Height-resolved velocity measurement of the boundary flow during liquid impact on dry and wetted solid substrates

Abstract

The impact of a droplet onto a dry or wet surface leads to a rapid formation of a shear flow at the boundary. We present a novel method to experimentally resolve this flow in time at different heights above the solid. The radial flow field close to the substrate is reconstructed by evaluation of streak images of fluorescent tracer particles in the liquid. By using a microscope objective with a narrow depth of field, it is possible to scan through the flow in thin horizontal layers of 5 μm thickness. We focus on the flow close (≤40 μm) to the boundary during the impact of elongated drops with diameters of 0.3–0.4 mm and speeds in the range of 2–3 m s⁻¹. The spatial resolution is obtained from several individual events of the repeatable impact process and varying the focal plane. Fluorescent streaks formed by the suspended particles are recorded with high-speed photography at up to 20,000 frames per second. The impact of water and of ethanol is investigated both on dry glass and on glass covered with a thin film of the same liquid. Results are given as spatio-temporal maps of radial flow velocity at different heights, and the maximum shear stress at the substrate is evaluated. The implications of the results are discussed with respect to cleaning applications.

Appendix

For estimation of the depth of field $\delta z$ we used equation (4) from Meinhart et al. (1999)

$$\begin{aligned} \delta z=\frac{n\lambda }{NA^2}+\frac{ne}{M\,NA} \end{aligned}$$

with index of refraction $n=1$, fluorescence wavelength $\lambda = 612 \,\hbox {nm}$, numerical aperture $NA=0.725$, smallest resolvable distance of the image detector $e = 20 \, \upmu \hbox {m}$, and magnification $M=20$. This results in a value of 2.4 $\upmu \hbox {m}$ for the depth of field.