Monday, 22 August 11:00am – 12:00pm

This seminar will be delivered in Online Zoom Please email chemistry.researchsupport@sydney.edu.au for zoom link and password.

Speaker: Mr Christopher Vega Sanchez; The University of Sydney

Supervisor: Prof. Chiara Neto

Title: Fluid slip on lubricant-infused surfaces

Abstract: Fluid slip on surfaces is a highly desirable property for many applications. It reduces the frictional drag, thereby reducing the energy associated with fluid transport. Lubricant-infused surfaces (LIS), in which a gas or liquid is entrapped within the surface roughness, have been proposed as a passive method to enhance fluid slip. After more than two decades of research, gas-infused surfaces (superhydrophobic surfaces) have been found to be unstable under hydrostatic pressure, flow, and impurities, which causes the fluid slip to vanish. Liquid-infused surfaces are a valid alternative, as the incompressible liquid lubricant is stable under high pressures, but they are affected by other issues: mainly that the liquid lubricant can be depleted under shear imposed by the external flow, leading to the disappearance of the slip.

In this Thesis, three outstanding aspects of slip on lubricant-infused surfaces were investigated: 1) the impact of total or partial lubricant depletion on the effective slip; 2) the development of experimental methods to accurately quantify slip on structured surfaces, and 3) reconciling experimental measurement of slip with existing theoretical models.

Firstly, the effect of lubricant depletion, lubricant-fluid interface shape and lubricant viscosity on the effective slip was studied using a two-phase Couette flow numerical simulation. The results indicate that the effective slip on LIS is affected by the shape of the lubricant-fluid interface and the lubricant-fluidviscosity ratio only when the surface is fully infused. However, even a slight (20%) lubricant loss decreases slip to the point of making the remaining (80%) of the lubricant superfluous, irrespective of the lubricant viscosity and interface shape.

Secondly, pressure drop measurements were carried out using a microfluidic setup to quantify slip on LIS. Previous works have reported slip length values for liquid-infused surfaces larger than predicted by theoretical models. Here, the pressure drop vs flow rate method was optimized in order to reduce the experimental uncertainty in the measured effective slip to 0.2 μm to 0.9 μm. The slip on silicone oil-infused surfaces was always larger than predicted and increased with increasing dissolved air content in the working fluid.

Finally, atomic force microscopy maps of the lubricant film and the surface topography revealed that the mechanism responsible for this unexpectedly large slip on oil-infused surfaces is the spontaneous nucleation of nanobubbles, which fill the surface roughness. A numerical simulation showed that the height and distribution of these lubricating layers quantitatively explain the large fluid slip observed experimentally.