Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A14: Colloidial Systems |
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Sponsoring Units: DSOFT Chair: Ryan McGorty, University of San Diego Room: Room 206 |
Monday, March 6, 2023 8:00AM - 8:12AM |
A14.00001: A microstructural analysis of shear thickening William C Buchholtz, H. A. Vinutha, Jeffrey S Urbach, Daniel L Blair, Emanuela Del Gado We investigate the possible microstructural underpinnings of shear thickening of dense suspensions with particular focus on simple particle structures. As a model, we consider a suspension of soft frictional disks (without lubrication or stress activated friction) at densities below shear jamming that exhibits contact driven thickening at low shear rate and overlap driven thinning at high rate. We find that the thickening flow exhibits several instances of distinct microstructure when compared with the thinning. In particular, the pair correlation function has sharp peaks at distances corresponding to particle chains, the heights of which are constant with rate, while during thinning these peaks decrease and broaden. From a statistical analysis of a range of microstrucrual patterns, we also find that straight particle chains and equilateral triangles tend to increase with rate during thickening but decreases during thinning. We try to link the presence of these structures and their capacity to percolate and transmit stresses to the toggling of the system between high and low shear stress states that occur during thickening. |
Monday, March 6, 2023 8:12AM - 8:24AM |
A14.00002: Using Magnetic Beads to Poke Dense Colloidal Samples Jessica Eyeson, Piotr Habdas, Eric R Weeks We are studying a dense suspension of colloids near the colloidal glass transition using magnetic beads as local probes. We experimentally study the displacement of magnetic beads under different magnetic force magnitudes such that the magnetic bead is displaced and returns to its equilibrium position. We vary both the duration and strength of the magnetic forces. We observe that for small forces and/or short durations, the magnetic bead returns to its equilibrium position. However, we find a threshold force at which the magnetic bead plastically deforms the sample and thus does not return to the equilibrium position. We examine how this threshold force increases at larger volume fractions of the colloidal sample. |
Monday, March 6, 2023 8:24AM - 8:36AM |
A14.00003: Oscillatory Perturbation of Dense Colloidal Suspensions Piotr Habdas, Rachel E Courtland, Eric R Weeks Using microscopic magnetic particles, we locally perturb dense colloidal suspensions by moving an external magnet in a rotational motion. We use confocal microscopy to track the position of the magnetic probe particle and surrounding colloidal particles. We use the known applied external force on the probe particle and the probe’s measured oscillatory amplitude to calculate the storage and loss moduli of colloidal suspensions with various volume fractions. We find that these quantities agree well with prior results obtained from traditional rheology. To characterize the response of the colloidal particles to the external perturbation, we obtain the oscillatory amplitude of the colloidal particles as a function of distance r from the probe particle and find that the amplitudes decay as 1/r. Finally, we investigate the possibility of determining the speed of sound in colloidal suspensions by measuring the phase lag of the colloidal particles’ oscillations with respect to the phase of the probe particle. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A14.00004: Tracking the Kinetics of Colloidomer Sedimentation with Holographic Video Microscopy Jatin Abacousnac, Angus McMullen, Jacqueline Sustiel, Jasna Brujic, David G Grier Colloidomers consist of micrometer-scale emulsion droplets bound by programmed surface interactions into flexible chains. The fluid-fluid boundary condition at a colloidomer's surface differs from the non-slip boundary conditions presented by solid colloidal spheres, and influences how a colloidomer sediments through viscous fluids. We use holographic video microscopy to track the three-dimensional motions of colloidomers as they relax from initial extended configurations created through holographic optical trapping. These measurements reveal the role of hydrodynamic coupling in the kinetics of colloidomer settling under non-equilibrium conditions. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A14.00005: Temperature and concentration dependence of the microgel Counter-ion Cloud configuration with increasing particle stiffness studied with Small-Angle Neutron Scattering (SANS) Boyang Zhou, Urs Gasser, Alberto Fernandez-Nieves Microgels are stimuli-sensitive colloids formed by cross-linked polymer networks and are a good model system for soft colloids. Unlike hard colloids, their interaction and phase behavior is not well understood, especially at high concentrations. Although pNIPAM is an uncharged polymer, pNIPAM microgels are peripherally charged due to charges remaining from particle synthesis. The corresponding counterion cloud has been found to play a crucial role in the observed spontaneous deswelling behavior at high concentrations; particles deswell before reaching random closing packing density Φrcp. Our recent SANS measurements have confirmed that the counterion cloud indeed locates at the particle surface, which corroborates our model explaining the observed spontaneous deswelling at high concentrations. Here, we have studied microgels with various softness, set by the crosslink density of 0.05%, 2.5% and 5%, and suspensions with concentrations from ζ=0.02 to ζ=1 were measured. We replace the counterions with either Na+ or NH4+ via dialysis, which changes the microgel form factor and allows obtaining detailed information on the configuration of the counter-ions and charged groups. All measurements were done at 18°C, 30°C, and 45°C, which are below, around and above the lowest critical solution temperature 32°C. We find that the deswelling ratio at high concentrations varies for these samples and the counterion cloud persists at the particle surface at all conditions. Our results allow exploring the temperature-, concentration-, and particle-stiffness dependence of the deswelling behavior, which leads to a grasp of the interplay among the osmotic pressures due to polymer-solvent mixing, chain elasticity and ionic contribution, which control the microgel volume phase transition. We find that the osmotic pressure due to the counterions has to be included for a comprehensive understanding of microgels at high concentrations and for formulating new models for their phase behavior that take spontaneous deswelling at high concentrations into account. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A14.00006: Experimental antiferromagnetic tetratic phase in confined quasi-2d hard-sphere system with depletants Michio Tanaka, Analisa Hill In this work, we experimentally demonstrate the self-assembly of a topologically ordered phase in a simple buckled system of colloidal spheres, i.e., without introduction of surface patterning or aspherical particles. Specifically, we realize an antiferromagnetic tetratic phase of hard-sphere, micron size polystyrene particles; the interparticle interactions of this system can be tuned using temperature-sensitive rod-like depletants made from the surfactant C12E6. We examine the topological order of the tetratic phase arising from the antiferromagnetic character of the system. We study the dislocations, in particular the presence/absence of single lattice dislocations which have been argued to be forbidden in the presence of strong antiferromagnetic interactions [D. Abutbul and D. Podolsky, Topological Order in an Antiferromagnetic Tetratic Phase, Phys. Rev. Lett. 128, 255501 (2022)]. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A14.00007: Hexagonal vortices during grain coarsening in colloidal crystals Helen K Chaffee, Sharon J Gerbode, Avani Anne, Eric Corona, Chris Couto In polycrystalline materials, a grain boundary loop arises when one crystal grain is embedded within a surrounding crystal. The enclosed grain shrinks to minimize free energy, and continuum theory predicts a linear rate of dissolution. However, both our experimental studies and brownian dynamics simulations of grain dissolution reveal a combination of two distinct behaviors: a fast process and a slow process. While the slower dissolution behavior matches with existing models for grain boundary motion, the faster process involves coordinated rotations of groups of particles, acting like mini-grains or "granules." These granule rotations can be understood by comparing the hexagonal patterns of particle displacement vorticity to the Moiré pattern obtained by overlaying the two misaligned crystal lattices. We estimate the free energy cost of grain dissolution by directly computing the free space available to each particle, and use this to interrogate the differences between granule rotation and typical grain boundary motion. Other studies have found hexagonal vorticity in magnetically-driven crystals, or during grain splitting, but here we find granule rotation is a generalized mechanism of grain boundary motion for undriven grain coarsening. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A14.00008: Dislocation reactions during granule rotation Eric Corona, Sharon J Gerbode, Chris Couto, Helen K Chaffee, Avani Anne Grain boundaries are composed of a sequence of dislocations, which diffuse and mutually interact. When one crystal grain is enclosed within another crystal, the resulting grain boundary loop shrinks as the enclosed grain dissolves over time to minimize free energy. In our colloidal crystal experiments, we observe a new mechanism for grain dissolution in which hexagonal particle clusters, or "granules", rotate to switch from one crystal orientation to another. The role of dislocations during granule rotation is not well understood, and is complicated by the presence of rapidly evolving grain boundaries. The conventional view of grain dissolution predicts that dislocations will glide until they meet and interact through dislocation reactions or dissociations. However our preliminary results indicate a more complicated behavior, in which the dislocation reactions are influenced by local variations in misorientation angle caused by granule rotation. These observations suggest that the existing framework for understanding grain boundary motion in terms of dislocations may be insufficient for modeling granule rotation. |
Monday, March 6, 2023 9:36AM - 9:48AM |
A14.00009: Polymer-grafted Gold Nano Capsules as Reversible Colorimetric Sensors Remi Dreyfus, Daeyeon Lee, Jaehyun Kim, Russell J Composto, Sean Choe, Joseph Rosenfeld, Yechan Kim Colloidal colorimetric microsensors have been shown to enable the in-situ detection of mechanical loads within materials [1,2]. Enhancing the sensitivity of these sensors to small scale loads and deformation while enabling reversibility of the sensing capability would expand their utility in various applications including biosensing and chemical sensing [3]. In this presentation, we introduce the synthesis of nano-sized multifunctional colloidal colorimetric sensors using a simple and versatile fabrication method. Multifunctional colloidal nano-sensors (NS) are prepared by emulsion-templated assembly of polystyrene grafted gold nanoparticles. We have explored a potential application of these NSs as mechanical sensors. Gold nanoparticle-based NSs are embedded in an elastomer matrix. The addition of a plasticizer imparts flexibility to the NSs. The plasmonic peak of the NSs shifts towards lower wavelengths upon application of uniaxial tension on the gold NSs/PDMS composite, indicating increased particle-to-particle distance. The composite recovers its original plasmonic peak upon the release of the strain, demonstrating reversibility. This study provides a robust and easily scalable method of producing sensors that can potentially detect the presence of chemicals or small strain present at the nanoscale in biological cells. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A14.00010: The pendant drop experiment for aggregates of adhesive granular particles Yasaman Heshmatzadeh, Jean-Christophe Ono-dit-Biot, Kari Dalnoki-Veress Classical experiments, typically performed using bulk continuous matter can be applied for granular systems to better understand their properties, and explore the analogies between granular and bulk continuous systems. While the classic pendant drop experiment can be used to measure the interfacial tension between fluids, here we perform the granular version of the pendant drop experiment. The system consists of aggregates of adhesive, monodisperse, frictionless oil droplets in an aqueous solution. Depending on the system parameters, the properties of aggregates resemble both liquid-like and solid-like systems. |
Monday, March 6, 2023 10:00AM - 10:12AM |
A14.00011: Morphology of an aggregate of adhesive oil droplets in a spinning cylinder Johnathan Hoggarth, Lisa Bhatia, Angela Moskal, Kari Dalnoki-Veress The spinning drop method is a classic technique to measure the interfacial tension of two immiscible liquids of different densities, such as a drop of oil in water. If an oil droplet is placed within a rotating container filled with water, the droplet will move to the centre of rotation. As the rotation speed increases, the oil droplet will elongate and, conversely, as the rotation slows down, the droplet will become more spherical. In this technique, the surface area of the droplet is set by a balance between the centripetal force and the interfacial tension. Here, we present an experiment studying a granular analogue of this system to probe the effective interfacial tension of an aggregate of microscopic monodisperse oil droplets in an aqueous bath. The droplets are adhesive, buoyant, and friction is negligible. In the experiments, oil droplets are deposited in a cylinder filled with an aqueous solution of surfactant and will accumulate at a barrier under the influence of buoyancy. The droplets form a cluster along the centre of rotation in response to centripetal forces. The cluster undergoes changes in shape as the rotation speed is varied. This 'spinning-aggregate' experiment allows us to investigate connections between continuum liquids and granular systems. |
Monday, March 6, 2023 10:12AM - 10:24AM Author not Attending |
A14.00012: Melting on a sphere Navneet Singh, Ajay K Sood, Rajesh Ganapathy Melting in two-dimensional flat-space is typically two-step and via the hexatic phase. How melting proceeds on a curved surface, however, is not known. Topology mandates that crystalline particle assemblies on these surfaces harbor a finite density of defects, which itself can be ordered, like the icosahedral ordering of five-coordinated disclination defects on a sphere. Thus, melting even on a sphere, the simplest closed surface, involves the loss of both crystalline and defect order. Here, by tuning interparticle interactions in-situ, we report the observation of an intermediate hexatic phase during the melting of colloidal crystals on a sphere. Remarkably, the vanishing of the shear modulus in the hexatic phase resulted in a precipitous drop of icosahedral defect order. Furthermore, unlike in flat-space, where disorder can fundamentally alter the nature of the melting process, on the sphere, we observed the signature characteristics of ideal melting. Our findings have profound implications for understanding, for instance, the self-assembly and maturation dynamics of viral capsids and also phase transitions on curved surfaces. |
Monday, March 6, 2023 10:24AM - 10:36AM |
A14.00013: pNIPAM microgel swelling behavior and suspension structure studied with small-angle neutron scattering Urs Gasser, Boyang Zhou, Alberto Fernandez-Nieves Microgels are of high interest as model systems for soft and deformable colloids, which show a richer and more complex interaction and phase behavior than hard colloids, because their deformability and compressibility provides additional degrees of freedom. This allows microgels to react to changes in their environment and to external stimuli in ways that are not available for hard colloids. Our detailed understanding of this responsiveness is the foundation to model their interaction and phase behavior and further for the development of tailored microgels for applications. We use SANS to measure the form- and structure factor of pNIPAM microgels in dilute and concentrated suspensions and find the microgels to keep a constant size up to a critical concentration and to deswell at higher packing. This happens before the random-close-packing concentration is reached. Our results are compatible with microgel deswelling triggered by the osmotic pressure set by counterions associated to charged groups in the microgel periphery, which sharply increases when the counterion clouds surrounding the microgels percolate. We find that this counterion cloud must be taken into account to model the particle-particle interaction and the phase behavior. Further, the suspension polydispersity must be considered to obtain accurate form- and structure factors. |
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