RESUMEN

We report on a dramatic order-of-magnitude increase in flow velocity within pinned evaporating droplets toward the end of their lifetime. The measurements were performed using high-speed microparticle image velocimetry. The study revealed interesting observations about the spatial and temporal evolution of the velocity field. The profile along the radius of the droplet is found to exhibit a maximum toward the three phase contact line with flow oscillations in time in this region. Additional optical measurements allowed further analysis of the observed trends. Analysis of the potential mechanisms responsible for the flow within the droplet demonstrated that these observations can be satisfactorily explained and accounted for by mass conservation within the droplet to compensate for evaporation.

Asunto(s)

Reología , Fenómenos Ópticos , Factores de Tiempo , Volatilización

RESUMEN

The flow field along the base of an evaporating ethanol-water droplet and its evolution time was measured by particle image velocimetry. Three stages are revealed, a first stage dominated by multiple vortices, a second transition stage characterized by a remarkable spike in outward flow not previously identified, and a third stage dominated by outward flow identical to that found for pure water. Stage I is thought to be driven by surface tension gradients arising from local concentration variation. The spike in outward flow is explained in terms of a transition corresponding to almost total depletion of ethanol. An exponential decay in vorticity during the transition stage is explained in terms of ethanol diffusion from the bulk to the interface. We speculate on the existence of a zero-concentration wave propagating from the apex to the contact line corresponding to the final total depletion of ethanol.