A moving contact line as a rheometer for nanometric interfacial layers.

Romain Lhermerout,Bruno Andreotti,Hugo Perrin,Etienne Rolley,Kristina Davitt

doi:10.1038/ncomms12545

Abstract

How a liquid drop sits or moves depends on the physical and mechanical properties of the underlying substrate. This can be seen in the hysteresis of the contact angle made by a drop on a solid, which is known to originate from surface heterogeneities, and in the slowing of droplet motion on deformable solids. Here, we show how a moving contact line can be used to characterize a molecularly thin polymer layer on a solid. We find that the hysteresis depends on the polymerization index and can be optimized to be vanishingly small (<0.07°). The mechanical properties are quantitatively deduced from the microscopic contact angle, which is proportional to the speed of the contact line and the Rouse relaxation time divided by the layer thickness, in agreement with theory. Our work opens the prospect of measuring the properties of functionalized interfaces in microfluidic and biomedical applications that are otherwise inaccessible.

Highlights

To cite this version: Romain Lhermerout, Hugo Perrin, Etienne Rolley, Bruno Andreotti, Kristina Davitt
The mechanical properties are quantitatively deduced from the microscopic contact angle, which is proportional to the speed of the contact line and the Rouse relaxation time divided by the layer thickness, in agreement with theory
It has long been understood that small-scale roughness and chemical heterogeneity on the surface are responsible for contact angle hysteresis as they create energy barriers to motion that pin the three-phase contact line[1,2]

Summary

Introduction

To cite this version: Romain Lhermerout, Hugo Perrin, Etienne Rolley, Bruno Andreotti, Kristina Davitt. How a liquid drop sits or moves depends on the physical and mechanical properties of the underlying substrate This can be seen in the hysteresis of the contact angle made by a drop on a solid, which is known to originate from surface heterogeneities, and in the slowing of droplet motion on deformable solids. It has long been understood that small-scale roughness and chemical heterogeneity on the surface are responsible for contact angle hysteresis as they create energy barriers to motion that pin the three-phase contact line[1,2]. For this reason, hysteresis is sometimes used as a measure of smoothness or ‘perfection’ of a surface. We use the idea that contact line motion contains information about the substrate to extract properties of an interfacial layer that are not accessible to standard rheometers, both because of the very short timescales involved and because the rheology of an interfacial layer differs from that of the bulk

Methods

Results

Conclusion