|Ph.D Student||Eyal Bittoun|
|Subject||Wetting on Super-Hydropohbic Surfaces|
|Department||Department of Chemical Engineering||Supervisor||Professor Emeritus Marmur Abraham|
|Full Thesis text|
A wide range of artificial and biological surface structures exhibit a very high contact angle (> ~150o) and a very small roll-off angle (< ~5o) when they come in contact with a water drop. The wetting properties of such surfaces are achieved by combining a low surface tension of the solid with a desired hierarchical surface roughness form the micro to the nanometer scale. The preparation of artificial and bio-mimetic non-wettable surfaces has been widely developed in the past decade. However, the physical mechanism governing the behavior of water drops on such surfaces is still incompletely understood.
The general objective of this research was to determine thermodynamic considerations for optimizing non-wettability. Basic theories of wetting on chemically heterogeneous and rough surfaces were validated experimentally and by numerical simulations. Furthermore, it was experimentally and theoretically shown that the measurement of contact angle hysteresis is sensitive to chemical heterogeneity even at the nano-meter scale.
Single and hierarchical surface topographies in two dimensions such as flat-top pillars, paraboloids, and fractal surfaces that mimic surfaces in nature were theoretically studied to optimize non-wettability. Three useful criteria were employed for comparing the non-wettability of surface designs from a wetting point of view. The first is the apparent contact angle at the transition point between the homogeneous (Wenzel) wetting regime and the heterogeneous (Cassie-Baxter) wetting regime. The second criterion is the wetted area of the solid surface in the heterogeneous wetting regime, and the third criterion is the height of the roughness feature. The results show that rounded protrusions, such as the paraboloids, are advantageous. This is an interesting conclusion, since the protrusions of the Lotus leaf are of a similar geometry. Moreover, it was demonstrated that multi-scale roughness does not necessarily guarantee non-wettable properties for the surface, unless it surface topography can thermodynamically stabilize the heterogeneous wetting regime.