Surface roughness and hydrodynamic boundary conditions

Abstract

We report results of investigations of a high-speed drainage of thin aqueous films squeezed between randomly nanorough surfaces. A significant decrease in the hydrodynamic resistance force as compared with that predicted by Taylor's equation is observed. However, this reduction in force does not represent the slippage. The measured force is exactly the same as that between equivalent smooth surfaces obeying no-slip boundary conditions, but located at the intermediate position between peaks and valleys of asperities. The shift in hydrodynamic thickness is shown to be independent of the separation and/or shear rate. Our results disagree with previous literature data reporting very large and shear-dependent boundary slip for similar systems.

Roughness Induced Boundary Slip in Microchannel Flows

“…A common setup to measure slip is to utilize a modified atomic force microscope (AFM) to oscillate a colloidal sphere in the vicinity of a boundary [12 -14]. Vinogradova and Yakubov demonstrated that assuming a wrong position of the surface during measurements can lead to substantial errors in the determined slip lengths [14]. They showed that measurements can be interpreted by assuming a modified sphere radius instead of Navier's slip condition, so that the position of a no-slip wall would be between peaks and valleys of the rough surface.…”

Liquid friction on charged surfaces: From hydrodynamic slippage to electrokinetics

“…The conclusions emerging from these studies are that, while the continuum hydrodynamics theory surprisingly remains valid up to very small length scales, the no-slip boundary condition (BC) for the fluid velocity at the solid surface may be violated in many situations (see e.g. [2,5,6,8,9]). Moreover, it has been shown that this violation of the usual no-slip BC is controlled by the wetting properties of the fluid on the solid surface: while the no-slip BC is fulfilled on hydrophilic surfaces, a finite velocity slip is measured on hydrophobic surfaces [2,5,8], originating in a low friction of the liquid at the wall.…”

“…therein), but mainly thanks to the development of new experimental techniques, such as optical velocimetry (see [3,4] and refs. therein), or dissipation measurements using Surface Force Apparatus and Atomic Force Microscope (see [5,6] and refs. therein).…”

Internal relaxation time in immersed particulate materials

“…In practice, however, there is usually a typical distance h 0 below which particle surfaces interact not only via the fluid, but also through a direct particleparticle force. This interaction can have various physical origins, among which direct contacts between asperities of rough solid surfaces [22,23,24,25] or repulsion between charged or polymeric surfactant molecules at the bubble or droplet surface [29]. Our present focus is not necessarily to capture any of the details of such effects, but rather to represent their common feature: a strong steric repulsion that works on the particle surfaces in parallel to the viscous force (Fig.1 a1).…”

“…The viscous friction between two smooth rigid spheres is a well known problem (see for instance [21]). However, the presence of asperities on rough surfaces has been shown to strongly affect lubrication, and this topic is still an active field of research [22,23,24,25]. Between bubbles or droplets, the flow of liquid is even more complex, as the surfaces are soft and can stretch [26,27], they can be permeable [28] and exhibit some repulsive electrostatic interactions leading to disjoncting pressure [29] as well as adhesion [2,30,31].…”