Dynamic Defrosting on Superhydrophobic and Biphilic Surfaces

Abstract

Ice formation and accretion present serious concerns for many building energy applications. In the heating, ventilation, air-conditioning, and refrigeration sectors, the most common approach to remove frost from a surface (defrost) is to reverse the system cycle direction and heat the working fluid. However, water retention on the heat exchanger during defrosting decreases the long term heat transfer performance. Here, we study the defrosting behavior of superhydrophobic and biphilic surfaces comprising of spatially distinct superhydrophobic (θa → 180°) and hydrophilic (θa = 78°) domains. Using top and side view optical imaging, we show that superhydrophobic regions undergo dynamic defrosting, where the frozen ice/frost layer undergoes spontaneous motion via the formation of a highly mobile slush. We experimentally show that the high mobility of dynamic defrosting enables the use of surface forces to ‘pull’ and remove the slush from the surface prior to it completely melting, even in the absence of forces such as gravity or shear. To explore the effect of pattern heterogeneity, we studied various biphilic patterns inspired from nature such as the banana leaf. We optimized the design space with respect to minimizing water retention during defrosting. Our work not only provides the fundamental understanding required for the design of heterogeneous defrosting coatings, but also elucidates the role of wettability gradients on defrosting dynamics for a plethora of energy applications.

Keywords: Biphilic Surfaces, defrosting, de-blooming, water retention, superhydrophobic, hydrophilic