Meso-scopic Simulation Of Droplet Spreading Under Squeezing Of Parallel Plates

The spreading behavior of a droplet(liquid bridge)under the squeezing of parallel plates is seen in the adhesion of micro-electronic components and the liquid lubrication between human joints,among which complex micro-scale flow behaviors exist,such as the moving contact line of triple phases.In order to solve this problem,a coarse-grained molecular dynamics method,i.e.,many-body Dissipative Particle Dynamics(MDPD),is used to simulate the droplet spreading under the squeezing of parallel plates,which may provide numerical guidance to practical application.Firstly,for the MDPD simulation of moving contact line(angle),the so-called YDF(Y-direction Distribution Function)algorithm is proposed for the measurement of the contact angle(line)of three-dimensional droplets deposited on a solid substrate,and the accuracy and robustness of the YDF algorithm are validated-Either hydrophobic or hydrophilic surface is modeled by varying the amplitude of the conservative force between droplet particle and wall particle.The specific rela-tionship between static contact angle(?static)of Newtonian fluid droplets and amplitude parameter Ast is obtained.In current simulation,the static contact angle can be constructed in the range of 40° 140°.The simulation results of moving contact angle(line)is validated with previous hy-drodynamic theory and a good agreement is obtained.The feasibility of the MDPD method in the droplet simulation is proved which lays a solid foundation for the study on the droplet spreading under the squeezing of parallel plates.Secondly,the spreading behavior of a Newtonian fluid droplet under the squeezing of parallel plates is simulated and analyzed as well as the dynamic behavior of contact angle(line)of droplets.In the quasi-static squeezing process,the whole squeezing process can be divided into two stages according to the evolution of the system free energy,which are the stage of the contact line retraction of the bottom plate(static plate)and the stage of the symmetrical contact line spreading of the upper(squeezing plate)and the bottom plates,respectively.The droplet spreading behavior in these two stages is thoroughly investigated and analyzed,and the effect of the squeezing speed of the upper plate is also studied.In addition,the effects of wetting property of the upper plate is also studied.The critical squeezing speed on which the contact line of the bottom plate without retraction under various working conditions is reported,and the whole process of the Newtonian fluid droplets spreading is described clearly.Finally,by designating two different types of MDPD particles and varying the conservative force between them,a yield-stress fluid model is constructed,in which topological microstructures are formed spontaneously.Moreover,the yield-stress fluid is in good agreement with the well-known Herschel-Bulkley fluid model.From the velocity profile of Poiseuille flow,the flow field can be divided into two parts,i.e.,the"solid" region and the"fluid" region.It is proved that the relationship between the local stress and the yield stress of the fluid model determines the attribution and division of the flow field.Based on this,a yield-stress fluid droplet can be constructed.The spreading behavior under the squeezing of parallel plates is further studied.It is concluded that under the same wettability of the plates,the maximum retraction displacements of moving contact lines of the yield-stress fluid droplet on the bottom plate is less than that of the Newtonian fluid droplet.