(483c) Elastic Deformation of Soft Supported Films of Finite Thickness during the Drainage of a Viscous Fluid

Elastic deformation of soft supported films of
finite thickness during the drainage of a viscous fluid

Joelle Frechette and Yumo Wang, Chemical and Biomolecular
Engineering, Johns Hopkins University, Baltimore MD 21218

Elastohydrodynamic deformation can
cause lift and reduce friction during sliding and alter the rheological
properties of soft colloidal particles. It also modifies the shape of
approaching surfaces, a determining factor for the adhesion dynamics to wet or
flooded surfaces. When studying elastohydrodynamics in soft matter it is a
challenge to measure simultaneously the hydrodynamic forces and the
deformation, both necessary to understand how contact is reached and the
coupling between deformation of the solid and viscous dissipation in the fluid.

In
this presentation the characterization of the spatiotemporal deformation of an
elastic film during the radial drainage of fluid from a narrowing gap will be
discussed. As predicted, we will show that the elastic deformation of a thick film
takes the form of a dimple and prevents full contact to be reached.¹ We reveal the
importance of measuring absolute surface separation when working with soft
materials by showing the effects of viscoelasticity of the solid in the fluid
film profiles, and point out that elastic compliance leads to lower hydrodynamic
forces at a given time while deformation leads to stronger forces at a given
fluid film thickness.With thin supported elastic films the stress caused by
fluid drainage becomes increasingly transferred to the underlying rigid
substrate and the dimple formation is suppressed, which allows the surfaces to
reach full contact (See top of Figure 1). For intermediate film thicknesses we
observe shapes that are more complex when close to contact (see bottom of Figure
1). These shapes originate from lubrication pressure and its corresponding
deformation distribution on a curved, layered compliant system. We extend existing
models for elastohydrodynamic deformation to the more general case of the
compliant material being part of a multi-layer system instead of a half-space.
When taking the finite thickness of the compliant film into account we predict the
same complex morphology as the ones observed in experiments. Our work is the
first reported and validated prediction that such complex surface profiles
exist in elastic solid and we draw analogies to the cases of “wimple” or
“pimple” that have been observed in droplets.

FIG.
1. (Left) Schematic of the elastohydrodynamic problem highlighted on effects of
film thickness. Grey core: rigid substrate; Red layer: compliant coating. Up:
thin film; down: intermediate thickness film. (Right) experimental deformation
profile of the compliant coatings caused by lubrication forces with
corresponding film thicknesses.

 Reference:

1. Wang, Y., C. Dhong, and
J. Frechette, Out-of-Contact
Elastohydrodynamic Deformation due to Lubrication Forces. Physical review
letters, 2015. 115(24): p. 248302.