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Understanding the motion of particles with ligand-receptors is important for biomedical applications and material design. Yet, even among a single design, the prototypical DNA-coated colloids, seemingly similar micrometric particles hop or roll, depending on the study. We shed light on this problem by observing DNA-coated colloids diffusing near surfaces coated with complementary strands for a wide array of coating designs. We find colloids rapidly switch between 2 modes: they hop - with long and fast steps - and crawl - with short and slow steps. Both modes occur at all temperatures around the melting point and over a wide array of designs. The particles become increasingly subdiffusive as temperature decreases, in line with subsequent velocity steps becoming increasingly anti-correlated. Overall, crawling (or hopping) phases are more predominant at low (or high) temperatures; crawling is also more efficient at low temperatures than hopping to cover large distances. We rationalize this behavior within a simple model: at lower temperatures, the number of bound strands increases, and detachment of all bonds is unlikely, hence, hopping is prevented and crawling favored. We thus reveal the mechanism behind a common design rule relying on increased strand density for long-range self-assembly: dense strands on surfaces are required to enable crawling, possibly facilitating particle rearrangements.

Concentrated colloidal suspensions and emulsions are amorphous soft solids, widespread in technological and industrial applications and studied as model systems in physics and material sciences. They are easily fluidized by applying a mechanical stress, undergoing a yielding transition that still lacks a unified description. Here, we investigate yielding in three classes of repulsive soft solids, using analytical and numerical modelling and experiments probing the microscopic dynamics and mechanical response under oscillatory shear. We find that at the microscopic level yielding consists in a transition between two distinct dynamical states, which we rationalize by proposing a lattice model with dynamical coupling between neighboring sites, leading to a unified state diagram for yielding. Leveraging the analogy with Wan der Waals's phase diagram for real gases, we show that distance from a critical point plays a major role in the emergence of first-order-like vs second-order-like features in yielding, thereby reconciling previously contrasting observations on the nature of the transition., Comment: 23 pages, 5 figures

The self-assembly of DNA-coated colloids into highly-ordered structures offers great promise for advanced optical materials. However, control of disorder, defects, melting, and crystal growth is hindered by the lack of a microscopic understanding of DNA-mediated colloidal interactions. Here we use total internal reflection microscopy to measure in situ the interaction potential between DNA-coated colloids with nanometer resolution and the macroscopic melting behavior. The range and strength of the interaction are measured and linked to key material design parameters, including DNA sequence, polymer length, grafting density, and complementary fraction. We present a first-principles model that quantitatively reproduces our experimental data without fitting parameters over a wide range of DNA ligand designs. Our theory identifies a subtle competition between DNA binding and steric repulsion and accurately predicts adhesion and melting at a molecular level. Combining experimental and theoretical results, our work provides a quantitative and predictive approach for guiding material design with DNA-nanotechnology and can be further extended to a diversity of colloidal and biological systems.

Total internal reflection microscopy (TIRM) measures changes in the distance between a colloidal particle and a transparent substrate by measuring the intensity of light scattered by the particle when it is illuminated by the evanescent field that is created from light totally internally reflected at the substrate interface. From these measurements, the height-dependent effective potential $\varphi(z)$ between the colloidal particle and the substrate can be measured. The spatial resolution with which TIRM can resolve the height $z$ and effective potential $\varphi(z)$ is limited by the intrinsic shot noise of the photon counting process used to measure the scattered light intensity. We develop a model to determine the spatial resolution with which TIRM can measure $\varphi(z)$ and verify its validity with simulations and experiments. We further establish the critical role of photon-counting statistics and the intensity integration time $\tau$ in TIRM measurements, which is a trade-off between narrowing the width of the photon counting distribution and capturing the instantaneous position of the probe particle., Comment: 10 pages, 7 figures

Emulsion droplets trapped in an ultrasonic levitator behave in two ways that solid spheres do not: (1) Individual droplets spin rapidly about an axis parallel to the trapping plane, and (2) coaxially spinning droplets form long chains aligned with their common axis of rotation. Acoustically-organized chains interact hydrodynamically, either to merge into longer chains or to form three-dimensional bundles of chains. Solid spheres, by contrast, form close-packed planar crystals drawn together by the sound-mediated secondary Bjerknes interaction. We demonstrate the chain-to-crystal transition with a model system in which fluid emulsion droplets can be photopolymerized into solid spheres without significantly changing other material properties. The behavior of this experimental system is quantitatively consistent with an acoustohydrodynamic model for spinning spheres in an acoustic levitator. This study therefore introduces acoustically-driven spinning as a mechanism for guiding self-organization of acoustically levitated matter., Comment: 10 pages, 4 figures

In periodically sheared suspensions there is a dynamical phase transition characterized by a critical strain amplitude $\gamma_c$ between an absorbing state where particle trajectories are reversible and an active state where trajectories are chaotic and diffusive. Repulsive non-hydrodynamic interactions between "colliding" particles' surfaces have been proposed as a source of this broken time reversal symmetry. A simple toy model called Random Organization qualitatively reproduces the dynamical features of this transition. Random Organization and other absorbing state models exhibit hyperuniformity, a strong suppression of density fluctuations on long length-scales quantified by a structure factor $S(q \rightarrow 0) \sim q^\alpha$ with $\alpha > 0$, at criticality. Here we show experimentally that the particles in periodically sheared suspensions organize into structures with anisotropic short-range order but isotropic, long-range hyperuniform order when oscillatory shear amplitudes approach $\gamma_c$.

Motility is a basic feature of living microorganisms, and how it works is often determined by environmental cues. Recent efforts have focused on develop- ing artificial systems that can mimic microorganisms, and in particular their self-propulsion. Here, we report on the design and characterization of syn- thetic self-propelled particles that migrate upstream, known as positive rheo- taxis. This phenomenon results from a purely physical mechanism involving the interplay between the polarity of the particles and their alignment by a viscous torque. We show quantitative agreement between experimental data and a simple model of an overdamped Brownian pendulum. The model no- tably predicts the existence of a stagnation point in a diverging flow. We take advantage of this property to demonstrate that our active particles can sense and predictably organize in an imposed flow. Our colloidal system represents an important step towards the realization of biomimetic micro-systems withthe ability to sense and respond to environmental changes, Comment: Published in Science Advances [Open access journal of Science Magazine]

We synthesize colloidal particles with various anisotropic shapes and track their orientationally resolved Brownian trajectories using confocal microscopy. An analysis of appropriate short-time correlation functions provides direct access to the hydrodynamic friction tensor of the particles revealing nontrivial couplings between the translational and rotational degrees of freedom. The results are consistent with calculations of the hydrodynamic friction tensor in the low-Reynolds-number regime for the experimentally determined particle shapes., Comment: 5 pages, 2 figures

A self-assembling structure using non-equilibrium driving forces leading to 'living crystals' and other maniputable particles with a complex dynamics. The dynamic self-assembly assembly results from a competition between self-propulsion of particles and an attractive interaction between the particles. As a result of non-equilibrium driving forces, the crystals form, grow, collide, anneal, repair themselves and spontaneously self-destruct, thereby enabling reconfiguration and assembly to achieve a desired property.