Wetting dynamics of thin liquid films and drops under Marangoni and centrifugal forces

Abstract

This paper presents an experimental study on thin liquid drops and films under the combined action of centrifugal forces due to rotation and radial Marangoni forces due to a corresponding temperature gradient. We shall examine thinning of a given liquid layer both with and without rotation and also consider the onset of the fingering instability in a completely wetting liquid drop. In many of the experiments described here, we use an interferometric technique which provides key information on height profiles. For thick rotating films in the absence of a temperature gradient, when an initially thick layer of fluid is spun to angular velocities where the classical Newtonian solution is negative, the fluid never dewets for the case of a completely wetting fluid, but leaves a microscopic uniform wet layer in the center. Similar experiments with a radially inward temperature gradient reveal the evolution of a radial height profile given by h(r) = A(t)r(α), where A(t) decays logarithmically with time, and [Formula: see text]. In the case where there is no rotation, small centrally placed drops show novel retraction behavior under a sufficiently strong temperature gradient. Using the same interferometric arrangement, we observed the onset of the fingering instability of small drops placed at the center of the rotating substrate in the absence of a temperature gradient. At the onset of the instability, the height profile for small drops is more complex than previously assumed.

Obtaining self-similar scalings in focusing flows

“…The container measures 13 cm in diameter and 2 cm in height. To fix the temperature dependent viscosity η and surface tension γ of the fluid, the container is uniformly heated to a temperature of 24 • C unless otherwise noted by running water at a set temperature through the double-walled rotating axis -for details see [15]. On the base of the container, a silicon wafer (University Wafers) is placed.…”

Wetting dynamics of a collapsing fluid hole

“…Backholm et al [12] have notably studied the interactions between multiple holes in viscous films. Recent experiments by Mukhopadhyay and Behringer [13] and Dijksman et al [14] utilize centrifugal forces by rotating an axisymmetric fluid reservoir. These forces drive fluid to the outer edge of the container thereby creating uniform and centered holes.…”

Thermal Marangoni-driven dynamics of spinning liquid films

“…The maximum thermal gradient is then achieved at the largest heating power we can provide, which is 100W. The edge temperature at 100W heating is about 60 • C. For a complete description of the setup, see [28]. This heating mechanism gives a azimuthally symmetric thermal profile in the bottom of the container, and hence on the silicon wafer, as shown in Fig.…”

“…The time dependent film height h(r, t) in the rotating container is described with a the time dependent axisymmetric lubrication approximation that includes surface tension, surface tension gradients, gravity, centrifugal force and disjoining pressure [9,10,28,31,32],…”