Odd Response-Induced Phase Separation of Active Spinners.

Due to the breaking of time-reversal and parity symmetries and the presence of non-conservative microscopic interactions, active spinner fluids and solids respectively exhibit nondissipative odd viscosity and nonstorage odd elasticity, engendering phenomena unattainable in traditional passive or active systems. Here, we study the effects of odd viscosity and elasticity on phase behaviors of active spinner systems. We find the spinner fluid under a simple shear experiences an anisotropic gas-liquid phase separation driven by the odd-viscosity stress. This phase separation exhibits equilibrium-like behavior, with both binodal-like and spinodal curves and critical point. However, the formed dense liquid phase is unstable, since the odd elasticity instantly takes over the odd viscosity to condense the liquid into a solid-like phase. The unusual phase behavior essentially arises from the competition between thermal fluctuations and the odd response-induced effective attraction. Our results demonstrate that the cooperation of odd viscosity and elasticity can lead to exotic phase behavior, revealing their fundamental roles in phase transition.

Morphology-Tailored Dynamic State Transition in Active-Passive Colloidal Assemblies.

Mixtures of active self-propelled and passive colloidal particles promise rich assembly and dynamic states that are beyond reach via equilibrium routes. Yet, controllable transition between different dynamic states remains rare. Here, we reveal a plethora of dynamic behaviors emerging in assemblies of chemically propelled snowman-like active colloids and passive spherical particles as the particle shape, size, and composition are tuned. For example, assembles of one or more active colloids with one passive particle exhibit distinct translating or orbiting states while those composed of one active colloid with 2 passive particles display persistent "8"-like cyclic motion or hopping between circling states around one passive particle in the plane and around the waist of 2 passive ones out of the plane, controlled by the shape of the active colloid and the size of the passive particles, respectively. These morphology-tailored dynamic transitions are in excellent agreement with state diagrams predicted by mesoscale dynamics simulations. Our work discloses new dynamic states and corresponding transition strategies, which promise new applications of active systems such as micromachines with functions that are otherwise impossible.

Regulation of Photogenerated Redox Species through High Crystallinity Carbon Nitride for Improved C-S Coupling Reactions.

A novel and efficient approach for the synthesis of α, ß-unsaturated sulfones through heterogeneous photocatalyzed C-S coupling reactions have been developed. The use of molten-salt method derived carbon nitride (MCN), a transition metal-free polymeric photocatalyst, combined with enhanced crystallinity and potassium iodide as an additive, effectively modulates photogenerated reactive redox species, markedly increasing the overall reaction selectivity. This method achieves the shortest reaction time (2 h) with high yield (up to 95 %) among the reported heterogeneous catalytic C-S bond formation reactions, matching the efficiency of the homogeneous photocatalysts. Furthermore, the application to challenging alkyne substrates has been demonstrated, underscoring the potential for a broad range of applications in pharmaceutical research and synthetic chemistry.

Active self-assembly of colloidal machines with passive rotational parts via coordination of phoresis and osmosis.

The organization of microscopic objects into specific structures with movable parts is a prerequisite for building sophisticated micromachines with complex functions, as exemplified by their macroscopic counterparts. Here we report the self-assembly of active and passive colloids into micromachinery with passive rotational parts. Depending on the attachment of the active colloid to a substrate, which varies the degrees of free freedom of the assembly, colloidal machines with rich internal rotational dynamics are realized. Energetic analysis reveals that the energy efficiency increases with the degrees of freedom of the machine. The experimental results can be rationalized by the cooperation of phoretic interaction and osmotic flow encoded in the shape of the active colloid, which site-specifically binds and exerts a torque to passive colloids, supported by finite element calculations and mesoscale simulations. Our work offers a new design principle that utilizes nonequilibrium interfacial phenomena for spontaneous construction of multiple-component reconfigurable micromachinery.

Orientational Order in Dense Colloidal Liquids and Glasses.

We construct structural order parameters based on local angular and radial distribution functions in dense colloidal suspensions. All the order parameters show significant correlations to local dynamics in the supercooled and glass regime. In particular, the correlations between the orientational order and dynamical heterogeneity are consistently higher than those between the conventional two-body structural entropy and local dynamics. The structure-dynamics correlations can be explained by a excitation model with the energy barrier depending on local structural order. Our results suggest that in dense disordered packings, local orientational order is higher than translational order, and plays a more important role in determining the dynamics in glassy systems.

Hydrodynamics-Induced Long-Range Attraction between Plates in Bacterial Suspensions.

We perform optical-tweezers experiments and mesoscale fluid simulations to study the effective interactions between two parallel plates immersed in bacterial suspensions. The plates are found to experience a long-range attraction, which increases linearly with bacterial density and decreases with plate separation. The higher bacterial density and orientation order between plates observed in the experiments imply that the long-range effective attraction mainly arises from the bacterial flow field, instead of the direct bacterium-plate collisions, which is confirmed by the simulations. Furthermore, the hydrodynamic contribution is inversely proportional to the squared interplate separation in the far field. Our findings highlight the importance of hydrodynamics on the effective forces between passive objects in active baths, providing new possibilities to control activity-directed assembly.

Constraint dependence of pressure on a passive probe in an active bath.

Mechanical pressure in active matter is generally not a state variable and possesses abnormal properties, in stark contrast to equilibrium systems. We here show that the pressure on a passive probe exerted by an active fluid even depends on external constraints on the probe by means of simulation and theory, implying that the mechanical pressure is not an intrinsic physical quantity of active systems. The active mechanical pressure on the passive probe significantly increases and saturates as its elastic constraint (realized by a trap potential) or kinematic constraint (realized by environmental friction) strengthens. The microscopic origin for the constraint-dependent pressure is that the constraints influence the probe dynamics, and hence change the frequency and intensity of the collisions between the probe and active particles. Our findings not only greatly advance the understanding of active mechanical pressure but also provide a new way toin situtune it.

High-Resolution Volumetric Imaging and Classification of Organisms with Standard Optical Microscopy.

Three-dimensional (3D) characterization of organisms is important for the study of cellular phenotypes, structural organization, and mechanotransduction. Existing optical techniques for 3D imaging rely on focus stacking or complex multiangle projection. Focus stacking has deleterious axial resolution due to the one-angle optical projection. Herein, we achieve high-resolution 3D imaging and classification of organisms based on standard optical microscopy coupled to optothermal rotation. Through a seamless fusion of optical trapping and rotation of organisms on a single platform, our technique is applicable to any organism suspended in clinical samples, enabling contact-free and biocompatible 3D imaging. Moreover, when applying deep learning to distinguish different types of biological cells with high similarity, we demonstrate that our platform improves the classification accuracy (96% vs 85%) while using one-tenth the number of training samples compared with conventional deep-learning-based classification.

Learned irrelevant stimulus-response associations and proportion congruency effect: A diffusion model account.

The increased Simon effect with increasing the ratio of congruent trials may be interpreted by both attention modulation and irrelevant stimulus-response (S-R) associations learning accounts, although the reversed Simon effect with increasing the ratio of incongruent trials provides evidence supporting the latter account. To investigate if learning irrelevant S-R associations is a common mechanism underlying the proportion congruency (PC) effect of the Simon task, we employed a variant of diffusion model, diffusion model for conflict tasks (DMC), to test which theory can simultaneously account for the mean reaction time (RT) and RT distribution patterns of the Simon effect in different PC conditions. Simulation results showed that the DMC modulating starting point according to learned irrelevant S-R associations rather than drift criterion or attention-related parameters (i.e., drift rate of controlled process, peak amplitude and time-to-peak of automatic activation) can simultaneously simulate the increase and reversal of the Simon effect and the different shapes of delta functions in different PC conditions. Moreover, when fitting to empirical data, the DMC adjusting starting point provided a good fit to the mean RT and RT distribution patterns of the Simon effect in different PC conditions. These results suggest learning irrelevant S-R associations (biasing starting point) may be a common mechanism underlying the PC effect of the Simon task. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Evidence for growing structural correlation length in colloidal supercooled liquids.

Using video microscopy, we measure the long-time diffusion coefficients of colloidal particles at different concentrations. The measured diffusion coefficients start to deviate from theoretical predictions based on random collision models upon entering the supercooled regime. The theoretical diffusion relation is recovered by assigning an effective mass proportional to the size of structurally correlated clusters to the diffusing particles, providing an indirect method to probe the growth of static correlation length scales approaching the glass transition. This method is tested and validated in the crystallization of mono-disperse colloids in quasi-two-dimensional experiments. The correlation length obtained for a binary colloidal liquid increases by a power law toward a critical packing fraction of ∼0.79. The system relaxation time exhibits a power-law dependence on the correlation length in agreement with dynamical facilitation theories.

Odd viscosity-induced Hall-like transport of an active chiral fluid.

Broken time-reversal and parity symmetries in active spinner fluids imply a nondissipative "odd viscosity," engendering phenomena unattainable in traditional passive or active fluids. Here we show that the odd viscosity itself can lead to a Hall-like transport when the active chiral fluid flows through a quenched matrix of obstacles, reminiscent of the anomalous Hall effect. The Hall-like velocity depends significantly on the spinner activity and longitudinal flow due to the interplay between odd viscosity and spinner-obstacle collisions. Our findings underscore the importance of odd viscosity in active chiral matter and elucidate its essential role in the anomalous Hall-like effect.

A Chemotactic Colloidal Motor.

Chemotaxis plays a crucial role in the realization of various functions of human life such as fertilization, immune function, inflammatory response, regeneration processes, etc. Inspired by the natural chemotaxis, colloidal motors with chemotactic ability can realize intelligent sense and targeted navigation, which bring a revolutionary method to biomedical applications like precision medicine. However, the application in the biomedical field requires the colloidal motors with submicrometer scale, strong chemotactic ability and clear chemotactic mechanism. In this Concept article, we introduce the recent progress of chemotactic colloidal motors, covering the fundamental theory behind experimental advancements. Particularly, the torque-driven reorientation motion of the submicrometer-sized colloidal motors during chemotaxis is discussed, and also their underlying mechanism is proposed. With the continuous research on chemotactic colloidal motors, it is believed that the emerging chemotactic colloidal motors will broaden practical applications in the biomedical field.

Photocatalytic cyclization of nitrogen-centered radicals with carbon nitride through promoting substrate/catalyst interaction.

The use of metal-free carbon nitride and light to drive catalytic transformations constitutes a sustainable strategy for organic synthesis. At the moment, enhancing the intrinsic activity of CN catalysts by tuning the interfacial coupling between catalyst and substrate remains challenging. Herein, we demonstrate that urea-derived carbon nitride catalysts with the abundant -NH2 groups and the relative positive charged surface could effectively complex with the deprotonated anionic intermediate to improve the adsorption of organic reactants on the catalyst surface. The decreased oxidation potential and upshift in its highest occupied molecular orbital position make the electron abstraction kinetics by the catalyst more energetically favorable. The prepared catalyst is thus utilized for the photocatalytic cyclization of nitrogen-centered radicals for the synthesis of diverse pharmaceutical-related compounds (33 examples) with high activity and reusability, which shows competent performance to the homogeneous catalysts.

Light-driven single-cell rotational adhesion frequency assay.

The interaction between cell surface receptors and extracellular ligands is highly related to many physiological processes in living systems. Many techniques have been developed to measure the ligand-receptor binding kinetics at the single-cell level. However, few techniques can measure the physiologically relevant shear binding affinity over a single cell in the clinical environment. Here, we develop a new optical technique, termed single-cell rotational adhesion frequency assay (scRAFA), that mimics in vivo cell adhesion to achieve label-free determination of both homogeneous and heterogeneous binding kinetics of targeted cells at the subcellular level. Moreover, the scRAFA is also applicable to analyze the binding affinities on a single cell in native human biofluids. With its superior performance and general applicability, scRAFA is expected to find applications in study of the spatial organization of cell surface receptors and diagnosis of infectious diseases. Supplementary Information: The online version contains supplementary material available at 10.1186/s43593-022-00020-4.

Transparent, photothermal and stretchable alginate-based hydrogels for remote actuation and human motion sensing.

Multifunctional hydrogels show potential applications in actuators and wearable sensors. However, it is still a challenge to develop a photothermal responsive conductive hydrogel with high transparency, mechanical properties, broad sensing range, and low-temperature resistance. In this work, a transparent, photothermal responsive, and highly stretchable alginate-based hydrogels was feasibly constructed by adding two-dimensional non-layered molybdenum dioxide nanosheets (2D-MoO2) to sodium alginate/polyacrylamide mixture and then soaking into the calcium chloride solution. The introduction of 2D-MoO2 renders the hydrogels excellent photothermal properties and controllable photomechanical deformation under near-infrared irradiation, while maintaining high transparency (~60 %).The calcium ions give the hydrogel excellent mechanics, conductivity, and freezing tolerance concurrently. The transparent hydrogel-based sensor shows wide sensing range (0-1800 %) and cycling stability in detecting deformations and real-time human motions even in harsh environments. Therefore, this work provides a new route for generating transparent multifunctional hydrogels towards the applications of remote actuation and strain sensing.

Spontaneous population oscillation of confined active granular particles.

Spontaneous collective oscillation may emerge from seemingly irregular active matter systems. Here, we experimentally demonstrate a spontaneous population oscillation of active granular particles confined in two chambers connected by a narrow channel, and verify the intriguing behavior predicted in simulation [M. Paoluzzi, R. Di Leonardo and L. Angelani, Self-sustained density oscillations of swimming bacteria confined in microchambers, Phys. Rev. Lett., 2015, 115(18), 188303]. During the oscillation, the two chambers are alternately (nearly) filled up and emptied by the self-propelled particles in a periodic manner. We show that the stable unidirectional flow induced due to the confined channel and its periodic reversal triggered by the particle concentration difference between two chambers jointly give rise to the oscillatory collective behavior. Furthermore, we propose a minimal theoretical model that properly reproduces the experimental results without free parameters. This self-sustained collective oscillation could serve as a robust active granular clock, capable of providing rhythmic signals.

Lattice Induced Short-Range Attraction between Like-Charged Colloidal Particles.

We perform experiments and computer simulations to study the effective interactions between like-charged colloidal tracers moving in a two-dimensional fluctuating background of colloidal crystal. By a counting method that properly accounts for the configurational degeneracy of tracer pairs, we extract the relative probability of finding a tracer pair in neighboring triangular cells formed by background particles. We find that this probability at the nearest neighbor cell is remarkably greater than those at cells with larger separations, implying an effective attraction between the tracers. This effective attraction weakens sharply as the background lattice constant increases. Furthermore, we clarify that the lattice-mediated effective attraction originates from the minimization of free energy increase from deformation of the crystalline background due to the presence of diffusing tracers.

Active depletion torque between two passive rods.

The active depletion torque experienced by two anisotropic objects in an active bath is a conceptional generalization of the equilibrium entropic torque. Using Brownian dynamics simulations, we compute the active depletion torque suffered by two passive rods immersed in an ensemble of active Brownian particles. Our results demonstrate that the active depletion torque is qualitatively different from its passive counterpart. Interestingly, we find that the active depletion torque can be greatly affected by the external constraint applied on the rotational degree of freedom of the rods, and even the direction may be changed with the orientational constraint, which is in contrast to the equilibrium depletion torque. The main reason for the remarkable features of the active depletion torque is that the active particles can significantly accumulate in the vicinity of the rods due to persistent self-propulsion, which is sensitively dependent on the constraint strength and the rod configurations. Our findings could be relevant for understanding the self-assembly and dynamics of anisotropic macromolecules in living environments.

Migration of an active colloidal cell in inhomogeneous environments.

Living cells on a substrate with mechanical inhomogeneities often migrate along or against the mechanical gradient, i.e., mechanotaxis, which inspires us to ask how biomimetic cells without biochemical signaling processes respond to environmental inhomogeneity. Here, we perform computer simulations to study the migration of a 2D active colloidal cell (ACC), which consists of active particles enclosed by a passive vesicle, in a heterogeneous environment composed of two adjoining uniform regions with different attributes (influencing the persistent length of the active particle). We find that the ACC can migrate unidirectionally across the interface separating the heterogeneous region and behave tactically. Interestingly, the tactic motion of the ACC is qualitatively different from that of the constituent active particles themselves. In addition, the ACC may also experience a directed drift along the interface of the heterogeneous environment. The tactic behavior of the ACC can be explained by analyzing the pressure distribution on the cell membrane exerted by the enclosed active particles. The findings provide insights into understanding the taxis of biological cells and designing biomimetic cells with environment-sensitive capabilities.