Dynamic wetting on non-smooth surfaces

Wetting is a ubiquitous phenomenon in nature. We see it in daily life such as coffee stains, lotus effects, etc. The lotus leaf  takes advantage of rainfall to effectively clean its surface by microstructures which makes the leaf hydrophobic allowing dirt to be captured in water beads that roll off the leaf. In the field of industry, we also find inkjet printing and spray coating and other microfluidics applications. Better controllability of wetting will improve the efficiency of these processes, and may open possibilities for new applications.

Because of its ubiquity and importance in the engineering field, enormous efforts have been made to understand the underlying physics. Yet, wetting on the complex structures has not been revealed, especially in dynamic situations, i.e. when contact line is moving. At a contact line, where three phases meet, complex and multi-scale phenomena take place. Because of this, continuum expressions are not capable of capturing moving contact line, and energy dissipation at a contact line must be taken into consideration. Furthermore, it gets even more complex when the contact line follows non-flat geometries, such as surfaces with slant pillars or flexible fibers.

We investigate wetting dynamics on such structures combining experimental observations and numerical simulations. For the first step we perform direct measurements of droplet spreading. The surfaces was made of Off-stoichiometry thiol-ene (OSTE) polymer with UV lithography and spreading is recorded with High speed camera.  OSTE and UV lithography technique allow us to investigate wide varieties of structures. In order to follow experimental results, we perform numerical simulations with the corresponding geometries based on Navier-Stokes and Cahn-Hilliard equation. The simulation is done efficiently with Maple-based toolbox "femLego", which was developed by Gustav Amberg and his co-workers.

Water droplet spreading on a surface with slant pillar arrays