Vaporization and Wetting Behaviors of Liquid Droplet on Solventphilic Patterned Surface
Date Issued
2014
Date
2014
Author(s)
Chen, Ding-Long
Abstract
Our research was focus on the wetting behavior of liquid droplet on solventphilic patterned surface. Contact angle of liquid droplet on a surface below than 90° was referred to the solventphilic surface. We chose octanol as the liquid droplet, PDMS as the solid surface. The advancing contact angle of octanol was 38°, the receding contact angle of octanol was 25°. The contact angle showed the affinity between the PDMS and octanol. The experiment included two parts: vaporization process of an octanol droplet deposited on the patterned PDMS surfaces and contact angle measurement. We used PDMS as the substrate to create series of different width and spacing pillar surface. When we observed the vaporization process of an octanol, it could be found that different pattern geometry showed different vaporization process. The vaporization process could be characterized into three kinds: Wenzel state, Cassie impregnating wetting state and Mixed state. Hemi-wicking and vaporization sequence of droplet differed from each other. The hemi-wicking wasn’t found in the process and receding with a constant angle when droplet was in Wenzel state. Hemi-wicking was the key feature of Cassie impregnating wetting state. But receding contact angle couldn’t be seen. Mixed state had both characteristics of Wenzel state and Cassie impregnating wetting state. No hemi-wicking and receding contact angle couldn’t be observed when droplet in Mixed state. From the observation of vaporization process, we know pattern geometry could affect the wetting states. Narrower pillar spacing and taller pillar would change the vaporization process from Wenzel state to Mixed state then the Cassie impregnating wetting state. We used two kinds of method to measure the advancing and receding contact angle. One was captive bubble method, the other was embedded-needle method. We got different tendency in contact angle with these two methods. When captive bubble was conducted, the advancing contact angle decreased as the surface roughness was increased. Eventually, the advancing contact angles become a constant when the surface roughness is further increased, i.e., the region of Cassie impregnating wetting state. However, when embedded-needle method was used, the advancing contact angle increased as the surface roughness was increased. Eventually, the advancing contact angles became constant when the surface roughness was further increased. The advancing contact angle measured by embedded-needle method didn’t meet the Wenzel model, but the result can be reproduced even we changed other kind of liquid. There was still no clear explanation to why captive bubble method could get the tendency of advancing contact angle decreased as the roughness increased. We still have long way to clarify the captive bubble method. This research provides another opinion when we deal with the wetting behavior and vaporization process in solventphilic surface.
Subjects
contact angle
advancing contact angle
solventphilic
vaporization
patterned surface
Type
thesis
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