Abstract:
self-cleaning and anti-icing. Superhydrophobic
surfaces have extreme water repellent properties associated with very high static contact angles
and low resistance to liquid motion on surfaces. These properties result from the combination of
the chemical hydrophobicity of the topmost layers of the substrate and its roughness, the latter
being the dominant factor. The superhydrophobic behavior arises on rough surfaces when drops
are suspended on microstructures, so that their contact with the substrate is minute. This
suspended state (Cassie-Baxter state) is however tends to collapse to wetted state (Wenzel state)
if the liquid-air interface is perturbed. Also, multiple metastable Cassie-wetting states may exist
separated by an energy barrier from Wenzel state. In this study, numerical method is applied to
investigate the wetting properties of micro-patterned rough surfaces with a particular focus on
the stability of the Cassie-Baxter state as well as to find the optimal texture to support the
superhydrophobic configuration.
Using open source software, Surface Evolver, 3D drop-shape models are developed to
numerically investigate the shapes and energies of Cassie drops on substrates with micro-posts
for a wide range of parametric space. The wettability of the pillar-patterned surfaces is
numerically quantified with an analysis of apparent static contact angle of droplets. The effect of
pillar spacing on contact angles is explored and the results are found to be in good agreement
with the classical wetting theory as well as with experimental data.
A normalized form of interfacial energy is used to compare the stabilities of droplet on substrates
with square pillars. The sequence of stable drop configurations with increasing droplet volume
on substrate is analyzed and an explanation of superhydrophobic drop spreading has been
provided for both isotropic and anisotropic wetting behavior. Numerical analysis shows that droplet spreading from one isotropic wetting configuration to one anisotropic configuration is not
favorable unless the spreading of the droplet is restricted to be anisotropic.
Applying dimensional variation in texture characterized by different pillar spacing and pillar
width, the key parameter which plays dominant role in the stability of droplet is investigated.
From numerical analysis it reveals that the solid fraction that avails at the drop-base is vital for
the stable size of the droplet. If droplet of different volumes are deposited confirming same dropbase
area on two distinct surfaces with difference in pillar spacing or width, the surface that
allows less solid fraction at drop-base will give stability to larger droplets compared to that
attains stability on the other surface.
Finally, to investigate the effect of pillar structure on superhydrophobicity, drop shapes on
cylindrical pillars and square pillars are explored under exactly same wetting configuration and it
has been found that the pillar structure does not cause notable effect in wetting if the droplet size
is too much large compared to the roughness structures.
In general, the developed simulation techniques provide reliable prediction of the wetting
behavior which will significantly aid the design of superhydrophobic surfaces through
optimizing the geometric parameters such as pillar size, shape and spacing as well as the nature
of surface coatings required.
