A simple method to alter the binding specificity of DNA-coated colloids that crystallize

DNA-coated colloids can crystallize into a multitude of lattices, ranging from face-centered cubic to diamond, opening avenues to producing structures with useful photonic properties. The potential design space of DNA-coated colloids is large, but its exploration is hampered by a reliance on chemically modified DNA that is slow and expensive to commercially synthesize. Here we introduce a method to controllably tailor the sequences of DNA-coated particles by covalently appending new sequence domains onto the DNA grafted to colloidal particles. The tailored particles crystallize as readily and at the same temperature as those produced via direct chemical synthesis, making them suitable for self-assembly. Moreover, we show that particles coated with a single sequence can be converted into a variety of building blocks with differing specificities by appending different DNA sequences to them. This method will make it practical to identify optimal and complex particle sequence designs and paves the way to programming the assembly kinetics of DNA-coated colloids.</p

A simple method to alter the binding specificity of DNA-coated colloids that crystallize

DNA-coated colloids can crystallize into a multitude of lattices, ranging from face-centered cubic to diamond, opening avenues to producing structures with useful photonic properties. The potential design space of DNA-coated colloids is large, but its exploration is hampered by a reliance on chemically modified DNA that is slow and expensive to commercially synthesize. Here we introduce a method to controllably tailor the sequences of DNA-coated particles by covalently appending new sequence domains onto the DNA grafted to colloidal particles. The tailored particles crystallize as readily and at the same temperature as those produced via direct chemical synthesis, making them suitable for self-assembly. Moreover, we show that particles coated with a single sequence can be converted into a variety of building blocks with differing specificities by appending different DNA sequences to them. This method will make it practical to identify optimal and complex particle sequence designs and paves the way to programming the assembly kinetics of DNA-coated colloids.</p

Predator-Prey Interactions between Droplets Driven by Nonreciprocal Oil Exchange

Chemotactic interactions are ubiquitous in nature and can lead to nonreciprocal and complex emergent behavior in multibody systems. Here we show how chemotactic signaling between microscale oil droplets of different chemistries in micellar surfactant solutions can result in predator-prey-like chasing interactions. The interactions and dynamic self-organization result from the net directional, micelle-mediated transport of oil between emulsion droplets of differing composition and are powered by the free energy of mixing. The nonreciprocal behavior occurs in a wide variety of oil and surfactant conditions, and we systematically elucidate chemical design rules for tuning the interactions between droplets by varying oil and surfactant chemical structure and concentration. Through integration of experiment and simulation, we also investigate the active behavior and dynamic reorganization of multi-droplet clusters. Our findings demonstrate how chemically-minimal systems can be designed with controllable, non-reciprocal chemotactic interactions to generate emergent self-organization and collective behaviors reminiscent of biological systems

Emulsion patterns in the wake of a liquid–liquid phase separation front

Miscible liquids can phase separate in response to a composition change. In bulk fluids, the demixing begins on molecular-length scales, which coarsen into macroscopic phases. By contrast, confining a mixture in microfluidic droplets causes sequential phase separation bursts, which self-organize into rings of oil and water to make multilayered emulsions. The spacing in these nonequilibrium patterns is self-similar and scale-free over a range of droplet sizes. We develop a modified Cahn–Hilliard model, in which an immiscibility front with stretched exponential dynamics quantitatively predicts the spacing of the layers. In addition, a scaling law predicts the lifetime of each layer, giving rise to a stepwise release of inner droplets. Analogously, in long rectangular capillaries, a diffusive front yields large-scale oil and water stripes on the time scale of hours. The same theory relates their characteristic length scale to the speed of the front and the rate of mass transport. Control over liquid–liquid phase separation into large-scale patterns finds potential material applications in living cells, encapsulation, particulate design, and surface patterning

Emulsion patterns in the wake of a liquid–liquid phase separation front

Miscible liquids can phase separate in response to a composition change. In bulk fluids, the demixing begins on molecular-length scales, which coarsen into macroscopic phases. By contrast, confining a mixture in microfluidic droplets causes sequential phase separation bursts, which self-organize into rings of oil and water to make multilayered emulsions. The spacing in these nonequilibrium patterns is self-similar and scale-free over a range of droplet sizes. We develop a modified Cahn–Hilliard model, in which an immiscibility front with stretched exponential dynamics quantitatively predicts the spacing of the layers. In addition, a scaling law predicts the lifetime of each layer, giving rise to a stepwise release of inner droplets. Analogously, in long rectangular capillaries, a diffusive front yields large-scale oil and water stripes on the time scale of hours. The same theory relates their characteristic length scale to the speed of the front and the rate of mass transport. Control over liquid–liquid phase separation into large-scale patterns finds potential material applications in living cells, encapsulation, particulate design, and surface patterning

Tunable Persistent Random Walk in Swimming Droplets

We characterize the motility of athermal swimming droplets within the framework of a persistent random walk. Just like active colloids, their trajectories can be modeled with a constant velocity V and a slow angular diffusion, but the random changes in direction are not thermally driven. Instead, V is determined by the interfacial tension gradient along the droplet surface, while reorientation of the surfactant gradient leads to changes in direction with a persistence time τ. We show that the origin of locomotion is the difference in the critical micellar concentration in the front and the back of the droplet, ΔCMC. Tuning this parameter by salt controls V from 3 to 15 diameters d/s. Surfactant concentration has little effect on speed, but leads to a dramatic decrease in τ over 4 orders of magnitude. The corresponding range of the persistence length ℓ=Vτ extends beyond the realm of synthetic or living swimmers, in which V is limited by fuel consumption and τ is set by thermal fluctuations or biological activity, respectively. Our tunable swimmers are ideal candidates for the study of the departure from equilibrium to high levels of activity. We show that their collective behavior exhibits the formation of active clusters of a well-defined size

Predator–prey interactions between droplets driven by non-reciprocal oil exchange

Chemotactic interactions are ubiquitous in nature and can lead to non-reciprocal and complex emergent behaviour in multibody systems. However, developing synthetic, inanimate embodiments of a chemomechanical framework to generate non-reciprocal interactions of tunable strength and directionality has been challenging. Here we show how chemotactic signalling between microscale oil droplets of different chemistries in micellar surfactant solutions can result in predator–prey-like non-reciprocal chasing interactions. The interactions and dynamic self-organization result from the net directional, micelle-mediated transport of oil between emulsion droplets of differing composition and are powered by the free energy of mixing. We systematically elucidated chemical design rules to tune the interactions between droplets by varying the oil and surfactant chemical structure and concentration. Through the integration of experiment and simulation, we also investigated the active behaviour and dynamic reorganization of multidroplet clusters. Our findings demonstrate how chemically minimal systems can be designed with controllable, non-reciprocal chemotactic interactions to generate emergent self-organization and collective behaviours reminiscent of biological systems. [Figure not available: see fulltext.

Solute-mediated interactions between active droplets

Concentration gradients play a critical role in embryogenesis, bacterial locomotion, as well as the motility of active particles. Particles develop concentration profiles around them by dissolution, adsorption, or the reactivity of surface species. These gradients change the surface energy of the particles, driving both their self-propulsion and governing their interactions. Here, we uncover a regime in which solute gradients mediate interactions between slowly dissolving droplets without causing autophoresis. This decoupling allows us to directly measure the steady-state, repulsive force, which scales with interparticle distance as F ∼ 1/r2. Our results show that the dissolution process is diffusion rather than reaction rate limited, and the theoretical model captures the dependence of the interactions on droplet size and solute concentration, using a single fit parameter, l = 16 ± 3 nm, which corresponds to the length scale of a swollen micelle. Our results shed light on the out-of-equilibrium behavior of particles with surface reactivit

Solute-mediated interactions between active droplets

Concentration gradients play a critical role in embryogenesis, bacterial locomotion, as well as the motility of active particles. Particles develop concentration profiles around them by dissolution, adsorption, or the reactivity of surface species. These gradients change the surface energy of the particles, driving both their self-propulsion and governing their interactions. Here, we uncover a regime in which solute gradients mediate interactions between slowly dissolving droplets without causing autophoresis. This decoupling allows us to directly measure the steady-state, repulsive force, which scales with interparticle distance as F ∼ 1/r2. Our results show that the dissolution process is diffusion rather than reaction rate limited, and the theoretical model captures the dependence of the interactions on droplet size and solute concentration, using a single fit parameter, l = 16 ± 3 nm, which corresponds to the length scale of a swollen micelle. Our results shed light on the out-of-equilibrium behavior of particles with surface reactivit

Photoresponsive hydrogel microcrawlers exploit friction hysteresis to crawl by reciprocal actuation

Mimicking the locomotive abilities of living organisms on the microscale, where the downsizing of rigid parts and circuitry presents inherent problems, is a complex feat. In nature, many soft-bodied organisms (inchworm, leech) have evolved simple, yet efficient locomotion strategies in which reciprocal actuation cycles synchronize with spatiotemporal modulation of friction between their bodies and environment. We developed microscopic (∼100 μm) hydrogel crawlers that move in aqueous environment through spatiotemporal modulation of the friction between their bodies and the substrate. Thermo-responsive poly-n-isopropyl acrylamide hydrogels loaded with gold nanoparticles shrink locally and reversibly when heated photothermally with laser light. The out-of-equilibrium collapse and reswelling of the hydrogel is responsible for asymmetric changes in the friction between the actuating section of the crawler and the substrate. This friction hysteresis, together with off-centered irradiation, results in directional motion of the crawler. We developed a model that predicts the order of magnitude of the crawler motion (within 50%) and agrees with the observed experimental trends. Crawler trajectories can be controlled enabling applications of the crawler as micromanipulator that can push small cargo along a surface