  Flow structure near three phase contact line (TPCL) of evaporating liquids
plays a significant role in liquid wetting and dewetting, liquid film
evaporation and boiling, etc. Despite the wide focus it receives, the
interacting mechanisms therein remain elusive and in specific cases,
controversial. Here, we reveal the profile of internal flow and elucidate the
dominating mechanisms near TPCL of evaporating droplets, using mathematical
modelling, microPIV, and infrared thermography. We indicate that for less
volatile liquids such as butanol, the flow pattern is dominated by capillary
flow. With increasing liquid volatility, e.g., alcohol, the effect of
evaporation cooling, under conditions, induces interfacial temperature gradient
with cold droplet apex and warm edge. The temperature gradient leads to
Marangoni flow that competes with outwarding capillary flow, resulting in the
reversal of interfacial flow and the formation of a stagnation point near TPCL.
The spatiotemporal variations of capillary velocity and Marangoni velocity are
further quantified by mathematically decomposing the tangential velocity of
interfacial flow. The conclusions can serve as a theoretical base for
explaining deposition patterns from colloidal suspensions, and can be utilized
as a benchmark in analyzing more complex liquid systems.
