Surface temperature of acoustically levitated water microdroplets measured using infra-red thermography

A 58 kHz acoustic levitator was fitted with an infra-red thermography camera to examine the drying behaviour of water microdroplets at various drying-air temperatures. The evaporation rate was greater with larger initial droplet size at otherwise identical drying-air temperature (Tda)(Tda). Measurement of droplet aspect ratio indicate that this is caused by differing acoustic field strengths. The measured droplet surface temperature in dry air showed no dependence on initial droplet size, but deviated from the wet bulb and also from the droplet temperature predicted by acoustic levitation theory. The degree of deviation of drying rate from that predicted by the d2d2-law using the wet bulb was dependent on TdaTda. Use of measured droplet surface temperature instead of the wet bulb gave, however, good agreement with the d2d2-law in dry air. No substantial effects of acoustic field streaming on drying rate could therefore be seen, even at the sound pressure levels of 106–165 dB used. Interpretation of evaporation rates of acoustically-levitated droplets requires therefore the measured droplet surface temperature.