Role of entrapped vapor bubbles during microdroplet evaporation
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(a) High-speed images of vapor bubble entrapment on an isothermal Al thin-film (T S ≅ 189 °C). The 120 μm scale-bar applies to all images. The time-stamp corresponds to the time elapsed after impingement (e.g., at 370 μs the water microdroplet is undergoing impingement oscillations at 5.1 kHz, whereas, at 17.4 ms48 ms, the impingement oscillations are damped and the vapor bubble is oscillating at 23.8 ± 0.3 kHz). (b) Frequency of thermally excited capillary waves with comparison to measured values (symbols). The dashed-line is the predicted dispersion behavior for a clean water surface (Eq. (1)). Data are from several evaporation experiments (175 °C215 °C). The open-circle symbols are for oscillating vapor bubbles that are trapped inside evaporating water microdroplets (). The triangle-symbols correspond to thermally induced microdroplet oscillations in the absence of vapor bubble entrapment (). Dynamic multimedia videos of (a) and the formation of coffee-ring patterns are provided (enhanced online). [URL: http://dx.doi.org/10.1063/1.4745009.1] [URL: http://dx.doi.org/10.1063/1.4745009.2]10.1063/1.4745009.110.1063/1.4745009.2
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Evaporation data as a function of heating frequency for water microdroplets on a laser heated Al thin-film (25 °C, 144 ± 2 mW, 33.6 μm). (a) Average evaporation rate () measured during the microdroplet lifetime. (b) Average evaporation rate per unit length of solid-liquid-vapor contact line. Data are based on the analysis of six or more evaporation experiments. RH is the relative humidity of the room. For the circle data (RH = 5% ± 2%): R0 = 47.9 ± 1.4 μm and θ0 = 93.4° ± 2.5°. For the triangle data (RH = 50% ± 2%): R0 = 46.7 ± 1.8 μm and θ0 = 95.7° ± 3.1°. A dynamic multimedia of vapor bubble oscillations at 777 Hz is provided (enhanced online). [URL: http://dx.doi.org/10.1063/1.4745009.3]10.1063/1.4745009.3
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