Evaporation characteristics of water droplets on super-hydrophobic surface
Figure1. The preliminary results are obtained with this device fabricated in CNMS.
Figure 2. Time ?lapse visualization of water droplets during natural convection on the (a) super-hydrophobic, (b) hydrophobic, and (c) hydrophilic surfaces. With the same volume of water droplets, the longer evaporatio time was observed on the superhydrophobic conditioins.
Many researchers studied about the characteristic of evaporating droplet on hydrophilic and hydrophobic surface for several decades. It is found that the variation of contact angle, contact radius, and height of the evaporating droplet are critical parameters in order to determine the surface characteristics. Two well-known equations, Cassie & Baxter and Wenzel, are used to predict the contact angle. They show a good agreement with microscale structures of array. However, our preliminary results of an array having 200 nm nominal diameter and 700 nm pitch show different phenomena from the generally-accepted equations addressed above. In this proposal we request access to EBL and the processing facilities to fabricate a series of arrays having different diameters and heights of posts and chemical coating for a more comprehensive future study of phenomena of evaporating droplet on hydrophobic surface. With these devices, we would like to perform two separate experiments. First we would like to find out the critical parameters to determine evaporation characteristics of water droplets on the hydrophobic and super-hydrophobic surfaces and modify, if possible, traditional equations widely accepted. Secondly, we would like to examine nano-particle motion /flow in the water droplets caused by both natural and forced convection on the hydrophilic and hydrophobic surfaces fabricated in this proposal.
The evaporation characteristics depending on the post-pattern array will be examined outside of the CNMS. The flow or convection phenomena will be examined by using mciro-PIV in the department of Mechanical Engineering ? Engineering Mechanics, Michigan Technological University. The contact angle measurement to determine surface conditions of hydrophilic, hydrophobic, or super-hydrophobic will be performed PI¡¯s developed optical system, while the numerical analysis has been performed at the Chung-ang University, Seoul, Korea.