Walking and rolling of crystals induced thermally by phase transition

Abstract

The mechanical motion of materials has been increasingly explored in terms of bending and expansion/contraction. However, the locomotion of materials has been limited. Here, we report walking and rolling locomotion of chiral azobenzene crystals, induced thermally by a reversible single-crystal-to-single-crystal phase transition. Long plate-like crystals with thickness gradient in the longitudinal direction walk slowly, like an inchworm, by repeated bending and straightening under heating and cooling cycles near the transition temperature. Furthermore, thinner, longer plate-like crystals with width gradient roll much faster by tilted bending and then flipping under only one process of heating or cooling. The length of the crystal is shortened above the transition temperature, which induces bending due to the temperature gradient to the thickness direction. The bending motion is necessarily converted to the walking and rolling locomotion due to the unsymmetrical shape of the crystal. This finding of the crystal locomotion can lead to a field of crystal robotics.

Fig. 1

Molecular structure of chiral azobenzene trans-(S)-1

Fig. 2

Characterization of the single-crystal-to-single-crystal phase transition. a Differential scanning calorimetry (DSC) curve measured in the temperature range of 60–160 °C at a rate of 10 °C min⁻¹ for heating and subsequent cooling. Red and blue lines represent heating and cooling, respectively. b–f Temperature dependence of lattice constants (−100, 20, 100, 125, and 160 °C); lengths of (b) a, (c) b, (d) c axes, (e) angle β, and (f) volume V. g Occupancies of two conformers A and B. h Dihedral angles between azobenzene plane and the phenyl ring of the side chain in each conformer. i, j Conformers A (green) and B (yellow) at (i) 125 and (j) 160 °C. The phenyl ring of the side chain of conformer A is shown in blue and red at 125 and 160 °C, respectively. Hydrogen atoms are omitted for clarity

Fig. 3

Changes of crystal shape and molecular arrangement under heating and cooling. a (100) Top view before and after phase transition (Supplementary Movie 1). The scale bar is 100 μm. b (010) cross-section view (Supplementary Movie 2). The scale bar is 50 μm. c Molecular packing on (100) face. d NH---O=C hydrogen bond chains along the b axis, which are derived from the two-fold helical molecular packing on the (010) face along the b axis (see Supplementary Fig. 3). e (001) side view (Supplementary Movie 3). The scale bar is 100 μm. Bending motion along the b axis was observed on heating and then cooling. In each panel, dotted lines in the pictures (drawn in white) indicate the initial size of the β crystal. f Molecular packing on (001) face. In packing diagrams, dotted lines (drawn in black) represent NH---O=C intermolecular hydrogen bonds along the b axis. Hydrogen atoms are omitted for clarity

Fig. 4

Inchworm-like walking of a long plate-like crystal with a thickness gradient. a Inchworm walking by repeated heating and cooling cycles (Supplementary Movie 5). The scale bar is 1 mm. b Experimental setup for simultaneous observation by microscope and infrared (IR) thermography camera. c, d Sequential snapshots during inchworm walk on heating and cooling, by optical microscope (c) and IR thermography (d) (Supplementary Movie 6). Dotted white lines in c indicate initial positions of the left and right edges. The scale bar is 1 mm. e Time dependence of surface temperatures at the selected points on the crystal and the glass plate, shown as a cross mark in d

Fig. 5

Fast rolling locomotion of a long, thin plate-like crystal. a Rolling of a long, thin plate-like crystal with a width gradient on heating (Supplementary Movie 7). b Sequential snapshots during the rolling locomotion on cooling (Supplementary Movie 8). Dotted lines indicate the initial position. c Slant upper side view of rolling locomotion of the same crystal on cooling (Supplementary Movie 9). The scale bar in a–c is 1 mm

Fig. 6

Schematic diagrams of the possible mechanisms of locomotion. a Inchworm-like walking of a long crystal with a thickness gradient by repeated heating and cooling cycles. Displacement to the right is indicated by the circle. b Rolling locomotion of a thin, long crystal with a width gradient under only one process of heating or cooling