In physics and chemical engineering, the term Pouillet effect refers to an exothermic reaction that takes place when a liquid is added to a powder. Strictly speaking, the heat generated is caused by adhesion of the liquid to the surface of the particles rather than by a chemical reaction.^[1] It was first observed in 1802 by Leslie, who noted that heat was evolved when dry sawdust was wetted with water.^[2] Claude Pouillet later described this phenomenon in 1822, and it subsequently became known as the "Pouillet effect" in France, and then elsewhere.^[3][4]

Negative Pouillet effect

Under certain conditions, a negative Pouillet effect is possible, i.e., heat can be absorbed instead of being released. G. Schwalbe showed that in the case of water below 4 degrees Celsius, the temperature of the system decreases.^[5] Joseph Mellor argued that this is due to the negative thermal expansion coefficient of water between 0 and 4 degrees Celsius,^[6] with the temperature change ${\displaystyle \Delta T}$ given by

${\displaystyle \Delta T={{\alpha T} \over {C_{p}D}}\Delta P}$

where ${\displaystyle \alpha }$ is the thermal expansion coefficient, ${\displaystyle C_{P}}$ is the specific heat, ${\displaystyle D}$ is the specific gravity, and ${\displaystyle \Delta P}$ is the applied pressure due to the addition of the liquid. According to this formula, any liquid with a negative thermal expansion coefficient would be expected to exhibit a drop in temperature.

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