Drainage of thin liquid films between relatively smooth surfaces

Abstract

Using a new surface force apparatus, static and dynamic measurements have been conducted to investigate the squeeze of a liquid from the contact between a relatively smooth sphere and a plane. Simple liquids and polymer melts (polyisoprene) have been studied, with different solid surfaces (mica, cobalt, gold, platinum, and steel). It has been found that an ‘‘immobile’’ layer of fluid is present on each solid surface which does not participate in the hydrodynamic flow of the liquid. The thickness of this film is relatively independent of the roughness of the solid, corresponds to a small number of molecular layers (1 to 5), and is influenced by the structure of the molecule. These fluids shows two different behaviors: for small molecules the thickness of the ‘‘immobile’’ layer is proportional to the viscosity; for larger molecules the thickness is proportional to the one-tenth power of the viscosity. During the squeeze process, when the solid surfaces are close, a ‘‘confined’’ layer occurs due to molecule entanglement. This phenomenon is described for a semirigid hydrocarbon.

Boundary Condition in Liquid Thin Films Revealed through the Thermal Fluctuations of Their Free Surfaces

“…Oppositely, for a n-hexadecane film on a glass substrate, the dynamics of the surface fluctuations is slowed down, revealing the presence of a few nanometer thick immobile layer close to the solid surface, in agreement with previous findings [2,3]. .…”

“…However, the immobile fluid layers observed in the 80's by several authors for wetting liquids [2,3] remain to be confirmed. Such layers are expected to be only a few nanometers thick, and an even better resolution than in ref.…”

“…Significant modifications of the properties of liquids at nanometric distances from a solid wall have been evidenced in the past decades [1], but a unifying picture of the different reported features (see for instance [2][3][4][5][6][7]) is still lacking. The questions at stake are in particular crucial for transport properties at small scales, involved e.g.…”

Microfluidics: The No-Slip Boundary Condition

“…Be it at the molecular size [55] or on larger scales [17,58,82,125,194], roughness and geometrical features have been observed to influence the behavior at liquid-solid interfaces. Not only does roughness leads to an ambiguity as to the exact location of the surface, but it impacts the dynamics of the nearby fluid, leading experimentally either to an increase [58,82,125,194] or a decrease [17] of friction with roughness.…”

“…The development of the Surface Force Apparatus in the 1970's [79,81,154] (see §3.1) has allowed for more than thirty years of precise probing down to the nanometer scale of both structure and dynamics of many Newtonian liquids against mica [28,57,58,72,77,80,89,90,97,129,130]. Experimental methods have included squeeze and/or shear flow for a variety of polar and non-polar liquids displaying a wide range of wetting conditions and shear rates.…”