Fig. 1: Autonomously propagating wave generated by the coherent motion of titanate nanosheets (TiNSs) in their aqueous dispersion.
From: Propagating wave in a fluid by coherent motion of 2D colloids
a Schematic illustrations of wave propagation by the coherent motion of negatively charged titanate nanosheets (TiNSs). In water, colloidally dispersed TiNSs are strongly correlated with each other, adopting a cofacial geometry with competitive electrostatic repulsion and van der Waals attraction. When a 10 T magnetic field is applied to this aqueous dispersion in a quartz cuvette along its z-axis, all the nanosheets align parallel to the xy-plane of the cuvette and form a monodomain cofacial assembly (left). After the magnetic field is turned off, ionic species are added from the open end of the cuvette, generating an ion gradient that progressively attenuates the electrostatic repulsion between TiNSs. Consequently, the van der Waals attraction between TiNSs prevails, thereby causing a contraction of the TiNS distance through their tilting motion. Owing to the strong correlation between TiNSs, this colloidal motion occurs coherently over a large scale, generating a wave, which then propagates unidirectionally along the direction of ion diffusion (right). b Ion-induced contraction of the TiNS distance elucidated by the DLVO theory. Theoretically calculated total potential (VA + VR) at different ion concentrations (0.27–0.32 mM) is plotted as a function of the TiNS distance. When ionic species are added to the colloidal dispersion, the electrostatic repulsion between TiNSs is attenuated due to the screening effect, thereby reducing the TiNS distance at the local minimum of the potential. c Time-dependent optical images of a magnetically oriented TiNS dispersion ([TiNS] = 0.5 wt%) in a quartz cuvette (40 × 10 × 1 mm) at 25 °C in air (0.04% CO2) after turning off the magnetic field.
