  Droplet evaporation has been intensively investigated in past decades owing
to its emerging applications in diverse fields of science and technology. Yet
the role transport mechanisms has been the subject of a heated debate,
especially the presence of Marangoni flow in water droplets. This work aims to
draw a clear picture of the switching transport mechanisms inside a drying
pinned sessile water droplet in both the presence and absence of
thermocapillarity by developing a comprehensive model that accounts for all
pertinent physics in both phases as well as interfacial phenomena at the
interface. The model reveals a hitherto unexplored mixed radial and buoyant
flow by shedding light on the transition from buoyancy induced Rayleigh flow to
the radial flow causing coffee ring effect. Predictions of the model
excellently match previous experimental results across varying substrate
temperatures only in the absence of Marangoni flow. When thermocapillarity is
accounted for, strong surface flows shape the liquid velocity field during most
of the droplet lifetime and the model starts to overestimate evaporation rates
with increasing substrate temperature.
