Single-crystal silver dendrite structures, which possess a high surface area along with narrow gaps and sharp edges, have been widely explored as surface enhanced Raman scattering substrates and for catalytic and biosensing applications. With one of the simplest galvanic reactions, the reaction of AgNO3 with Cu, Ag dendrites can easily be produced. Here, we report new findings about this reaction and dendrite growth mechanisms. A series of experiments revealed a much more complicated reaction mechanism: metal Ag could form through reduction of Ag+ by intermediate nitrite (NO2 –) ions inside the solution on a surface away from Cu. It was also found that Ag dendrites developed through a particle-mediated growth process. This new reaction mechanism can be utilized to generate completely freestanding, pure, and clean single-crystal Ag dendrites at room temperature within a few minutes.
<p>The study of active colloidal microswimmers with tunable phoretic and self-organizational behaviors is important for understanding out-of-equilibrium systems and the design of functional, adaptive matter. Solubilizing, self-propelling droplets have emerged as a rich chemical platform for exploration of active behaviors, but isotropic droplets rely on spontaneous symmetry breaking to sustain motion. The introduction of permanent asymmetry, e.g. in the form of a biphasic Janus droplet, has not been explored previously as a comprehensive design strategy for active droplets, despite the widespread use of Janus structures in motile solid particles. Here, we uncover the chemomechanical framework underlying the self-propulsion of active, biphasic Janus oil droplets solubilizing in aqueous surfactant. We elucidate how droplet propulsion is influenced by the degree of oil mixing, droplet shape, and oil solubilization rates for a range of oil combinations. A key finding is that for droplets containing both a mobile (solubilizing) and non-mobile oil, the degree of partitioning of the mobile oil across the Janus droplets’ oil-oil interface plays a pivotal role in determining the droplet speed and swimming direction. As a result, we observe propulsion speeds of Janus droplets more than an order-of-magnitude faster than chasing pairs of single emulsion droplets which lack an oil-oil interface. In addition, spatiotemporal control over droplet swimming speed and orientation is demonstrated through the application of local thermal gradients applied via induced via joule heading and laser spot illumination. We also explore the interactions between collections of Janus droplets including the spontaneous formation of multi-droplet spinning clusters that rotate predictably based on symmetry. Our findings provide key insights as to how the chemistry and structure of multiphase fluids can be harnessed to design microswimmers with programmable active and collective behaviors.</p><br>
Newton’s third law, action = reaction, is a foundational statement of classical mechanics. However, in natural and living systems, this law appears to be routinely violated for constituents interacting in a nonequilibrium environment. Here, we use computer simulations to explore the macroscopic phase behavior implications of breaking microscopic interaction reciprocity for a simple model system. We consider a binary mixture of attractive particles and introduce a parameter that is a continuous measure of the degree to which interaction reciprocity is broken. In the reciprocal limit, the species are indistinguishable, and the system phase separates into domains with distinct densities and identical compositions. Increasing nonreciprocity is found to drive the system to explore a rich assortment of phases, including phases with strong composition asymmetries and three-phase coexistence. Many of the states induced by these forces, including traveling crystals and liquids, have no equilibrium analogs. By mapping the complete phase diagram for this model system and characterizing these unique phases, our findings offer a concrete path forward toward understanding how nonreciprocity shapes the structures found in living systems and how this might be leveraged in the design of synthetic materials.
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