Cleaning of viscous drops on a flat inclined surface using gravity-driven film flows

Abstract

We investigate the fluid mechanics of cleaning viscous drops attached to a flat inclined surface using thin gravitydriven film flows. We focus on the case where the drop cannot be detached either partially or completely from the surface by the mechanical forces exerted by the cleaning fluid on the drop surface. Instead a convective mass transfer establishes across the drop-film interface and the fluid in the drop dissolves into the cleaning film flow, which then transports it away. The characteristic time scale of dissolution is much longer than the advection time scale in the film flow. Thus, the shape and size of the drop can be considered as quasi-steady. To assess the impact of the shape and size of the drop on the velocity of the cleaning fluid, we have developed a novel experimental technique based on particle image velocimetry. We show the velocity distribution at the film surface in the situations both where the film is flowing over a smooth surface, and where it is perturbed by a solid obstacle representing a very viscous drop. We find that at intermediate Reynolds numbers the acceleration of the starting film is overestimated by a plane model using the lubrication approximation. In the perturbed case, the streamwise velocity is strongly affected by the presence of the obstacle. The upstream propagation of the disturbance is limited, but the disturbance extends downstream for distances larger than 10 obstacle diameters. Laterally, we observe small disturbances in both the streamwise and lateral velocities, owing to stationary capillary waves. The flow also exhibits a complex three-dimensional converging pattern immediately below the obstacle.

Particle image velocimetry and modelling of horizontal coherent liquid jets impinging on and draining down a vertical wall

“…The surface velocities were measured using a novel PIV technique based on artificial pearlescence in water dyed opaque so that only particles located at the flow surface were tracked (Landel et al, 2015). The jet velocities studied were relatively low compared to industrial cleaning jet flows, as this gave stable films and little splashing.…”

“…In section 3 we present our experimental procedure and apparatus. We also describe briefly the velocity measurement technique of Landel et al (2015).…”

“…We compare predictions of different models, including a model based on that of Mertens et al (2005) proposed in this paper, with our experimental measurements for the geometry and the velocity field of the flow. The experimental investigation of the velocity field of the thin film flow is made possible for the first time thanks to the novel technique of Landel et al (2015).…”

“…These are flat, titanium dioxide coated mica particles with size 5-25 m: they exhibit a silver-pearl colour when mixed with water. The dye concentration was chosen so that the solution was opaque and only particles on or very near the surface of the flow would be imaged, so that the PIV captured surface velocities (see Landel et al, 2015, for more details).…”

“…This is the first time that detailed experimental measurements of the velocity field of a thin falling film produced by an impinging jet are presented. The novel particle imaging velocimetry technique of Landel et al (2015) can capture, at high time and space resolutions, the twodimensional velocity field at the surface of a thin film that is not constrained in a channel. In our experiments, it is essential that the film remains unconstrained at the edges, but rather flows freely on a planar surface.…”