Evaporation of alcohol droplets in humid environments

It is common to spray surfaces with alcohol to remove any potential source of infection. Once on the surface, the alcohol droplets quickly evaporate and leave behind a dry and cleaner surface. Here, we use experimental observations and theoretical modeling to show that this simple daily observation hides a wealth of fascinating fluid dynamics. We show that while the alcohol droplet evaporates, water condenses into the drop and later evaporates again. At low ambient relative humidities, the condensation of water does not have a considerable effect on the dynamics of the evaporating alcohol drop. However, beyond a critical ambient relative humidity, condensation of water creates strong solutal Marangoni flows that significantly deform the drop, making it pancake-like in shape. We show that at these high relative humidities the alcohol content of the drop quickly decays, leaving behind a thin film of water on the surface that evaporates much more slowly. Our observations suggest that when evaluating the effectiveness of alcoholic sprays for disinfecting surfaces it is necessary to account for the ambient relative humidity.