Usually, cool water moves away from hot areas by virtue of the well-known Marangoni effect, but a non-intuitive reversal of this phenomenon may take place, if small enough droplets are used, as reported recently by us. In a recent discovery, we have explained that 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. As they evaporate, there is a lower concentration of molecules on the warmer side, so those from the cooler side start moving toward the heat. The localised cooling, which increases the local surface tension where the droplet touches the surface, causes the droplet to slide along towards the hotter region. Discovery of this phenomenon may revolutionize the way in which thermal management of electronic devices may be made in the future.
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