Direct Measurement of Slip Velocities Using Three-Dimensional Total Internal Reflection Velocimetry

Abstract

The possible existence of slip of liquids in close proximity to a smooth surface is studied experimentally via the dynamics of small particles suspended in a shear flow. Sub-micron fluorescent particles suspended in water are imaged and analyzed using Total Internal Reflection Velocimetry (TIRV). For water flowing over a hydrophilic surface, the measurements are in agreement with previous experiments and indicate that slip, if present, is minimal at low shear rates, but increases slightly as the shear rate increases. Furthermore, surface hydrophobicity can be attributed for additional shear-rate dependent boundary slip. Issues associated with the experimental technique and the interpretation of results are also discussed.

Using surface force apparatus, diffusion and velocimetry to measure slip lengths

“…Recently developed methods of PIV using TIRF illumination have highlighted the possibility to locally probe a flow at a few hundreds of nanometres above a surface (Pouya et al 2005;Huang et al 2006). This has led to the proposal of the experimental approach detailed below to locally measure the properties of nanoflows in an approximately 300 nm layer with a resolution in the direction orthogonal to the flow of approximately 30 nm (Bouzigues & Tabeling 2007, submitted).…”

Large apparent slip at a moving contact line

“…The glass was coated with octadecyltrichlorosilane (OTS) to render it hydrophobic with a contact angle of 95 • ± 7 • (Figure 1). Previous experiments using substrates prepared in a similar manner 9 have measured the rms surface roughness to be 0.35 nm. The flow was illuminated from below using a 514 nm argon ion laser delivered through a 100X 1.45 NA microscope objective (Nikon).…”

“…9,18,19 However, within 10 µm of the contact line, the apparent slip length rises rapidly. The flood illumination results give very large estimates for the slip length, reaching several micrometers at the contact line.…”

“…A further refinement that is possible with the TIRF data is to use the tracer particle intensity to localize the velocity vector at a given height from the substrate. 9,[14][15][16] To do this, we divided the near-wall region into four zones centered at z = 60, 120, 180, and 240 nm. The raw particle images were binned according to their intensity which, due to the exponentially decaying illumination field, is reliably attributed to the particle's distance from the substrate.…”

“…In the first mode, the laser beam is brought in normal to the substrate, while in the second mode, the beam is introduced at a high incidence angle, above the critical angle, so that the liquid phase is illuminated by an evanescent field, penetrating approximately 300 nm into the liquid phase. 9 The motion of the tracer particles was imaged in fluorescence, coupled via a relay lens to a dual-stage image intensifier (Hammamatsu V9501U-74-G240) and a high speed CMOS camera (Photron APX). In the case of the flood illumination, the working fluid was a glycerol-water mixture (20% glycerol, by weight) with a viscosity, µ, of 10.8 mPa s, and a surface tension, σ, of 68 mN/m.…”