Impact of microcapsules wettability on thermal conductivity of microencapsulated phase change material(MPCM) suspensions

Abstract

The purpose of this paper is to investigate the effect of the wettability on MPCM suspensions thermal conductivity. The wettability of the capsules, characterized by contact angle between solid capsules and carrying fluid, was modified by mixing two selected surfactants into the suspensions, i.e.,cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate(SDS) and changing such surfactants' additive amount. Meanwhile, the Hot Disk 2500s thermal analyzer was applied to test the static thermal conductivity of the MPCM suspensions in which the mass fraction of capsules is 10wt.% and obtained the relation between the thermal conductivity and the contact angle. The results indicated, when the mass fraction of surfactants falls into the range of 0 -0.05wt.%, the effect of the CTAB is more significant than the SDS. However, when the mass fraction of surfactants continue to increase from 0.05wt.%, the significance of the SDS exceeded the CTAB. It was also found that the decrease in the contact angle led to the growth in thermal conductivity for both Maxwell model's theoretical value and experimental results. When the contact angle fell into the range of 45°-95°, the Maxwell model's theoretical results of the thermal conductivity could predict the thermal conductivity of MPCM suspensions with a good accuracy, but inversely when the contact angles were smaller than 45°, a significant gap was found between the two results.To remove such gap a correction factor "A" which is associated with contact angles was proposed.