Measurement And Hysteresis Of Contact Angle Based On Lattice Boltzmann Method

In computational fluid dynamics,the lattice Boltzmann method,which originated from lattice gas automata and molecular dynamics theory,has developed into a reliable new method,and its efficiency,accuracy and robustness are widely confirmed.It is no longer dependent on the Navier-Stokes equations of macroscopic continuum,and the macroscopic behavior of complex fluid is simulated from microscopic model,and it is successfully applied in the study of complex fluid motion such as particle flow,heat flow and microfluidics.Especially in the study of multiphase flow,the lattice Boltzmann method does not need the boundary integral,which avoids the difficulties encountered by the traditional computational fluid dynamics method in tracking a large number of dispersed phase interfaces.Based on the principle of thermodynamics,the task force proposed a thermodynamic consistency and Galilean invariance of the multiphase flow model,which laid the foundation for the study of surface wetting.Surface wetting is a common natural phenomenon.As a characteristic of surface wettability,the contact angle is the result of the three-phase interaction of gas,liquid and solid surface,and it is also the physical quantity commonly used in research and application.Although the lattice Boltzmann method simulates the study of surface wetting has been a great success,but there is no reliable algorithm to calculate the dynamic contact angle;even the static contact angle can only use the ideal spherical crown model calculation or the use of external tools measuring.First,the use of chemical potential boundary conditions,the droplets directly on the substrate,no longer need to transition zone,contact angle can be measured directly from the three contact points.Then,by analyzing the microcosmic morphology of the contact angle,the tangent is determined by the intersection of the gas-liquid interface and the second row grid,and the method of calculating the dynamic contact angle in real time is proposed.Under the condition of neglecting gravity,the contact angle calculated by this method is compared with the theoretical calculation result.At temperatures of 0.7 and 0.8,the range of 30 to 160 degrees is very good,indicating that the calculation of the method is accurate.At the same solid interface,the droplet radius is from 20 to 200 lattice,and the calculated contact angle remains constant,indicating that the calculated result of the method is stable.In the case of containing gravity,droplets and drape droplets of different sizes from 0.1 cm to 0.5 cm were calculated.The results show that the deformation degree of the droplet is increasing with the increase of the size,but the contact angle of the droplet calculated by the current method is kept constant.This is consistent with the theoretical expectations,but also a true reflection of the microscopic nature of the contact angle.Further,the droplets are placed on the solid surface in which the hydrophobic water is alternately arranged,and when the degree of inclination of the surface is not too large,the droplet is tilted under the action of gravity,resulting in the contact angle hysteresis.The contact angle hysteresis was calculated using three different hydrophobic configurations.The results show that although the hydrophobic mode surface has a greater contact angle than the hydrophilic surface,and the droplets are less prone to instability,however,the contact angle hysteresis is basically the same as the tendency of the surface to increase.When the unsteady sliding,the droplets will continue to slip through the hydrophobic water strip,the advance angle will gradually increase and then suddenly decline,the angle will gradually decrease and then suddenly increase,the two changes are generally not synchronized,resulting in contact hysteresis dynamic state of fluctuation.The dynamic waveform of the contact angle hysteresis is related to the phase of the forward and backward angles,and contains the high order fluctuations due to the jitter of the droplet.These studies on contact angles and hysteresis facilitate deep understanding and practical application of wetting.The lateral migration of particles in the flow of pipelines is a wonderful natural phenomenon.In recent years,it has developed into a research hotspot in the field of microfluidics.Using the multi-relaxation lattice Boltzmann method,we study the inertial aggregation of elliptical particles.Similar to the classical Segre-Silberberg effect,elliptical particles also exhibit lateral migration and equilibrium.However,due to its non-circular geometry,elliptical particles also contain non-uniform rotation and periodic vibration.we have studied the effects of Reynolds number,blocking ratio and aspect ratio on the elliptical trajectory,and found that the change of the Reynolds number to the elliptical equilibrium position is very small.when the Reynolds number changes from 3 to 300,the change rate of the equilibrium position is only 3%the larger the blocking ratio is,the closer the equilibrium position is to the centerline of the pipe,and the effect of the particles on the flow field is stronger.as the aspect ratio increases,the rotation period of the elliptical particles becomes shorter and the equilibrium position exhibits a saddle,In the aspect ratio close to 0.5 to reach the lowest point.this work has a positive effect on the study of blood flow in birds with oval or oval red blood cells.