The movement of liquid molecules along a solid surface is called hydrodynamic slip. This event is central to understanding how fluids are transported at the smaller scales.
Between a solid and liquid interface, friction is present. When this friction is extremely high, the velocity of the fluid at this interface can be considered zero - this is called the “no-slip” boundary. This can be used to assume fluid flow at macroscopic scales, however there has been much focus in the last few decades to understand this at the more microscopic scale.
A cross-disciplinary research collaboration* aimed to develop a fundamental understanding of the physics of fluid flow. They investigated a long-standing question in fluid dynamics by trying to understand the factors that control friction at a solid/liquid interface. The group did so by conducting experiments using novel techniques that allowed them to precisely measure nanoscale fluid flow.
When discussing their experimental setup, Dr Mark Ilton said: “We use several Linkam stages in the labs, all in the THMS family. The Linkam stages provide a standardised way to thermally anneal our samples across the various labs involved in the collaboration. The simplicity, quick ramp-rates, and remarkable long-term stability are all key features. Since the viscosity of the polymeric fluids, a crucial parameter in our measurements, is highly sensitive to temperature, the precision of the Linkam stages is integral to the experiments. The size of the sample stage provided enough room to have a control sample side-by-side with a sample of experimental interest. This was a crucial part of our experimental protocol and enabled the data quality that supported our conclusions.”
Their experiments demonstrated that solid substrates that are considered “ideal” (coated silicon wafers, where the solid/liquid interactions are weak compared to uncoated substrates) can still have consequential friction due to transient adsorption of liquid molecules. This has important repercussions for products that use such coatings as they may not be as ideal as first thought.
By Tabassum Mujtaba
*McMaster University, University of Massachusetts Amherst, University of Bordeaux, Global Institution for Collaborative Research and Education, Hokkaido University, Laboratoire de Physico-Chimie Théorique, PSL Research University, Max Planck Institute for Dynamics and Self-Organization & Ecole Polytechnique.