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 °C ≲ TS ≲ 70 °C and 80° ≲ θ ≲ 110° 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 (φLG) scales inversely with contact angle (i.e., φLG ∝ θ−1) with maximum values of φLG near the triple-line. We also find - via mass transfer calculations based on the kinetic theory of gases - that the evaporation coefficient (∊e, also referred to as the evaporation efficiency or the accommodation coefficient for evaporation) scales inversely with contact angle (i.e., ∊e ∝ θ−1); yielding values for ∊e within 0.52 × 10−3 ≳ ∊e ≳ 0.18 × 10−3 for 80° ≲ θ ≲ 110°, respectively.