Dynamic Effects on Force Measurements. 2. Lubrication and the Atomic Force Microscope

Abstract

We present the results of investigations of high-speed drainage of a thin film confined between a microscopic colloidal probe and a substrate performed with a new atomic force microscope-related setup. Theoretical calculations are used to formulate the governing equation (force balance) for instantaneous deflection of a cantilever spring, which is due to both concentrated forces acting on a colloidal probe and viscous drag force on a cantilever itself. The suggested way to subtract the latter contribution allows design of a lubrication experiment. Two pairs of interacting solids, characterized by different wettability and smoothness, immersed into water-electrolyte solutions have been studied. Results for hydrophilic silica surfaces are in excellent agreement with the Reynolds theory of hydrodynamic lubrication. Faster drainage of a thin film confined between hydrophobic rough polystyrene surfaces is consistent with the theory of film drainage between slippery surfaces. The slip lengths are found to be of the order of the size of asperities, and do not depend on the separation and shear rate. The results are important for colloidal dynamics and nanofluidics.

Microfluidics: The No-Slip Boundary Condition

“…The conclusions have been confirmed by Molecular Dynamics simulations [135] and are consistent with studies of flow in capillaries with diameters of tens of nanometers [3,91]. In this context, the large number of recent published experiments reporting some form of (apparent) slip with λ ∼ 1 nm−1 µm in the flow of Newtonian liquids is surprising [9,16,17,18,29,30,32,33,38,39,40,67,87,88,68,108,116,125,126,150,162,163,178,192,193,194,195], and has allowed to re-discover a few early [29] Poly(carbonate)+PVP SDS solutions studies reporting some degree of slip [21,34,46,141,160]. In part, this chapter is an attempt to describe and interpret these more recent experimental results.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Comparing measured force curves with calculations a significant effective slip was found, which could be described by a slip length of 10-14 nm. In contrast, in experiments with two particles glued to the cantilever to increase the distance of the cantilever, Vinogradova and Yakubov found no slip [1034].…”

Electro-osmosis on Anisotropic Superhydrophobic Surfaces

“…Since the EO flow amplification scales as (1 + bκ), and b can be of the order of tens of nanometers [9,10,11,12], for typically nanometric Debye length an order of magnitude enhancement might be expected.…”