Evaporative microclimate driven hygrometers and hygromotors

A strip of paper placed on a hand spontaneously curls upwards. This simple observation illustrates the ability of a relatively homogeneous hygroscopic structural material, paper, to sense and respond to the microclimate near a non-equilibrium system, a moist evaporative boundary layer. We quantify this interaction using a simple experiment and show that it can be understood in terms of a minimal model. A small modification of this paper hygrometer that makes one or another surface partly hydrophobic using a crayon or tape allows us to create a hygro-oscillator or a hygromotor that converts transverse moisture gradients into lateral oscillations or directed motion. Our study shows how treating paper as a responsive structural material allows us to extract information and work from a microclimatic boundary layer, transforming a messenger to a machine.