Measurement of thermal boundary resistance between water and superhydrophobic surfaces by the bi-directional differential 3ω method

Abstract

This study describes a new approach for measuring thermal boundary resistance between water and superhydrophobic surfaces with a thin air layer in between. This new bi-directional differential 3ω method was used to detect the small thermal boundary resistance due to superhydrophobicity. The superhydrophobic surface was prepared by spin-coating a commercial colloidal solution (Glaco Mirror Coat Zero) on the borosilicate glass substrate with a thin film-heater line for the 3ω measurements. The surface wettability was changed from superhydrophobic to superhydrophilic by plasma treatment of the coating surface to obtain the reference state without the air layer (thermal boundary resistance), required for the differential measurement. The obtained thermal boundary resistance was 7.5 × 10−6 [(K･m2)/W], and its equivalent air layer thickness was 0.2 μm and was consistent with the surface topology observed using an atomic force microscope. This measurement technique is considered useful for characterizing the thermal resistance of superhydrophobic structures used in heat transfer applications, such as phase-change heat transfer surfaces and microchannel heat exchangers.