Evaporation of ethanol/water mixture droplets on a pillar-like PDMS surface

Evaporation of about 6.0 mu L ethanol/water droplets with different ethanol concentrations on both planar and pillar-like polydimethylsiloxane (PDMS) surfaces was studied. A small fraction of the as-fabricated pillar-like PDMS surface is Wenzel-wetted while other part Cassie-Baxter-wetted. When ethanol/water droplet evaporates on a planar PDMS surface, the contact radius continuously decreases with time, and the contact angle first increases to the maximum, then keeps unchanged and finally decreases. However, when a mixture droplet evaporates on a pillar-like PDMS surface, the droplet first experiences a relatively long constant contact radius (CCR) stage without or with the addition of ethanol. There is a spreading process in the final stage of a mixture droplet with an initial ethanol concentration of 25%. A spontaneous spreading is also found at the end of the CCR stage when a 50% ethanol/water mixture droplet evaporates on a pillar-like PDMS surface. To better understand the spreading behavior, a KH-8700 microscopy was used to record the process from top view and it was found that the solid-liquid area developed to an octagon. The average spreading speed was measured to be 0.3-0.9 mu m/s. Using Taylor's expansion theorem, excess free energy was calculated and it is concluded that evaporating droplet will spontaneously spread on a pillar-like PDMS surface when the excess energy becomes larger than the energy barrier.