Session T01: Steerable Particles: New Ways to Manipulate Fluid-Mediated Forces

Steering particles in turbulent flows

Particle shape is a primary design parameter used to control particle motion in fluid flows. When the flow is turbulent, the relative alignment between non-spherical particles and local flow structures plays a central role in particle motion. After introducing the ways particle shape couples to relative velocity, relative rotation, and local velocity gradients, this talk will explore how we design particles that will respond in desired ways in turbulence experiments. Elongated particles align with the local stretching direction which aligns with vorticity due to vortex stretching. If rod shaped particles have chiral ends with opposite handedness, they show a preferential rotation direction in particle coordinates. Ramified particles made of symmetric configurations of slender arms allow separate control over particle size and sedimentation rate. When sedimenting, particles tend to align with a long axis horizontal due to inertial torques. Turbulence competes with this preferential alignment, so by controlling particle shape, size, and sedimentation rate, one can control the orientation distribution and hence the lateral spreading rate.   

Synchronization and collective motion of interfacial capillary spinners

When a millimetric body is deposited onto the interface of a vibrating liquid bath, the relative motion between the object and interface generates outward propagating waves which leads to steady propulsion. Prior work has shown that chiral objects or "spinners" in isolation can rotate at a steady rate, with both the direction of rotation and angular speed depending sensitively on the spinner geometry and driving parameters. Here, we consider the capillary wave-mediated interactions of multiple spinners. We first characterize the two-spinner problem, demonstrating that spinners are able to stably synchronize their rotation in certain parameter regimes via their mutual wavefield. The many-body problem is then considered where a rich variety of static and dynamic interaction modes are found. A model of the spinner interaction is proposed, allowing us to draw analogy to other systems exhibiting spontaneous synchronization. This highly tunable and accessible system represents a new platform for studying active and driven systems interacting through deformable substrates.   

Steering Colloids using Chemical Gradients

Dynamically programmable colloidal materials (DPCMs) are an emerging class of materials that change their structure based on external stimuli. With the correct programming, DPCMs could be influenced to organize into arbitrary structures currently unattainable, with applications in photonics, tissue engineering, and nanorobotics. However, realizing such applications requires spatiotemporal control of colloids to a combined precision and scale that is currently unattainable. Inspired by the complex feedback of non-equilibrium chemical signals in biological systems, it has been suggested that colloids can be controlled using internal chemical feedback loops. By combining chemical feedback with a previous method of steering colloids with an electric field, we have devised a new technique for steering individual colloidal particles using externally applied chemical gradients. Our simulations use model predictive control to steer particles on various trajectories using a chemical solute gradient. We also present a new method of "stakeholder control", in which a chemically active particle is used to move other nonreactive particles into desired locations. This new method solves some of the problems of scale-up that arise from chemical gradient control.   

Surface charge heterogeneity directed particle migration and assembly

Detecting and delivering colloidal particles to a spot with surface charge heterogeneity is important in many applications, such as coating quality control, surface modification, and cargo delivery. Since the location of the spot with surface charge heterogeneity is not known, it is challenging to map variations in the surface charge density in a large area simultaneously in an inexpensive way. Here we demonstrate a system to automatically assemble particles in areas with surface charge heterogeneity thus detecting and delivering particles to the target location at the same time. We utilize colloidal particles that self-emit solutes that interact with the non-uniformly charged surface to drive them diffusiophoretically. The physical mechanism for the particle motion is unveiled and a theoretical framework is laid out to predict the particle migration behavior with different types of surface charge density patterns. We have demonstrated experimentally that initially uniformly distributed colloidal particles on surfaces assemble and form patterns that conform to the surface charge density pattern. This unique system opens new avenues to sense the surface heterogeneity and harness it for targeted cargo delivery.

    

Divine Proportions and the Fractal Dimension of DLCA Aggregates

Aggregation is a non-equilibrium process of fundamental importance for all dispersed particulate systems. Here I present a restricted hierarchical model of diffusion limited cluster-cluster aggregation (DLCA). The model yields an analytical calculation of the fractal dimensions and self-preserving cluster shapes in two and three spatial dimensions in excellent agreement with those found in nature and simulations. Remarkably, the shape is described by the Fibonacci series and the divine proportion in two dimensions and d-dimensional generalization of the Fibonacci series and the divine proportion in three and higher dimensions.

    

Real-time micromanipulation through 3D stress-free flows

Modern micromanipulation techniques, such as optical tweezers, have been established as essential approaches to examining many crucial biophysical phenomena. These approaches, however, often stem from the concept of trapping particles through certain stress fields, which restricts their applications to stress-sensitive phenomena. By employing polyhedral symmetries in a multi-channel microfluidic design, we show that the tasks of displacing and trapping a particle can be indeed separated into two distinct sets of flow operations, characterized by their unique groups of symmetries. Combining all "displacing" modes, targeted particles are entrained by uniform flows in all possible directions, giving rise to stress-free micromanipulation in 3D, without any need for traps. To examine the capacity of such trap-free micromanipulation, we engineered complex microscale paths of manipulated particles by programming the flow within each channel, as controlled in real-time via pressure regulators. In contrast to those trap-based manipulations, all particles in the stress-free flows follow the desired path, without the need to supervise any traps based on the particles' locations. Additionally, we optimized the actual paths followed by the manipulated particles by incorporating curvature-dependent flow speed, using, for instance, a conformal-mapping strategy.   

The lifetime of swimming droplets

Active droplets serve as a model system for studying chemotaxis, solute-mediate interactions, and locomotion of micro-organisms, e.g., algae and bacteria. They are intrinsically isotropic and self-propel by spontaneously breaking symmetry [1], by contrast to Janus particles that swim as a result of built-in design asymmetries. Here we study the motion of swimming droplets made of diethyl phthalate oil (DEP) in varying concentrations of aqueous sodium dodecyl sulphate (SDS) solution above the critical micellar concentration [2]. During the course of a lifetime of a droplet, three motility types are identified - self-avoidance, oscillations, and active Brownian particle (ABP) behavior. Analyzing experimental trajectories allows us to map out the probability distribution of the said motility types as a function of both the SDS concentration and the shrinking droplet size. Comparing the experimental data with distinct theoretical models describing ABP, non-Markovian [3], or true self-avoiding random walks (TSAW) [4] opens the path to understanding the microscopic origin of the locomotion of the droplets. 

    

Protocols for Assembly of Passive Solutes in Active Solvent

We investigate the assembly of passive solutes in a bath of self-propelled active Brownian particles. The nonequilibrium nature of the active bath leads to long-range non-monotonic interactions between the passive solutes that depend on the solute shape and solvent activity. Using Brownian dynamics simulations we construct a data-driven Gaussian field theory, based on tools from equilibrium liquid state theories, that quantitatively describes the interactions between the solutes. With this theoretical approach, we construct protocols that lead to the robust assembly of solutes through the design of the solute shape and the dynamic tuning of the solvent activity, hence enabling novel pathways for the directed self-assembly of particles.   

Synchronous oscillatory electro-inertial focusing of synthetic and biological particles

The manipulation of microparticles and living cells using active and passive techniques in microfluidic devices is useful for a variety of applications including filtration of contaminants, flow cytometry, and drug delivery. Here, we present experimental results on the focusing of synthetic and biological particles using synchronous oscillatory electro-inertial flow in a microfluidic device. By varying the phase difference between the oscillatory flow and AC electric field, we show that the focusing efficiency and the focusing positions of microparticles can be controlled. Specifically, we are able to focus 1-micron polystyrene particles or bacteria within a microfluidic channel of only 2 cm in length. Furthermore, this technique is suitable for preserving the viability of biological particles, including epithelial cells, due to the low shear stress experienced in the device. These results show that synchronous oscillatory electro-inertial microfluidics offers novel capabilities for manipulating microscale biological particles based on their physical properties, such as size and zeta potential.   

Steering colloids using solute gradients and flow

Controlling the motion of colloids is important in a wide range of applications, from cell sorting to filtration and water purification. Recent studies have utilized the diffusiophoretic migration of colloids due to solute gradients to manipulate and guide them. However, solute gradients next to solid surfaces also drive diffusioosmotic flows, which are often ignored. Here, combining experimental observations and numerical simulations of flow through a T-junction, we show that the coupling between diffusiophoretic migration of colloids and diffusioosmotic flows next to the solid surfaces can lead to unexpected focusing patterns.   

Ultra-fast ballistic supercavitating plasmonic nanoparticles driven by optical pulling forces

Here, we report, ballistic plasmonic Au nanoparticles that swim at unprecedented speeds, ~245,000 μm s−1 in a water medium, against the propagation direction of a loosely focused Gaussian beam. The beam waist being ~2 orders larger than the radius of the nanoparticles, negates the effect of the optical gradient forces, and can be approximated as a linearly polarized plane wave. Additionally, in water medium, the optical forces experienced by the Au nanoparticles are ~2 orders of magnitude smaller than what Strokes' Law permits to reach such high speeds. The Au nanoparticle can generate a nanoscale bubble, when excited especially at the plasmon resonance peak and can encapsulate the Au nanoparticle (supercavitation), creating a virtually frictionless environment (water vapor), which can support such speeds. Certain nanoparticle-nanobubble configurations can create unique optical conditions, which leads to the phenomenon known as 'optical pulling' to drive the Au nanoparticles against the propagation direction of the beam. The nanoparticle inside the bubble, can reach temperature (~850 K) much higher than the critical temperature of water, and as the nanoparticle reach the nanobubble surface it can instantly vaporize the water thereby extending the bubble boundary. We believe, this light driven ultra-fast movement of the nanoparticles may benefit a wide of range of nano and bio-applications while providing new insights to the field of optical manipulation.   

Direct observation of nanoplastics in seawater via shrinking surface bubble deposition

Plastics production surpasses all other synthetic materials globally, with 0.4-4 million tons of them entering the oceans every year, posing serious environmental challenges. After disposal, these plastics can be fragmented by UV irradiation and mechanical means. While microplastics are detected in abundance, the morphological information on nanoplastics in our environment has never been confirmed, although they are believed to be significantly more toxic to living organisms compared to their microscopic counterparts. Here we report the direct observation of nanoplastics in ocean water around the world leveraging a unique shrinking surface bubble deposition (SSBD) technique. We identified nanoplastics with a variety of compositions, including polycaprolactam (Nylon), polystyrene (PS) and polyethylene terephthalate (PET), which are common materials for daily consumables (e.g., textiles, coffee cup lids and water bottles). They also possess diverse morphologies, such as nanoflakes, nanofibers and ball-stick nanostructures. The identification of a variety of nanoplastic particles from the water samples collected from several locations on the coastlines of the U.S., China, South Korea and deep (>300 m) in the Gulf of Mexico suggests their widespread distribution in ocean. These results should promote investigations into the environmental impact of plastic disposal.

    

Inertial torque on a squirmer

A small spheroid settling in a quiescent fluid experiences an inertial torque that aligns it so that it settles with its broad side first. Here we show that an active particle experiences such a torque too, as it settles in a fluid at rest. For a spherical squirmer, the torque is  $T^prime = -(9/8) m_f (v_s⁽⁰⁾ x v_g⁽⁰⁾) where v_s⁽⁰⁾ is the  swimming velocity, v_g⁽⁰⁾ is the settling velocity in the Stokes approximation, and m_f is the equivalent  fluid mass. This torque aligns the swimming direction against gravity: swimming up is stable, swimming down is unstable. This talk is based on the preprint F. Candelier, J. Qiu, L. Zhao, G. Voth & B. Mehlig, Inertial torque on a squirmer, arXiv:2209.03129.