Paper MN-TuP2
Wetting Behavior During Evaporation and Condensation of Water Microdroplets on Superhydrophbic Patterned Surfaces

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water repellent properties. The superhydrophobic surfaces may be generated by the use of hydrophobic coating, roughness and air pockets between solid and liquid. The geometric effects and dynamic effects, such as surface waves can destroy the composite solid-air-liquid interface. The relationship between the water droplet size and geometric parameters governs the creation of composite interface and affects transition from solid-liquid interface to composite interface. Therefore, it is necessary to study the effect of droplets of various sizes. We have studied the effect of droplet size on contact angle by using evaporation studies with droplet radii ranging from about 700 to 300 µm. Experimental and theoretical studies of wetting properties of silicon surfaces patterned with pillars of two different diameters and heights with varying pitch values are presented. We propose a criterion where transition from Cassie and Baxter regime to Wenzel regime occurs when the droop of the droplet sinking between two asperities is greater than depth of the cavity. Based on the experimental data and the proposed transition criteria, the trends are explained. For the first time, environmental scanning electron microscopy (ESEM) is used to form smaller droplets of about 20 µm radius and measure the contact angle on the patterned surfaces. The investigation has shown that ESEM provides a new approach to wetting studies on the microscale.