Abstract
This work analyzes the dynamic viscosity, surface tension and wetting behavior of phase change material nano–emulsions (PCMEs) formulated at dispersed phase concentrations of 2, 4 and 10 wt.%. Paraffin–in–water samples were produced using a solvent–assisted route, starting from RT21HC technical grade paraffin with a nominal melting point at ~293–294 K. In order to evaluate the possible effect of paraffinic nucleating agents on those three properties, a nano–emulsion with 3.6% of RT21HC and 0.4% of RT55 (a paraffin wax with melting temperature at ~328 K) was also investigated. Dynamic viscosity strongly rose with increasing dispersed phase concentration, showing a maximum increase of 151% for the sample containing 10 wt.% of paraffin at 278 K. For that same nano–emulsion, a melting temperature of ~292.4 K and a recrystallization temperature of ~283.7 K (which agree with previous calorimetric results of that emulsion) were determined from rheological temperature sweeps. Nano–emulsions exhibited surface tensions considerably lower than those of water. Nevertheless, at some concentrations and temperatures, PCME values are slightly higher than surface tensions obtained for the corresponding water+SDS mixtures used to produce the nano–emulsions. This may be attributed to the fact that a portion of the surfactant is taking part of the interface between dispersed and continuous phase. Finally, although RT21HC–emulsions exhibited contact angles considerably inferior than those of distilled water, PCME sessile droplets did not rapidly spread as it happened for water+SDS with similar surfactant contents or for bulk–RT21HC.
Highlights
Phase change materials (PCMs) are substances that absorb/release large amounts of so−called “latent heat” when they undergo a phase change transition such as melting and freezing processes [1,2].Thermal energy storage systems based on PCMs allow larger densities of energy storage within reduced temperature ranges, when compared to those reservoirs working with conventional thermal fluids
In the case of nano–emulsions stabilized with ionic surfactants, effective/relative dynamic viscosity was observed to increase with dispersed phase volume work), effective/relative dynamic viscosity was observed to increase with dispersed phase volume fraction much more rapidly than for emulsions of larger droplets fraction much more rapidly than for emulsions of larger droplets
Phase change material nano–emulsions produced following a solvent–assisted route were characterized in terms of dynamic viscosity and surface properties
Summary
Phase change materials (PCMs) are substances that absorb/release large amounts of so−called “latent heat” when they undergo a phase change transition such as melting and freezing processes [1,2]. Some rheological studies on PCM nano–emulsions [4,5,9,13,15,16,17] or suspensions containing micro–encapsulated PCMs [18,19,20,21] analyzed changes in dynamic viscosity occurring during the solidifyication/melting of phase change material droplets/micro–capsules Most of those investigations observed deformations in shear viscosity–temperature evolution at temperatures around solid–liquid transitions. Has been found valuable in the characterization of polymers containing suspended particles [24,25], for example When it comes to PCM slurries, an analysis of the temperature evolution of normal stress in shear flow could contribute to a better understanding of possible structural transformations occuring during solid–liquid phase change of PCM droplets/micro–capsules. With this work we intend to complete the characterization of this nano–emulsion system started in our recent publication [32], in which phase change transitions, thermal conductivities and volumetric behavior were analyzed
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
