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Water-film thickness imaging based on time-multiplexed near-infrared absorption with up to 500 Hz repetition rate

M. Lubnow, T. Dreier, C. Schulz, and T. Endres

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Abstract

We demonstrate high-repetition-rate imaging of the liquid-film thickness in the 50–1000 µm range resulting from impinging water droplets on a glass surface. The pixel-by-pixel ratio of line-of-sight absorption at two time-multiplexed near-infrared wavelengths at 1440 and 1353 nm was detected with a high-frame-rate InGaAs focal-plane array camera. Frame rates of 1 kHz and thus measurement rates of 500 Hz could be achieved, well suited to capture the fast dynamics of droplet impingement and film formation. The droplets were sprayed onto the glass surface using an atomizer. Suitable absorption wavelength bands for water droplet/film imaging were determined from Fourier-transform infrared (FTIR) spectra of pure water between 298 and 338 K. At 1440 nm, the water absorption is nearly temperature-independent, making the measurements robust against temperature fluctuations. Time-resolved imaging measurements capturing the dynamics of the water droplet impingement and evolution were successfully demonstrated.

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Supplementary Material (4)

NameDescription
Visualization 1       Video of surface wetting by a water spray.
Visualization 2       Video of a water droplet bursting on the substrat surface and covering it with a thin water film.
Visualization 3       Video of a water spray hitting the substrate after a previous injection.
Visualization 4       A fast moving droplet flying over a ~80-µm thick water film. The fast movement of the droplet results in an position offset in the ratioed images and thus in a “shadow” cast by the droplet seen here on its right side.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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