Modeling the Wetting Behavior of Heterogeneous Patterned Surface by Numerical Method

There are two sections in our studies. First part is a simple and cheap method for fabrication of superhydrophobic surface and the wetting behaviors on structured hydrophilic or hydrophobic surfaces are also studied. The other part is the study of wetting behavior on 2 μm striped heterogeneous surface.

Wetting behavior plays an important role in various industries, and has been studied for a long while. Lower the surface energy and enhance the surface roughness are the method to prepare superhydrophobic surface. We are able to combine and modify a soft-embossing technique and surface modification of a self-assembled monolayer to establish a standard operation procedure for manufacturing superhydrophobic surfaces. Two issues are addressed in this part of study. First, the water contact angles of surfaces without modification are discrepant with the Wenzel and Cassie models. Second, the different polymer surfaces with modification can change their characteristics from hydrophilicity to hydrophobicity. Further, different polymer surfaces have the same contact angle only if they have the same structure before modification. And the results are quite consistent with Wenzel and Cassie models.

Besides, we examine contact angle of liquid droplet on 2 μm striped surface by the Matlab modeling and also determine equilibrium contact angle (ECA), advancing contact angle (ACA) and receding contact angle (RCA) from free energy curve obtained by the Surface Evolver. The ECA (78°) from the Matlab modeling is totally consistent with the Cassie model, but is quite different from the results (83°~85°) shown in the Surface Evolver. However, the tendencies of ECAs from both methods are the same – oscillation with small liquid and approaching a constant with larger liquid. In our system, ACA is always a constant (110°) but RCA is volume-dependent. RCA oscillates at first, and finally falls into a certain narrow region (70°~80°). Our result also indicates that the contact line of liquid drop inclines to be a slightly ellipse-like shape, not a perfect circle-like shape. Further, we make a discussion about the thickness of contact angle transition region. The dimensionless thickness from hydrophobic to hydrophilic surface is almost 0.22, and the dimensionless thickness from hydrophilic to hydrophobic surface decreases from 0.02 to 0.0027 as increasing liquid volume.