Numerical Simulation And Theoretical Analysis On Droplet Motion In Microchannels Driven By Wettability Gradient Surfaces

The self-motion of droplets driven by wettability gradient is a common phenomenon in diverse areas,such as biomedicine,pollution detection,heat transfer enhancement and petrochemical industry,and the dynamic characteristics of droplet spreading,shrinking or disturbing have great significance for practical applications.In this thesis,the self-motion of droplets in microchannels with different sizes were numerically studied by using COMSOL Multiphysics based on an improved level set method firstly.The effects of contact angle,wettability gradient,wetting characteristic,channel size,working fluid,and number of droplets on the droplet motion were investigated in detail.The variation of droplet displacement,velocity and vortex during its self-motion were presented.Secondly,the mathematical model of droplet self-propelling on a solid surface was proposed in terms of mechanical and energy perspectives.And then the self-propelling mechanism of droplets in parallel microchannels was analyzed and discussed in detail to verify the accuracy of the earlier simulation,which provides certain guiding significance for future works.The main work and conclusions are drawn as follows:(1)The dynamic behaviors of droplet self-propelling in microchannels with the contact angle of wetting wall decreased linearly from 110° to 101° were numerically simulated and the droplet self-motion process was obtained by the software of COMSOL.When the wettability gradient was established onto the channel wall,the droplet self-motion can be achieved towards the high wetting side of the channel accompanied by droplet shrinking and spreading forward.It was found that there existed many vortex in the passing area and the vortex near the three-phase contact line was especially larger.Moreover,there existed slight difference in the vortex distribution between the upper and lower wetting walls,which may be due to the gravity action.During the whole movement process,at the beginning the droplet velocity increased dramatically and then towards a constant value,featured by local fluctuations.(2)The linear continuous,exponential and step-wettability equations describing the wettability gradient forms of channel surfaces were introduced,respectively,and the dynamic behaviors of droplet self-propelling in microchannels were numerically simulated and compared among the three contact angle equations.It was found that the linear contact angle function provided a strong driving force without causing a large drop in speed and could consume less computing resources according to the result comparisons with other two equations.Therefore,the linear continuous equation was chosen as the optimal form to describe the wettability gradient form of surfaces.(3)The influences of wettability gradient,wetting characteristic,channel size,working fluid,and droplet number on the droplet movement were investigated and compared in detail.The results showed that the droplet self-motion was enhanced at a smaller initial contact angle under the same wettability gradient and a greater wettability gradient under an identical initial wetting condition,which effectively increased the kinetic energy and droplet velocity.The droplet velocity varied in the range of several to tens of centimeters per second at wettability gradients ranging from 3.5 to 10 °/mm.Besides,the channel width(75,120,and 180 ?m)also provided profound impact on the dynamic behavior of droplets,and the droplet speed increased with the channel width due to the reduction of flow resistance.The influence of working fluid can be attributed to the ratio of surface tension to dynamic viscosity and it was found that the droplet moved faster at larger surface tension.Furthermore,the droplets also moved faster as the number of droplets increased owing to their interaction which enhanced the disturbance of velocity field and weaken the motion resistance as well.It was also considered that channel surfaces of droplet passing region were wetted,which in turn weaken the viscous resistance effectively.(4)Based on the theoretical analysis,a mathematical model,including the derivation of kinematic equation and the analysis of energy conservation equation,was established to analyze the mechanism of droplet self-motion in parallel microchannels.Then,the numerical results were obtained by the software of MATLAB and was compared with the above simulation results to verify its reliability.It was found that the changes of channel size,wettability gradient,and fluid type greatly affected the droplet velocity.According to the law of conservation of energy,the feasibility and accuracy of droplets self-motion in parallel microchannels were verified with the energy method.Moreover,the comparison between the numerical results derived from numerical simulation and theoretical model,respectively,largely demonstrated the reliability of computational model.