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

Abstract

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.

The Use of the VOF‐Model to Study the Wetting of Solid Surfaces

“…Precise measuring methods of the rivulet thickness that are discussed in the literature need calibration when reflecting a fluorescence signal on a black background [7,8] or transmitting signals through glass [11,17] and can thus not be directly used with an inclined metal plate or structured packing element. For this reason the liquid position is allocated spatially using a needle, which is moved perpendicular …”

Characterisation of liquid film in a microstructured falling film reactor using laser scanning confocal microscopy

“…Examples include conductivity probes [15][16][17], ultrasound detectors [18], fibre-optic sensors [19], microwave techniques [20,21], laser interferometry [22,23], fluorescence [24], laser-induced fluorescence [25,26], optical measurement methods [27][28][29], rainbow refractometry [30] and infrared thermography [31]. The use of conductivity probes was not considered for this investigation because they require physical contact with the liquid film which would greatly affect flow characteristics due to the small flow channels and liquid film thicknesses encountered.…”

Experimental and Numerical Models of Three-Dimensional Gravity-Driven Flow of Shear-Thinning Polymer Solutions Used in Vaginal Delivery of Microbicides

“…Previous experiments have been designed for other applications (e.g., geophysical flow [18] and industrial flow [26]), and use several possible strategies to obtain a local fluid thickness (i.e., surface topography) as a function of time, such as an anamorphic Schlieren system [27,28], a laser focus displacement instrument [29], or a fluorescence imaging technique [30,31]. However, the previous methods do not meet all of the criteria we require: a tabletop apparatus and software, a temperature/humidity-controlled environment, a release mechanism to produce a contact-free initial condition for shape, and an algorithm for obtaining spreading measurements and approximating surface topography.…”