Experimental Investigation Of Internal Flow Inside Evaporating Droplet On PDMS Surface

With the emergence of new materials and continuous optimization of fabrication technology,some materials such as polydimethylsiloxane(PDMS)play an extremely important role in micro/Nano fabrication,micro/nano electromechanical systems and micro/nano-fluid systems,etc.Meanwhile,droplet is widely used in these systems as carrier.However,it is usually evaporative or volatile.On the one hand,its evaporation behavior can have a great influence on the working performance of these systems.On the other hand,the behavior can be used to derive some new functions.Under the action of vertical component of liquid-vapor interfacial tension,there will be a surface deformation.Such a deformation will influence the evaporation characteristics of sessile droplet and then affect the internal flow inside the evaporating droplet.In this dissertation,internal flow inside an evaporating droplet on PDMS surfaces was studied experimentally.The main contents of this dissertation include as following:(1)Evaporation of water droplet on PDMS surfaces was experimentally investigated.It was found that the larger the curing ratio,the lower the Young's modulus and the longer the duration of constant contact area(CCR)stage.The analytical solution of sessile evaporating droplet in CCR stage was obtained under the assumptions of spherical droplet and uniform concentration gradient.(2)The internal flow inside evaporating droplet on PDMS surfaces was studied using micro-particle imaging velocimetry(Micro-PIV)to reveal the influence of the curing ratio on convection and the distribution of velocity and vorticity.Deposition patterns of suspension droplet with two kinds of concentration after evaporation were observed using laser confocal microscopy,and the results showed obvious difference between the two cases.(3)The internal flow at the solid-liquid interface was studied using particle tracking velocimetry(PTV)technology.It was found that there is an internal flow from the edge of the droplet to the center at the early evaporation stage and then the movement is reversed.The movement characteristic of the depinning contact line was analyzed using the least square method.Comparing the forces acting on microparticles near the depinning contact line with the inertial force,we found that these microparticles were nearly in force equilibrium.At last,we established a novel pinningand depinning mechanisms of microparticles confined at the contact line.