Writing in Water

Abstract

Writing is an ancient communication technique dating back at least 30,000 years. While even sophisticated contemporary writing techniques hinge on solid surfaces for engraving or the deposition of ink, writing within a liquid medium requires a fundamentally different approach. We here demonstrate writing of lines, letters and complex patterns in water by assembling lines of colloidal particles. Exploiting gravity, we roll an ion-exchange bead (pen) across a layer of sedimented colloidal particles (ink). The pen evokes a hydrodynamic flow collecting ink-particles into a durable, high-contrast line along its trajectory. Deliberate substrate-tilting sequences facilitate pen-steering and thus to draw and write. We complement our experiments with a minimal model that quantitatively predicts the observed parameter dependence for writing in fluids and highlights the generic character of writing by line-assembly. Overall, our approach opens a versatile route for writing, drawing and patterning fluids, even at the micro-scale.Comment: Article 23 p, 5 figs, 53 refs, Supl. Info with 13 fig