The drainage of thin liquid films between solid surfaces

TLDR

We present measurements of liquid film thickness over time as the films are squeezed between molecularly smooth mica surfaces. Three Newtonian, nonpolar liquids (octamethylcyclotetrasiloxane, n‑tetradecane, and n‑hexadecane) were studied, with film thicknesses measured to 0.2 nm accuracy as they drained from ~1 µm to a few molecular layers. The drainage follows Reynolds lubrication theory for thicknesses above 50 nm, but below that the flow slows due to a shear‑free layer near each surface, and in very thin films the continuum theory fails, producing stepwise drainage matching molecular layers; additionally, trace water on hydrophilic surfaces causes film rupture.

Abstract

We present measurements of the thickness as a function of time of liquid films as they are squeezed between molecularly smooth mica surfaces. Three Newtonian, nonpolar liquids have been studied: octamethylcyclotetrasiloxane, n-tetradecane, and n-hexadecane. The film thicknesses are determined with an accuracy of 0.2 nm as they drain from ∼1 μm to a few molecular layers. Results are in excellent agreement with the Reynolds theory of lubrication for film thicknesses above 50 nm. For thinner films the drainage is slower than the theoretical prediction, which can be accounted for by assuming that the liquid within about two molecular layers of each solid surface does not undergo shear. In very thin films the continuum Reynolds theory breaks down, as drainage occurs in a series of abrupt steps whose size matches the thickness of molecular layers in the liquid. The presence of trace amounts of water has a dramatic effect on the drainage of a nonpolar liquid between hydrophilic surfaces, causing film rupture which is not observed in the dry liquids.