Marangoni instabilities in a sessile droplet of 1cSt silicone oil evaporating at constant contact angle mode on a heated copper substrate were investigated through a series of experiments. Results reveal that with the increase of the temperature of substrate

three kinds of convection modes occur in sequence

i.e.

thermocapillary convection

steady Bénard-Marangoni (BM) convection and irregular oscillatory BM convection

respectively. For the steady BM convection

with the receding of the wetting radius

the cell number decreases one by one while the morphology of the BM cell does not vary. When the cell number is less than five

the individual cell becomes to be circular. With the contact angle increases

the critical Marangoni number (Mac) for the onset of the steady BM convection increases. The dimensionless cell number increases with the dimensionless wetting radius

which is independent on the contact angle.