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. Copyright c © EPLA, 2014 The ability to effectively measure an environmental parameter or actuate a device relies on a large, reversible response to a small external stimulus. Sensors often utilize the disproportionate trade-off between small mechanical response and large change in electrical properties, such as in the case of piezoelectric and capacitive sensors. In order to obtain reversible, large mechanical response, ancient strategies in biology and recent advances in engineering have utilized soft materials, such as in the actuation of natural and synthetic muscles [1–3]. A complementary theme in mechanoreception can be found in diverse biological situations involving slender objects which allow small varying lateral strains to cause large changes in shape via bending. Indeed natural examples include the opening/closing of pine cones [4] and curling of wheat awns [5] and have also inspired artificial analogs for potential engineering applications [3,6–8]. A natural candidate for a responsive slender structure that might serve as a hygromorphic sensor or actuator is paper, a fibrous porous sheet of cellulose fibers that is inexpensive, versatile, and sustainable. As a disordered material, paper derives its rigidity from entanglement and adhesion of its composite fibers. The athermal and frictional nature of its relatively large fibers (length ≈ 1mm, diameter ≈ 10μm) fundamentally (a)E-mail: [email protected] distinguishes its response [9,10] from otherwise analogous thermal polymeric solids; indeed the relation to these solids is similar to that between jammed granular systems and molecular glasses. During the fabrication of paper, the alignment of fibers along the direction of deposition by fluid leads to a marked mechanical anisotropy [9]. This, together with the fact that cohesion between composite cellulose fibrils is mediated by interand intra-molecular hydrogen bonds sensitive to stimuli such as humidity, heat, solvent concentration, and ionic strength [11–13], means that varying these parameters leads to expansion or contraction of the network, preferentially in the direction perpendicular to the alignment direction. Though typical strains may be small, strain gradients across the thickness can lead to large out-of-plane deflections. Figure 1(a) shows how a thin sheet of paper placed on one’s palm curls as it swells on one side in response to the exudation of moisture from the skin. When skin is replaced by a moist sponge from which water evaporates at a controlled rate, we observe a similar phenomenon (see the appendix for experimental details). In fig. 1(b), (c), we show two small pieces of paper, a lightweight yellow paper and a heavyweight white paper, placed on the sponge spontaneously curl and bend upward from their edges. Both papers preferentially bend perpendicular to the fiber alignment, the effect being more pronounced in the heavier paper, which has greater fiber alignment