Session F18: Fluids IV

Gravitational settling of active droplets

The gravitational settling of oil droplets solubilizing in an aqueous micellar solution contained in a capillary channel is discussed. The motion of these active droplets reflects a competition between gravitational and Marangoni forces, the latter due to interfacial tension gradients generated by differences in filled-micelle concentrations along the oil-water interface. This competition is studied by varying the surfactant concentration, the density difference between the droplet and the continuous phase, and the viscosity of the continuous phase. The Marangoni force enhances the settling speed of an active droplet when compared to the Hadamard-Rybczynski prediction for a (surfactant free) droplet settling in Stokes flow. The Marangoni force can also induce lateral droplet motion, suggesting that the Marangoni and gravitational forces are not always aligned. The decorrelation rate () of the droplet motion, measured as the initial slope of the velocity autocorrelation and indicative of the extent to which the Marangoni and gravitational forces are aligned during settling, is examined as a function of the droplet size: correlated motion (small values of ) is observed at both small and large droplet radii, whereas significant decorrelation can occur between these limits. This behavior of active droplets settling in a capillary channel is in marked contrast to that observed in a dish, where the decorrelation rate increases with the droplet radius before saturating at large values of droplet radius. A simple relation for the crossover radius at which the maximal value of  occurs for an active settling droplet is proposed.

    

Spontaneous shock waves in pulse-stimulated Quincke rollers

Microscopic Quincke rollers – colloids suspended in a weak electrolyte and energized by an electric field – are a popular realization of synthetic active matter: interacting self-propelled particles. The pulsating electric field generates a multitude of novel patterns not observed in the system stimulated by a constant field. Shock waves emerge spontaneously in local high-density regions and move faster than the average particle speed. The shock waves occur when the roller's translational and rotational decoherence times become comparable. In turn, this time ratio is controlled by the electric pulse duration. The experiment is supported by the computational modeling highlighting the role of the particle collisions and hydrodynamic flows. Our results provide insight into the design of reconfigurable active matter systems.   

Emerging states of isotropic autophoretic disks: from crystalline solids to active turbulence

Active droplets swim autonomously in viscous fluids due to the nonlinear interplay between solute transport with self-generated Marangoni flows. This mechanism is also responsible for the spontaneous propulsion of disk-shaped camphor boats on a liquid-air interface. Here, we study the collective motion of isotropic autophoretic disks representing a paradigmatic system for suspensions of active droplets and camphor boats. We conducted extensive two-dimensional particle-resolved simulations considering full hydro-chemical interactions, spanning a two-parameter space of Péclet number and area fraction. Varying the two parameters, the disk suspensions exhibit multiple emerging states: triangular lattice crystal, liquid phase, gas of clusters, and active turbulence. A narrow range of hexatic phase between the liquid and solid phases has been identified, the emergence of which is captured by our far-field scaling theory. Our simulations have reproduced a few experimental observations, including the crossing and reflecting trajectories of two active droplets, and the stationary crystalline structure formed by or turbulent motion of camphor boats.  

    

Author not Attending

The nonlinear relationship between the form and function of physical structures in our built environment raises challenges for design. Modern design methods, such as topology optimization, provide structural solutions but obscure the relationship between the form of the solution and the formulation of the underlying design problem. Here, we show that embedding computational structure design in statistical physics provides unprecedented insight into the origin and organization of design features. We show how our "hyperoptimization" approach, a generalized, superset of molecular dynamics and standard simulated annealing optimization, surmounts known design problems including grayscale ambiguity, manufacturing inaccuracy, and artificially over-specified criteria in computational morphogenesis.   

Using liquid-in-liquid 3D printing for fabrication of bijels induced by solvent transfer induced phase separation

Bicontinuous interfacially jammed emulsion gels (bijels) are a new class of soft material first introduced in 2005. These materials have beneficial properties primarily because of the larger contact area along the interface of the two interwoven phases and within a small volume.  Despite such promising properties, the fabrication of bijels has proved to be time-consuming and challenging due to the limits in selecting constituent materials, which confines their wide applications. Furthermore, bijels reported in the current literature are created in bulk and lack complexity in designs and shapes, which is another limiting reason. In this study, the fabrication of bijels is presented using solvent transfer induced phase separation methods (referred to as STRIPS) with various oil components (such as fatty acids and phthalates), nanoparticles (with different sizes and charges), and surfactants (cationic and anionic). Using the liquid-in-liquid 3D printing (LL3DP) technique, various structures with different designs and structural features were printed, and confocal/light microscopy images confirmed the formation of bicontinuous microstructures. Shear rheometry on the printed bijels characterized their mechanical properties and rheological behavior, suggesting robust shear-thinning gels. With the advancement presented in this work, bijels can find their way into various fields and for different applications, including energy, tissue engineering, catalysis, and separation.

    

Using Phase Field Models to Simulate Colloidal Chemohydrodynamics in Bulk and at Phase Boundaries

Colloidal particles can migrate in a solution in response to a solute concentration field, a phenomenon known as diffusiophoresis. Chemically active colloids can modify the concentration field of its surrounding, thus harvesting energy from the environment to self-propel or to change the trajectory of neighboring colloids. To date, the most efficient methods to simulate these active systems rely on Green's functions of the Laplace and Stokes operators that are only valid in the steady and dilute limits. However, many active systems of interest display interesting feedback behavior in dense and unsteady systems. We have recently developed a method using phase field models that performs full chemohydrodynamics simulations of such dense and unsteady systems and incorporates colloidal particles as highly viscous fluid phases. We domonstrate the feasability of this by simulating particles in both homogenous and herterogenous fluidic environments and compare to known theoretical results. We also demonstrate the ability of the method to simulate self-diffusiophoresis by adding asymmetric chemical reactions to colloidal systems. 

    

Encountering obstacles: microrollers interacting in complex and structured environments.

Micro swimmers in a fluid can either self-propel or be driven by external forces. The interactions between micro swimmers and their surroundings lead to a wide range of phenomena: self-assembly, phase separation, hydrodynamic trapping, etc. They can also sense the microstructure of the fluid or the boundary geometries, which changes their motion. In our experiments, I explore how magnetically driven microrollers maneuver in a structured environment. When the rollers are spun by an external magnetic field, they pump fluid around themselves, driving them to move in the desired direction. This also generates strong and long-range flows. I will demonstrate how such flows interact with a complex fluid and generate new structure, and how the fluid structure influences the roller motion. I will also discuss how the boundary geometries interact with microrollers through the flow and manipulate the microroller motion.   

Active depletion forces and colloidal clustering in an active bath

We analyze active depletion interactions between a pair of colloids suspended in an active bath of non-interacting run-and-tumble micro-swimmers using a minimal model for colloid-swimmer interactions. Using stochastic simulations, we confirm the existence of an effective attractive force, which is the active analog of the classic passive depletion force arising in suspensions of polymer coils. We use simulations to characterize the dependence of this active depletion force on colloid separation distance, density of the active bath, and mean run length of the micro-swimmers. In suspensions of multiple colloids, these interactions tend to drive colloidal clustering and aggregation, the kinetics of which we study for colloids subject to short-ranged adhesive interactions.   

Dynamics of nanoparticles in glassy matrices of different tracer-matrix size ratios

The dynamics of particles in a dense suspension slow dramatically and become increasingly cooperative as the glass transition volume fraction Φ_g is approached. How these changes in the dynamics affect motion of penetrant particles remains incompletely understood. Using confocal microscopy, we investigate the dynamics of tracer nanospheres in glassy nanoparticle liquids at different tracer-to-particle size ratios δ. The dynamics of the tracer particles were calculated from trajectories obtained using particle-tracking algorithms, and differential dynamic microscopy was used to characterize the relaxation behavior of the matrix. Near Φ_g, the mean-square displacements of the tracer particles plateau on short time scales due to caging. For matrices with Φ < Φ_g, the cages relax on time scales and the tracers recover diffusive dynamics on long time scales. For matrices with Φ > Φ_g, however, tracers remain caged on all experimental time scales. For a given matrix Φ, the dynamics slow upon increasing δ. These results provide insight into how the ratio of tracer and matrix particle sizes affects the transport mechanism of particles confined in complex matrices, which can find applications in cell biology.   

Transfer Kinetics of Cargo Items among Nanocarriers

Nanocarriers such as micelles, liposomes, nanoshells and nanocages, dendrimers, carbon nanotubes, and nanoparticles are used in applications to transport cargo items--often drug molecules--to a target site. When nanocarriers collide with each other, cargo items are able to migrate from one to another nanocarrier. We employ chemical reaction kinetics to characterize how the distribution of cargo items among all nanocarriers in a mixture of different nanocarrier types depends on time. In the continuum limit, valid when each nanocarrier contains a sufficiently large number of cargo items, we express the kinetic equations as a system of partial differential equations--diffusion equations with additional demixing terms--that evolve into Gaussian distributions over time. We solve the partial differential equations and thus determine the kinetic behavior for any initial distribution of cargo in this multi-type nanocarrier system. The model can be generalized to address related problems, including the account of sink conditions, spatial variations analogous to diffusion-reaction phenomena, the release of cargo directly into the solution, and aggregation of cargo items inside carriers.

 

    

Author not Attending

Over the past decade, by engineering soft materials from active units, physicists have learned how to drive fluids from within. The generic strategy consists in assembling orientationally ordered liquids from self-propelled particles. Their resulting mesmerizing flows are now understood in basic geometries such as linear channels or circular chambers. However, unlike viscous flows, active flows are intrinsically multistable. This intrinsic nonlinear behavior results in highly degenerated flow patterns, even in simple geometries.

Building on model experiments based on Quincke roller fluids, we reveal and elucidate two qualitatively different classes of flow degeneracy when active fluids explore honeycomb hydraulic networks. Our work explains how the local strategies used by active flows to resolve geometrical frustration determine the large scale morphology of their flow fields.   

Nanoparticle dynamics in semiflexible ring polymer solutions

Understanding the transport of nanoparticles (NPs) in polymer solutions is important for applications such as targeted drug delivery and enhanced oil recovery. When the size of NP and polymers are comparable, the polymer solution cannot be treated as a continuum fluid and the NP dynamics deviate from the prediction of the generalized Stokes-Einstein relation. We studied the effects of polymer backbone stiffness on the dynamics of NP in semi-dilute ring polymer solutions using hybrid molecular dynamics–multiparticle collision dynamics simulations. The long-time diffusion coefficients of NPs decrease with an increase in the stiffness of ring polymers and deviate from the predictions of a polymer coupling theory by Cai et al. [Macromolecules 44, 7853–7863 (2011)].  At intermediate time scales, the NP subdiffusive exponents in fully flexible ring polymer solutions are strongly correlated to those of the polymer center-of-mass (COM), suggesting NPs dynamics are coupled with dynamics of polymer COM. Upon increasing backbone stiffness, however, we observe that the NP and polymer COM dynamics begin to decouple. We discuss these observations in the context of expectations from polymer coupling theory and contrast with those from complementary studies of NP dynamics in solutions of linear polymer chains.

    

Tidal locking in structured active matter on elastic surfaces

Agents in active matter are typically rigid and thus treated as point particles. However, active matter agents can also contain structure such that an individual can be compressed or stretched. In the presence of external fields, the coupling between the structured agent and its environment can deform the agent as well as affect its trajectory. Here, we use a recently developed system in which 200-gram, 10 cm diameter wheeled vehicles locomote at constant speed on a large deformable spandex membrane (diameter 2.4 m). Agents exhibit rich dynamics, including precessing orbits of a single rigid body; the dynamics can be mapped to motion in a "fiducial" curved spacetime (Li et al., PNAS, 2022). We use this system to explore properties of structured active matter in a spatially varying field by connecting two vehicles via a linear spring and study the orbital dynamics of the vehicles on the membrane. For a range of relative car speeds the entity can be captured in a tidally-locked trajectory such that the same vehicle remains closest to the central depression during a trajectory, analogous to the state where the Moon maintains the same side facing towards the Earth. We posit that the membrane and its gradient field are integral to creating the tidal-locking trajectories, as tidal-locking is not observed without the membrane. Further, we observe that the capture and attraction to the tidally-locked state is modulated by the spring's restoring force, and the rate of capture depends on the spring constant and relative car speeds.   

Author not Attending

Non-equilibrium statistical mechanics connects random thermal fluctuations in equilibrium with macroscopic transport phenomena, as famously encapsulated in Onsager's regression hypothesis. Transport phenomena usually involve down-gradient fluxes; for example particles tend to diffuse from high to low concentration, and heat tends to flow from high to low temperature. In this talk, we examine "odd" transport phenomena, in which fluxes can be orthogonal to gradients, and thus need not affect the relaxation of the gradients. We particularly emphasize how odd transport phenomena such as odd diffusion, odd thermal conduction, and odd viscosity may arise in chiral active matter. By applying Onsager's regression hypothesis in the context of such steady states, and by reformulating this hypothesis at the level of the constitutive relations rather than that of the relaxation equations, we show that Green-Kubo relations of the standard form hold in general for odd transport coefficients. These relations reveal the connection between time-reversal symmetry breaking and odd transport, as well as yielding Onsager-Casimir reciprocal relations. Importantly, these Green-Kubo relations hold even in contexts where linear response (i.e. fluctuation-dissipation) relations break down. We conclude by demonstrating the applicability of these Green-Kubo relations through simulations of odd diffusion in concentrated solutions of chiral active particles.   

Swimming the chaotic seas: invariant manifolds, tori, and the transport of swimmers in vortex flows

We analyze the kinematics of micro-swimmers in an imposed microchannel

flow consisting of alternating fluid vortices. These swimmers could be

biological (e.g. bacteria or algae) or artificial (e.g. Janus

particles). Using dynamical systems techniques, we show that transport

from one vortex down the channel to another vortex is mediated by both

invariant tori and so-called Swimming Invariant Manifolds (SwIMs); SwIMs have

previously been emphasized as one-way barriers to swimmer transport,

but they also form chutes which guide swimmer passage between

vortices. The SwIM geometry thus plays a critical role in determining

transport rates of swimmers between vortices.  The invariant tori, on

the other hand, lead both to trapping within vortex cells and

ballistic transport between vortex cells.  Our theoretical framework

is applied to experiments on algae in microfluidic channels.