Pattern formation on hydrophobic PDMS substrates by evaporating droplets : combined effect of substrate stiffness, relative humidity and particle size

Desiccation patterns left by micro-droplets of water impregnated with particles on hydrophobic substrates have been analyzed with respect to variations in the elastic stiffness of the substrates, particle size and relative humidity. The complex and unique patterns obtained, have been analyzed and explained in terms of the time scales of moving Triple Phase Line (TPL) on substrate and substrate relaxation rate. The rate of TPL movement is found to depend on the relative humidity and substrate stiffness. In turn, this affects the contact angle hysteresis. Particle movement is a result of viscous drag and inertia apart from electrostatic interactions. We have successfully explained the myriad patterns obtained from drying droplets via systematic rheological measurements along with an understanding of the role of all the effective forces and their time scales of action.