Steady-state droplet evaporation: Contact angle influence on the evaporation efficiency

Steady-state water droplet evaporation on a heated substrate is investigated to systematically correlate the effects of a droplet's contact angle on the evaporation rate. Experiments are performed on laser patterned polymer substrates, using a syringe-pump fluid supply methodology-in combination with in situ infrared thermography and visible-image microscopy-to pin/fix the contact line and facilitate steady-state operation with controlled substrate surface temperatures and contact angles ranging between 22 degrees C less than or similar to T-s less than or similar to 70 degrees C and 80 degrees less than or similar to theta less than or similar to 110 degrees respectively. The results affirm that (1) the effects of substrate cooling (also referred to as evaporative cooling) cannot be neglected for evaporation on low thermal conductivity polymer surfaces and (2) the evaporative mass flux (phi(LG)) scales inversely with contact angle (i.e., phi(LG) proportional to theta(-1)) with maximum values of phi(LG) near the triple-line. We also find-via mass transfer calculations based on the kinetic theory of gases-that the evaporation coefficient (epsilon(e), also referred to as the evaporation efficiency or the accommodation coefficient for evaporation) scales inversely with contact angle (i.e., epsilon(e) proportional to theta(-1)); yielding values for epsilon(e) within 0.52 x 10(-3) greater than or similar to epsilon(e) greater than or similar to 0.18 x 10(-3) for 80 degrees less than or similar to theta less than or similar to 110 degrees, respectively. (C) 2016 Published by Elsevier Ltd.