Wetting and transport phenomena of substrateless plane and curvilinear hierarchical stripe structures

• Applicants: Privatdozent Dr. Johannes L´huillier; Professor Dr. Egbert Oesterschulze; Professorin Dr.-Ing. Clarissa Schönecker
• Subject Area: Fluid Mechanics ; Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 467661067

In this project we investigate innovative Cassie-Baxter (CB) wetting surface structures that feature no underlying substrate. In this way, we avoid the known disadvantages of such patterns with substrate: sagging of the meniscus and condensation of vapor enclosed in pockets of the structure that both lead to the complete wetting of the surface in the undesired Wenzel state. Substrateless wetting structures offer a further accessible interface and thus provide an additional degree of freedom which will be exploited to control the wetting state of the other interface. This is accomplished by enclosing the second interface into a fixed volume and choosing therein the appropriate medium (gas or oil as infusion medium) and pressure. Stripes are addressed as wetting patterns in a planar as well as cylindrical geometry because they exhibit excellent transport properties. For the improvement of both the meniscus stability and hydrophobic wetting properties of the stripes we will integrate overhang as well as nanostructures. The static and dynamic wetting behavior of these different structures will be investigated both experimentally and theoretically. The goal is the control of the meniscus topology by external gas pressure or oil infusion.Although we will fabricate planar stripe structures from silicon because of the reproducible and very accurate micromachining processes available in the clean room, we will also address planar as well as curvilinear surface structures made of metals obtained by laser micro machining. In this case thin foils made of various stainless steel metal structures are employed to establish and obtain finally technically relevant processes and structures like tubes. From our point of view this is an important issue because to our knowledge it was not in the focus of the literature of wetting phenomena at all. We will transfer the knowledge obtained from the experimental studies of the silicon structures to the metal based systems. In case of laser micro machining the processing of burrless surface structures polished or furnished with a nanotexture will be studied in detail for each material of interest. Static wetting analysis of all stripe surface structures will be conducted by contact angle measurements including the determination of contact angle hysteresis. Furthermore, we will apply optical coherence tomography (OCT) to visualize the topology of the meniscus of droplets or fluids between the stripes exploiting the depth profiling capability of OCT. For the investigation of transport phenomena, the surfaces will be incorporated in channel and tube systems. The experimental investigations will be complemented with theoretical analyses, spanning from the analytical modelling of the substrateless- and wetting state-specific slippage to numerical flow simulations and the evaluation of the achieved drag reduction.

• DFG Programme: Research Grants