as reported by new scientist, researchers at the indian institute of technology in kharagpur have discovered that nanometer-sized water droplets on water-repellent surfaces move towards heat, which could have significant applications in electronics cooling.

the simulations showed a microdroplet moving towards heat rather than away.
(wikimedia commons)

typically, cold water droplets move from high temperature to low temperature, but in computer simulations run at the institute show that this can be reversed at the nanoscale.

the article explained, “fluids tend to move when there is an imbalance of surface tension – the force that helps you blow soap bubbles. temperature can cause surface tension to change, with fluids generally flowing away from warmth. but get down to the nanoscale and the effect is reversed.”

it continued, “the effect is down to an increase in van der waals forces, which attract molecules in the droplet towards molecules in the surface on which it is resting. in this case, the enhanced force pulls water molecules out of the droplet, causing them to evaporate. although this force is present both on the hot and cold side of a droplet, the molecules on the hot side are more energetic so they can evaporate much faster.”

the evaporation lowers the concentration of molecules on the warmer side and propels molecules from the coolers side towards the heat. “that localized cooling, which increases the local surface tension where the droplet touches the surface, causing the droplet to slide along towards the hotter region,” the article added.

a researcher from the institute in bangalore questions whether this could work at a larger scale, which brings into question the applications for this simulation. the article noted that for the technology to be practical it would have to work on the micrometer scale and to exploit this property would require separate micro-channels or polymer layers to protect electronics.

next up for the researchers is to go beyond simulation and into lab experiments.

the research was recently published in nanoscale. the abstract stated:

“a sessile droplet or a film usually moves from hotter regions to colder regions, due to variations in interfacial tension. this, known as the so-called marangoni effect, is true for most pure liquids like water for which the surface tension decreases with increase in temperature. in stark contrast to this existing understanding, we bring forth the coupled effect of wettability and temperature gradient on the dynamics of the three-phase contact line.

“by simultaneously tracking the dynamic evolution of the three-phase contact line due to evaporation and diffusion of molecules through molecular dynamics simulations, we explore the coterminous effects of the change of surface tension coefficients and wetting parameter with temperature on sessile droplets residing on surfaces with different wettabilities.

“we demonstrate, for the very first time, that inverse marangoni effect, which is believed to be exclusively observed in mixtures and self-rewetting fluids, is feasible in pure water at scales where inertial effects are negligible.

“the results of the study finds application in electronic chip cooling where by combined tuning of surface characteristics and marangoni forces, droplets can be passively transported to warmer regions for efficient thermal management.”