Traveling waves are common in biological and synthetic systems, including the recent discovery that silver (Ag) colloids form traveling motion waves in H 2 O 2 and under light. Here, we show that this colloidal motion wave is a heterogeneous excitable system. The Ag colloids generate traveling chemical waves via reaction-diffusion, and either self-propel through self-diffusiophoresis (“ballistic waves”) or are advected by diffusio-osmotic flows from gradients of neutral molecules (“swarming waves”). Key results include the experimental observation of traveling waves of OH − with pH-sensitive fluorescent dyes and a Rogers-McCulloch model that qualitatively and quantitatively reproduces the key features of colloidal waves. These results are a step forward in elucidating the Ag-H 2 O 2 -light oscillatory system at individual and collective levels. In addition, they pave the way for using colloidal waves either as a platform for studying nonlinear phenomena, or as a tool for colloidal transport and for information transmission in microrobot ensembles.
BACKGROUND Recently, some smart superwetting materials with switchable wettability have attracted great interest due to their ability to controllably separate oil/water mixtures. However, the fabrication of most smart materials requires complex procedures and precious chemical reagents for surface modification, limiting large‐scale engineering applications of the fabricated smart surface. RESULTS In this study, a silver surface was prepared by extremely simple displacement deposition on copper mesh. The silver‐coated mesh exhibits superhydrophobicity and superoleophilicity after heating treatment, which allows oil to permeate while blocking water. Interestingly, the heated silver‐coated mesh gains superhydrophilic and underwater superoleophobic properties after UV irradiation, which allows water to permeate but prevents oil. As a consequence, the as‐fabricated mesh is capable of achieving two completely different oil/water separation processes. In addition, the silver‐coated mesh shows excellent environmental stability under a series of harsh conditions. CONCLUSION A novel smart engineering material with UV‐induced switchable wettability was successfully fabricated. In view of the rapid and simple fabrication approach, it is envisaged that this work will provide a promising prospect of large‐scale production in smart oil/water separation materials. © 2017 Society of Chemical Industry
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