R. A. Schluter

Experimental study of rivulet formation on an inclined plate by fluorescent imaging

The instability of a two-dimensional moving contact line is studied for a thin liquid film flowing down an inclined plane, leading to the formation of rivulets. A fluorescent imaging method was developed to facilitate accurate measurement of the spacing between rivulets, tip velocity, three-dimensional shape and dynamic contact line. A fluid circulation system produced steady films at constant volumetric flux, in contrast to time-varying films at constant total volume, as in previous measurements. Comparisons are made with the existing data for constant-volume films, and with theoretical predictions for the wavelength of the rivulets formed at constant inlet flow rate. Data were also obtained for rivulet shapes, tip speeds and contact angles as functions of the angle of inclination of the plate and liquid Reynolds number.

Fluorescent imaging system for global measurement of liquid film thickness and dynamic contact angle in free surface flows

Fluorescent dye dissolved in a liquid flow was used to outline liquid-gas free boundaries and, with digital imaging, to observe quantitatively surface wave propagation and pattern formation, as well as contact-line velocity and contact angle in thin film flows on horizontal and inclined substrates. Using the relatively inexpensive system described here, a fluid depth measurement with a precision of ±0.02 mm is obtained routinely in flows of several millimeters depth over an area of approximately one square meter, and essentially unlimited continuous time spans. Dynamic contact angles are measured, for the first time, on liquid fronts with significant three-dimensional curvature such as rivulets draining down an inclined plate at any speed or global location. Procedures to normalize results quantitatively for any nonuniformities of the incident illumination are given. Estimates of the contribution to the experimental error by other effects, such as variations in dye concentration and temperature, and image digital register capacity, are also discussed. Illustrative results for two fluids and several dyes are given. Refinements to decrease the local error further to ±0.005 mm or less are described.Fluorescent dye dissolved in a liquid flow was used to outline liquid-gas free boundaries and, with digital imaging, to observe quantitatively surface wave propagation and pattern formation, as well as contact-line velocity and contact angle in thin film flows on horizontal and inclined substrates. Using the relatively inexpensive system described here, a fluid depth measurement with a precision of ±0.02 mm is obtained routinely in flows of several millimeters depth over an area of approximately one square meter, and essentially unlimited continuous time spans. Dynamic contact angles are measured, for the first time, on liquid fronts with significant three-dimensional curvature such as rivulets draining down an inclined plate at any speed or global location. Procedures to normalize results quantitatively for any nonuniformities of the incident illumination are given. Estimates of the contribution to the experimental error by other effects, such as variations in dye concentration and temperature, and image...

Electrohydrodynamics of Thin Flowing Films

Thin films of oil flowing down a nearly-vertical plate were subjected to a strong normal electrostatic field. Steady-state height profiles were measured by fluorescence imaging. For electrode potentials less than that required to produce an instability, the two-dimensional response of the interface was < 1%. Calculations of the fluid height coupled with the electric field solution were identical to uncoupled calculations for electric fields below the stability threshold. Pressure profiles under the film and three-dimensional effects are also discussed.

Dynamics of a dry spot

Experimental results are presented for the motion of a dry spot in a thin viscous film on a horizontal surface. These include global and spatial measurements of dry spot diameter, front velocities, static and dynamic contact angle, and the shape of the liquid–solid interface. Data are presented as a function of initial fluid depth for both an advancing fluid front of a collapsing dry spot and a receding fluid front of an opening dry spot. Results for both cases show that the final or static hole diameter increases as the initial fluid depth decreases. Also, insight is obtained into the relationship between the contact angle and the velocity for both advancing and receding fluid fronts. The experimental results are compared to a lubrication model, and good agreement is obtained.

Use of an Electric Field in an Electrostatic Liquid Film Radiator

Abstract: Experimental and numerical work was performed to further the understanding of an electrostatic liquid film radiator (ELFR) that was originally proposed by Kim et al. 1 The ELFR design utilizes an electric field that exerts a normal force on the interface of a flowing film. The field lowers the pressure under the film in a space radiator and, thereby, prevents leakage through a puncture in the radiator wall. The flowing film is subject to the Taylor cone instability, whereby a cone of fluid forms underneath an electrode and sharpens until a jet of fluid is pulled toward the electrode and disintegrates into droplets. The critical potential for the instability is shown to be as much as an order of magnitude higher than that used in previous designs. 2 Furthermore, leak stoppage experiments indicate that the critical field is adequate to stop leaks in a working radiator.