet impacting solid boundary,and both the spreading and rebounding phenomena of the droplet are reproduced in the simulation.The droplet deformation,flow fields and pressure fields inside the droplet at different moments are analyzed.Two important factors,the initial velocity and diameter,are discussed in determining the maximum spreading factor,revealing that the maximum spreading factor increases with the increase of the impact velocity and droplet diameter respectively.展开更多
The rebound behaviors of droplets impacting on a self-fabricated superhydrophobic brass surface (WCA=I64.5°) were ob- served and studied by using high-speed-camera. In accordance with energy conversion, theoret...The rebound behaviors of droplets impacting on a self-fabricated superhydrophobic brass surface (WCA=I64.5°) were ob- served and studied by using high-speed-camera. In accordance with energy conversion, theoretical analysis of different behav- iors and rebound mechanism were given. At lower velocities, three behaviors in different velocity ranges were observed: par- tial rebounding, entire rebounding and ejecting during rebounding. At higher velocities, such two behaviors as rebound after splashing and rebound, ejecting after splashing, occurred alternately and exhibited certain periodicity. A function to predict the critical impact velocity is derived from energy conservation condition, and the prediction values tally with the experimental values, with the maximum relative error about 14%.展开更多
Droplet impacting liquid surface is not only the extremely prevalent phenomenon in the nature and industrial production but also the extremely complicated problem of strong non-linear transient impact and free-surface...Droplet impacting liquid surface is not only the extremely prevalent phenomenon in the nature and industrial production but also the extremely complicated problem of strong non-linear transient impact and free-surface flow. On the basis of the two-dimensional viscous incompressible N-S equations, this paper conducts a study of numerical simulation on the problem of droplet impacting liquid surface (water beads) of water container in certain initial velocity by the method of SPH (smoothed particle hydrodynamics). The effect of surface tension is considered between surface particles by searching the free surface particles in the course of study; the effect of initial impact has been solved by use of artificial viscosity; at the same time, the side-wall virtual particles and image virtual particles are both introduced to deal with the boundary condition, which has solved the boundary defects quite well and eliminated the instability of real particles dropped to the comer of container. The calculated results form the distribution chart of particles, flow field chart, pressure chart and the displacement and velocity variation curve of different particles. The comparison between simulated results and experimental photos shows that the simulation is effective. This paper compares the variational curves for fluctuations of liquid surface qualitatively through adopting the methods of level-set, BEM and SPH, respectively at last. The simulated results show that it will produce strong non-linear phenomena, such as the splash of liquid, discrete liquid surface, and strong wave of free liquid surface, when the droplet impacts liquid surface; in the course of impacting, the movement of liquid particles exhibits the characteristic of oscillation; the method of SPH has certain advantages of dealing with the large deformation problem of free surface.展开更多
This paper focused on the sessile droplet freezing and ice adhesion on aluminum with different wettability (hydrophilic, com- mon hydrophobic, and superhydrophobic surfaces, coded as HIS, CHS, SHS, respectively) ove...This paper focused on the sessile droplet freezing and ice adhesion on aluminum with different wettability (hydrophilic, com- mon hydrophobic, and superhydrophobic surfaces, coded as HIS, CHS, SHS, respectively) over a surface temperature range of -9℃ to -19℃. It was found that SHS could retard the sessile droplet freezing and lower the ice adhesion probably due to the interfacial air pockets (IAPs) on water/SHS interface. However, as surface temperature decreasing, some IAPs were squeezed out and such freezing retarding and adhesion lowering effect for SHS was reduced greatly. For a surface temperature of-19℃, ice adhesion on SHS was even greater than that on CHS. To discover the reason for the squeezing out of lAPs, forces applied to the suspended water on IAPs were analyzed and it was found that the stability of IAPs was associated with surface mi- cro-structures and surface temperature. These findings might be helpful to designing of SHS with good anti-icing properties.展开更多
Due to the property of water repellence, biomimetic superhydrophobic surfaces have been widely applied to green technologies, in turn inducing wider and deeper investigations on superhydrophobic surfaces. Theoretical,...Due to the property of water repellence, biomimetic superhydrophobic surfaces have been widely applied to green technologies, in turn inducing wider and deeper investigations on superhydrophobic surfaces. Theoretical, experimental and numerical studies on wetting transitions have been carried out by researchers, but the mechanism of wetting transitions between Cassie-Baxter state and Wenzel state, which is crucial to develop a stable superhydrophobic surface, is still not fully understood. In this paper, the flee energy curves based on the transition processes are presented and discussed in detail. The exis- tence of energy barriers with or without consideration of the gravity effect, and the irreversibility of wet- ting transition are discussed based on the presented energy curves. The energy curves show that different routes of the Cassie-to-Wenzel transition and the reverse transition are the main reason for the irre- versibility. Numerical simulations are implemented via a phase field lattice Boltzmann method of large density ratio, and the simulation results show good consistency with the theoretical analysis.展开更多
基金Project(50435030) supported by the National Natural Science foundation of ChinaProject supported by the Program for New Century Excellent Talents in Chinese University Project(GZ080010) supported by the Open Research Fund Program of Jiangsu Province Key Laboratory for Photon Manufacturing Science and Technology
文摘Twelve samples with periodic array square pillars microstructure were prepared on the silicon wafer by plasma etching techniques, on which space b of the square pillars increased from 5 to 60 μm. In order to study the effect ofb on the wettability of the rough surface, the effects of apparent contact angle (CA) and sliding angle (a) of the droplet on the rough surface were measured with the contact angle meter. The results show that the experimental values of CA well agree with the classical wetting theory and a decreases with the increase of b. Two drop shapes exist on the samples' surface, corresponding to the Cassie state and the Wenzel state respectively. The contact state in which a drop would settle depends typically on the size of b. On the role of gravitation, the irreversible transition of a drop from Cassie state to Wenzel state should occur at a certain space of the square pillars. Since the transition has implications on the application of super-hydrophobic rough surfaces, theoretically, the prediction of wetting state transition on square pillar array micro-structured surfaces provides an intuitionistic guidance for the design of steady superhydrophobic surfaces.
基金Supported by the National Natural Science Foundation of China(No.20236050)
文摘A numerical investigation of the unsteady motion of a deformed drop released freely in another quiescent liquid contaminated by surfactant is presented in this paper. The finite difference method was used to solve numerically the coupled time-dependent Navier-Stokes and convective-diffusion equations in a body-fitted orthogonal coordinate system. Numerical simulation was conducted on the experimental cases, in which MIBK drops with the size ranging from 1.24 mm to 1.97 mm rose and accelerated freely in pure water and in dilute sodium dodecyl sulphate (SDS) aqueous solution. The applicability of the numerical scheme was validated by the agreement between the simulation results and the experimental data. Both the numerical and experimental results showed that the velocitytime profile exhibited a maximum rising velocity for drops in SDS solutions, which was close to the terminal velocity in pure water, before it dropped down to a steady-state value. The effect of the sorption kinetics of surfactant on the accelerating motion was also evaluated. It is also suggested that introduction of virtual mass force into the formulation improved obviously the precision of numerical simulation of transient drop motion.
文摘The accurate prediction of the droplet size distribution(DSD)in liquid–liquid turbulent dispersions is of fundamental importance in many industrial applications and it requires suitable kernels in the population balance model.When a surfactant is included in liquid–liquid dispersions,the droplet breakup behavior will change as an effect of the reduction of the interfacial tension.Moreover,also the dynamic interfacial tension may be different with respect to the static,due to the fact that the surfactant may be easily desorbed from the droplet surface,generating additional disruptive stresses.In this work,the performance of five breakup kernels from the literature is assessed,to investigate their ability to predict the time evolution of the DSD and of the mean Sauter diameter,when different surfactants are employed.Simulations are performed with the Quadrature Method of Moments for the solution of the population balance model coupled with the two-fluid model implemented in the compressible Two Phase Euler Foam solver of the open-source computational fluid dynamics(CFD)code Open FOAM v.2.2.x.The time evolution of the mean Sauter diameter predicted by these kernels is validated against experimental data for six test cases referring to a stirred tank with different types of surfactants(Tween 20 and PVA 88%)at different concentrations operating under different stirrer rates.Our results show that for the dispersion containing Tween 20 additional stress is generated,the multifractal breakup kernel properly predicts the DSD evolution,whereas two other kernels predict too fast breakup of droplets covered by adsorbed PVA.Kernels derived originally for bubbles completely fail.
基金Supported by the National Natural Science Foundation of China(No.51079095)the Science Fund for Creative Research Groups of the National Natural Science Foundation of China(No.51021004)
文摘A droplet undergoes spreading,rebounding or splashing when it impacts solid boundary,which is a typical phenomenon of free surface flow that exists widely in modern industry.Smoothed particle hydrodynamics(SPH)method is applied to numerically study the dynamical behaviors of the droplet impacting solid boundary,and both the spreading and rebounding phenomena of the droplet are reproduced in the simulation.The droplet deformation,flow fields and pressure fields inside the droplet at different moments are analyzed.Two important factors,the initial velocity and diameter,are discussed in determining the maximum spreading factor,revealing that the maximum spreading factor increases with the increase of the impact velocity and droplet diameter respectively.
基金supported by The National Natural Science Foundation of China (Grant No.51109178)Science and Technology Innovation Foundation of NWPU (Grant No.JC20120218)
文摘The rebound behaviors of droplets impacting on a self-fabricated superhydrophobic brass surface (WCA=I64.5°) were ob- served and studied by using high-speed-camera. In accordance with energy conversion, theoretical analysis of different behav- iors and rebound mechanism were given. At lower velocities, three behaviors in different velocity ranges were observed: par- tial rebounding, entire rebounding and ejecting during rebounding. At higher velocities, such two behaviors as rebound after splashing and rebound, ejecting after splashing, occurred alternately and exhibited certain periodicity. A function to predict the critical impact velocity is derived from energy conservation condition, and the prediction values tally with the experimental values, with the maximum relative error about 14%.
基金supported by the National Natural Science Foundation of China (Grant Nos. 51079095, 51021004)
文摘Droplet impacting liquid surface is not only the extremely prevalent phenomenon in the nature and industrial production but also the extremely complicated problem of strong non-linear transient impact and free-surface flow. On the basis of the two-dimensional viscous incompressible N-S equations, this paper conducts a study of numerical simulation on the problem of droplet impacting liquid surface (water beads) of water container in certain initial velocity by the method of SPH (smoothed particle hydrodynamics). The effect of surface tension is considered between surface particles by searching the free surface particles in the course of study; the effect of initial impact has been solved by use of artificial viscosity; at the same time, the side-wall virtual particles and image virtual particles are both introduced to deal with the boundary condition, which has solved the boundary defects quite well and eliminated the instability of real particles dropped to the comer of container. The calculated results form the distribution chart of particles, flow field chart, pressure chart and the displacement and velocity variation curve of different particles. The comparison between simulated results and experimental photos shows that the simulation is effective. This paper compares the variational curves for fluctuations of liquid surface qualitatively through adopting the methods of level-set, BEM and SPH, respectively at last. The simulated results show that it will produce strong non-linear phenomena, such as the splash of liquid, discrete liquid surface, and strong wave of free liquid surface, when the droplet impacts liquid surface; in the course of impacting, the movement of liquid particles exhibits the characteristic of oscillation; the method of SPH has certain advantages of dealing with the large deformation problem of free surface.
基金supported by the National Natural Science Foundation of China(Grant Nos.21203089 and 51263018)International Science and Technology Cooperation Program of China(Grant No.2012DFA51200)+1 种基金Science and Technology Project of Jiangxi Province(Grant No.20123BDH80015)the Open Fund of Jiangsu Key Laboratory for Biomass-based Energy and Enzyme Technology(Grant No.JSBEET1224)
文摘This paper focused on the sessile droplet freezing and ice adhesion on aluminum with different wettability (hydrophilic, com- mon hydrophobic, and superhydrophobic surfaces, coded as HIS, CHS, SHS, respectively) over a surface temperature range of -9℃ to -19℃. It was found that SHS could retard the sessile droplet freezing and lower the ice adhesion probably due to the interfacial air pockets (IAPs) on water/SHS interface. However, as surface temperature decreasing, some IAPs were squeezed out and such freezing retarding and adhesion lowering effect for SHS was reduced greatly. For a surface temperature of-19℃, ice adhesion on SHS was even greater than that on CHS. To discover the reason for the squeezing out of lAPs, forces applied to the suspended water on IAPs were analyzed and it was found that the stability of IAPs was associated with surface mi- cro-structures and surface temperature. These findings might be helpful to designing of SHS with good anti-icing properties.
基金financial support of this work by the doctoral degree scholarship of China Scholarship Council and the University of Nottingham,UK
文摘Due to the property of water repellence, biomimetic superhydrophobic surfaces have been widely applied to green technologies, in turn inducing wider and deeper investigations on superhydrophobic surfaces. Theoretical, experimental and numerical studies on wetting transitions have been carried out by researchers, but the mechanism of wetting transitions between Cassie-Baxter state and Wenzel state, which is crucial to develop a stable superhydrophobic surface, is still not fully understood. In this paper, the flee energy curves based on the transition processes are presented and discussed in detail. The exis- tence of energy barriers with or without consideration of the gravity effect, and the irreversibility of wet- ting transition are discussed based on the presented energy curves. The energy curves show that different routes of the Cassie-to-Wenzel transition and the reverse transition are the main reason for the irre- versibility. Numerical simulations are implemented via a phase field lattice Boltzmann method of large density ratio, and the simulation results show good consistency with the theoretical analysis.