Quasi-static motion of microparticles at the depinning contact line of an evaporating droplet on PDMS surface

In this paper, evaporation of sessile water droplets containing fluorescent polystyrene(PS) microparticles on polydimethylsiloxane(PDMS) surfaces with different curing ratios was studied experimentally using laser confocal microscopy. At the beginning, there were some microparticles located at the contact line and some microparticles moved towards the line. Due to contact angle hysteresis, at first both the contact line and the microparticles were pinned. With the depinning contact line, the microparticles moved together spontaneously. Using the software ImageJ, the location of contact lines at different time were acquired and the circle centers and radii of the contact lines were obtained via the least square method. Then the average distance of two neighbor contact lines at a certain time interval was obtained to characterize the motion of the contact line. Fitting the distance-time curve at the depinning contact line stage with polynomials and differentiating the polynomials with time, we obtained the velocity and acceleration of both the contact line and the microparticles located at the line. The velocity and the maximum acceleration were,respectively, of the orders of 1 μm/s and 20-200 nm/s^2, indicating that the motion of the microparticles located at the depinning contact line was quasi-static. Finally, we presented a theoretical model to describe the quasi-static process, which may help in understanding both self-pinning and depinning of microparticles.

Effect of substrate elasticity on evaporation kinetics and evaporative deposition of aqueous polystyrene nanoparticles droplets

Evaporation of aqueous polystyrene(PS)nanoparticles droplets on silicon and polydimethylsiloxane(PDMS)surfaces was studied.Experimental results showed that softer PDMS surfaces yielded a longer constant contact radius(CCR)stage,which could be ascribed to surface deformation of PDMS induced by the vertical component of liquid-vapor interfacial tension.Ringlike depositions of nanoparticles with different crack patterns were found on both silicon and PDMS surfaces.In-situ observation of crack formation showed that nanoparticle movement on the silicon surface was impeded,resulting in radial cracks with periodic distribution.In contrast,nanoparticles were shown to move easily on the PDMS surface.This observation indicated the difference in crack patterns on surfaces could be attributed to the friction force between nanoparticles and the substrate.A large friction force between nanoparticles and the substrate prevented cracks from moving,resulting in a radial crack pattern with periodic distribution,while a small friction force produced multiple large cracks.