Katarzyna Kowal

Slippery flows in nature and industry

In various settings in nature and industry, certain types of substrates promote increased fluid flow through sliding. Large-scale examples include the flow of ice sheets, such as that of Greenland and Antarctica, over bedrock lubricated by a layer of subglacial till. Small-scale examples include the lotus leaf and fabricated hydrophobic surfaces, which are of use for various industrial applications, from microfluidics and flow delivery in the pharmaceutical and biotechnology industries, to drag reduction applications in the aeronautics and maritime industries, for example. What makes these types of surfaces slippery is their microstructure, which cushions the flow from below, promoting slip. This talk goes through the development of a novel theoretical and experimental framework for generating slip underneath viscous fluids experimentally. We build into it the freedom to adjust slip, as desired, for large-scale fluid-mechanical experiments motivated by environmental and industrial phenomena in which basal sliding is important. We base our framework on an analogy to classical hydrophobic surfaces and consider a range of substrates, including those that are lubricated from below, those that are deformable and those that are structured, consisting of a periodic sequence of two-dimensional, fluid-filled cavities, each giving rise to macroscopic sliding laws. We illustrate these theoretical results by means of a series of fluid-mechanical experiments.