Wetting Dynamics of Volatile Hydrofluoroether (HFE) Liquid Droplets*

The present work reports the wetting dynamics of a series of Hydrofluoroether liquids, having low surface tension and an appreciable volatility. As the series number increases from HFE-7100 to 7500, the volatility of the liquid decreases without any significant change in surface tension. The high-speed shadowgraphy technique is employed to observe the spreading of HFE-liquid droplets of different sizes on glass and sapphire substrates. Droplets of HFE-7500 liquid show the strongest wetting behavior compared to other HFE-liquids. For a given volume, the maximum wetted diameter of droplets decreases as the series number decreases from HFE-7500 to 7100. In contrast, the final (evaporation-driven) contact angle increases with the decrement in the series number. During the spreading stage, the higher volatile liquid spreads slower, with an apparent wetting exponent ranging from 1/11 to 1/18, which is smaller than the Tanner's law exponent of 1/10. Apart from immediate evaporative losses, this wetting behavior may also be affected by the larger evaporative cooling of the higher volatile liquid at the droplet interface, which induces thermal Marangoni stresses in the opposite direction of the contact line motion, thereby restricting the spreading of the droplet.