Field-Directed Motion, Cargo Capture, and Closed-Loop Controlled Navigation of Microellipsoids.

Microrobots have the potential for diverse applications, including targeted drug delivery and minimally invasive surgery. Despite advancements in microrobot design and actuation strategies, achieving precise control over their motion remains challenging due to the dominance of viscous drag, system disturbances, physicochemical heterogeneities, and stochastic Brownian forces. Here, a precise control over the interfacial motion of model microellipsoids is demonstrated using time-varying rotating magnetic fields. The impacts of microellipsoid aspect ratio, field characteristics, and magnetic properties of the medium and the particle on the motion are investigated. The role of mobile micro-vortices generated is highlighted by rotating microellipsoids in capturing, transporting, and releasing cargo objects. Furthermore, an approach is presented for controlled navigation through mazes based on real-time particle and obstacle sensing, path planning, and magnetic field actuation without human intervention. The study introduces a mechanism of directing motion of microparticles using rotating magnetic fields, and a control scheme for precise navigation and delivery of micron-sized cargo using simple microellipsoids as microbots.

Surface Science View of Perfluoroalkyl Acids (PFAAs) in the Environment.

Per- and polyfluoroalkyl substances (PFAS) constitute a notorious category of anthropogenic contaminants, detected across various environmental domains. Among these PFAS, perfluoroalkyl acids (PFAAs) stand out as a focal point in discussions due to their historical industrial utilization and environmental prominence. Their extensive industrial adoption is a direct consequence of their remarkable stability and outstanding amphiphilic properties. However, these very traits that have made PFAAs industrially desirable also render them environmentally catastrophic, leading to adverse consequences for ecosystems. The amphiphilic nature of PFAAs has made them highly unique in the landscape of anthropogenic contaminants and, thereby, difficult to study. We believe that well-established principles from surface science can connect the amphiphilic nature of PFAAs to their accumulation and transport in the environment. Specifically, we discuss the role of interfacial science in describing the stability, interfacial uptake (air-liquid and solid-liquid), and wetting capability of PFAAs. Surface science principles can provide new insights into the environmental fate of PFAAs, as well as provide context on their deleterious effects on both the environment and human health.

Effects of charge asymmetry on the liquid-liquid phase separation of polyampholytes and their condensate properties.

Liquid-liquid phase separation (LLPS) is the mechanism underlying the formation of bio-molecular condensates which are important compartments regulating intra- and extra-cellular functions. Electrostatic interactions are some of the important driving forces of the LLPS behaviors of biomolecules. However, the understanding of the electrostatic interactions is still limited, especially in the mixtures of biomolecules with different charge patterns. Here, we focus on the electrostatic interactions in mixtures of charge-asymmetric and charge-symmetric polyampholytes and their roles in the phase separation behaviors. We build charge-asymmetric and charge-symmetric model proteins consisting of both glutamic acid (E, negatively charged) and lysine (K, positively charged), i.e. polyampholytes of E35K15 (charge asymmetric) and E25K25 (charge symmetric). Pure E25K25 can undergo LLPS. To investigate the effects of charge-asymmetric polyampholytes on the mixtures of E25K25/E35K15, we perform coarse-grained simulations to determine their phase separation. The charge-asymmetric polyampholyte E35K15 is resistant to the LLPS of the mixtures of E25K25/E35K15. The condensate density decreases with the molar fraction of E35K15 increasing to 0.4, and no LLPS occurs at the molar fraction of 0.5 and above. This can be attributed to the electrostatic repulsion between the negatively charged E35K15 polymers. We further investigate the effects of charge asymmetry on the conformations and properties of the condensates. The E35K15 polymers in the condensates exhibit a more collapsed state as the molar fraction of E35K15 increases. However, the conformation of E25K25 polymers changes slightly across different condensates. The surface tensions of condensates decline with the increase of the molar fraction of E35K15 polymers, while the diffusivity of polymers in the condensed phases is enhanced. This work elucidates the role of charge-asymmetric polyampholytes in determining the LLPS behaviours of binary mixtures of charge-symmetric and charge-asymmetric proteins as well as the properties of condensed phases.

Selective Vapor Condensation for the Synthesis and Assembly of Spherical Colloids with a Precise Rough Patch.

Patchy particles occupy an increasingly important space in soft matter research due to their ability to assemble into intricate phases and states. Being able to fine-tune the interactions among these particles is essential to understanding the principles governing the self-assembly processes. However, current fabrication techniques often yield patches that deviate chemically and physically from the native particles, impeding the identification of the driving forces behind self-assembly. To overcome this challenge, we propose a new approach to synthesizing spherical colloids with a well-defined rough patch on their surface. By treating polystyrene microspheres with vapors of a good solvent, here an acetone-water mixture, we achieve selective polymer corrugation on the particle surface resulting in a chemically similar yet rough surface patch. The key step is the selective condensation of the acetone-water vapors on the apex of the polystyrene microparticles immobilized on a substrate, which leads to rough patch formation. We leverage the ability to tune the vapor-liquid equilibrium of the volatile acetone-water mixture to precisely control the polymer corrugation on the particle surface. We demonstrate the dependence of patch formation on particle and substrate wettability, with the condensation occurring on the particle apex only when it is more wettable than the substrate, which is consistent with Volmer's classical nucleation theory. By combining experiments and molecular dynamics simulations, we identify the role of the rough patch in the depletion interaction-driven self-assembly of the microspheres, which is crucial for designing programmable supracolloidal structures.

Magnetic Control of Nonmagnetic Living Organisms.

Living organisms inspire the design of microrobots, but their functionality is unmatched. Next-generation microrobots aim to leverage the sensing and communication abilities of organisms through magnetic hybridization, attaching magnetic particles to them for external control. However, the protocols used for magnetic hybridization are morphology specific and are not generalizable. We propose an alternative approach that leverages the principles of negative magnetostatics and magnetophoresis to control nonmagnetic organisms with external magnetic fields. To do this, we disperse model organisms in dispersions of Fe3O4 nanoparticles and expose them to either uniform or gradient magnetic fields. In uniform magnetic fields, living organisms align with the field due to external torque, while gradient magnetic fields generate a negative magnetophoretic force, pushing objects away from external magnets. The magnetic fields enable controlling the position and orientation of Caenorhabditis elegans larvae and flagellated bacteria through directional interactions and magnitude. This control is diminished in live spermatozoa and adult C. elegans due to stronger internal biological activity, i.e., force/torque. Our study presents a method for spatiotemporal organization of living organisms without requiring magnetic hybridization, opening the way for the development of controllable living microbiorobots.

Interfacial Activity and Surface pKa of Perfluoroalkyl Carboxylic Acids (PFCAs).

Perfluoroalkyl carboxylic acids (PFCAs) are widely used synthetic chemicals that are known for their exceptional stability and interfacial activity. Despite their industrial and environmental significance, discrepancies exist in the reported pKa values for PFCAs, often spanning three to four units. These disparities stem from an incomplete understanding of how pH influences the ionized state of PFCA molecules in the bulk solution and at the air-water interface. Using pH titration and surface tension measurements, we show that the pKa values of the PFCAs adsorbed at the air-water interface differ from the bulk. Below the equivalence point, the undissociated and dissociated forms of the PFCAs exist in equilibrium, driving to the spontaneous adsorption and reduced air-water surface tension. Conversely, above the equivalence point, the complete ionization of the headgroup into the carboxylate form renders PFCAs highly hydrophilic, resulting in reduced interfacial activity of the molecules. The distinction in the chemical environments at the interface and bulk results in differences in the pKa of PFCA molecules in the bulk phase and at the air-water interface. We explore the effects of the fluoroalkyl tail length of PFCAs on their surface pKa and interfacial activity across a broad pH range. We further demonstrate the influence of pH-dependent ionized state of PFCAs on their foamability and the rate of microdroplet evaporation, understanding of which is crucial for optimizing their industrial applications and developing effective strategies for their environmental remediation. This study underscores the potential significance of pH in directing the interfacial activity of PFCAs and prompts the inclusion of pH as a key determinant in the predictions of their fate and potential risks in the environment.

Quantification of solution-free red blood cell staining by sorption kinetics of Romanowsky stains to agarose gels.

The imaging and quantification of stained red blood cells (RBCs) are important for identifying RBCs in hematology and for diagnosing diseased RBCs or parasites in cytopathology. Romanowsky staining has been used traditionally to produce hues in blood cells using a mixture of anionic eosin Y and cationic methylene blue and azure B. While Romanowsky stains have been widely used in cytopathology, end-users have experienced problems with varying results in staining due to the premature precipitation or evaporation of methanol, leading to the inherent inconsistency of solution-based Romanowsky staining. Herein, we demonstrate that the staining and destaining of blood smears are controllable by the contact time of agarose gel stamps. While the extent of staining and destaining is discernable by the hue values of stamped red blood cells in micrographs, the quantification of adsorbed and desorbed Romanowsky dye molecules (in particular, eosin Y, methylene blue and azure B) from and to the agarose gel stamps needs a model that can explain the sorption process. We found predictable sorption of the Romanowsky dye molecules from the pseudo-second-order kinetic model for adsorption and the one phase decay model for desorption. Thus, the method of agarose gel stamping demonstrated here could be an alternative to solution-based Romanowsky staining with the predictable quantity of sorption and timing of contact.

Uptake and release of perfluoroalkyl carboxylic acids (PFCAs) from macro and microplastics.

Microplastics and per- and polyfluoroalkyl substances (PFAS) are two of the most notable emerging contaminants reported in the environment. Micron and nanoscale plastics possess a high surface area-to-volume ratio, which could increase their potential to adsorb pollutants such as PFAS. One of the most concerning sub-classes of PFAS are the perfluoroalkyl carboxylic acids (PFCAs). PFCAs are often studied in the same context as other environmental contaminants, but their amphiphilic properties are often overlooked in determining their fate in the environment. This lack of consideration has resulted in a diminished understanding of the environmental mobility of PFCAs, as well as their interactions with environmental media. Here, we investigate the interaction of PFCAs with polyethylene microplastics, and identify the role of environmental weathering in modifying the nature of interactions. Through a series of adsorption-desorption experiments, we delineate the role of the fluoroalkyl tail in the binding of PFCAs to microplastics. As the number of carbon atoms in the fluoroalkyl chain increases, there is a corresponding increase in the adsorption of PFCAs onto microplastics. This relationship can become modified by environmental weathering, where the PFCAs are released from the macro and microplastic surface after exposure to simulated sunlight. This study identifies the fundamental relationship between PFCAs and plastic pollutants, where they can mutually impact their thermodynamic and transport properties.

Correction: Magnetic field enabled in situ control over the structure and dynamics of colloids interacting via SALR potentials.

Correction for 'Magnetic field enabled in situ control over the structure and dynamics of colloids interacting via SALR potentials' by Hashir M. Gauri et al., Soft Matter, 2023, 19, 4439-4448, https://doi.org/10.1039/D3SM00354J.

Magnetic field enabled in situ control over the structure and dynamics of colloids interacting via SALR potentials.

Colloidal suspensions are an ideal model for studying crystallization, nucleation, and glass transition mechanisms, due to the precise control of interparticle interactions by changing the shape, charge, or volume fraction of particles. However, these tuning parameters offer insufficient active control over interparticle interactions and reconfigurability of assembled structures. Dynamic control over the interparticle interactions can be obtained through the application of external magnetic fields that are contactless and chemically inert. In this work, we demonstrate the dual nature of magnetic nanoparticle dispersions to program interactions between suspended nonmagnetic microspheres using an external magnetic field. The nanoparticle dispersion simultaneously behaves as a continuous magnetic medium at the microscale and a discrete medium composed of individual particles at the nanoscale. This enables control over a depletion attractive potential and the introduction of a magnetic repulsive potential, allowing a reversible transition of colloidal structures within a rich phase diagram by applying an external magnetic field. Active control over competing interactions allows us to create a model system encompassing a range of states, from large fractal clusters to low-density Wigner glass states. Monitoring the dynamics of colloidal particles reveals dynamic heterogeneity and a marked slowdown associated with approaching the Wigner glass state.

Synergistic Role of Temperature and Salinity in Aggregation of Nonionic Surfactant-Coated Silica Nanoparticles.

The adsorption of nonionic surfactants onto hydrophilic nanoparticles (NPs) is anticipated to increase their stability in aqueous medium. While nonionic surfactants show salinity- and temperature-dependent bulk phase behavior in water, the effects of these two solvent parameters on surfactant adsorption and self-assembly onto NPs are poorly understood. In this study, we combine adsorption isotherms, dispersion transmittance, and small-angle neutron scattering (SANS) to investigate the effects of salinity and temperature on the adsorption of pentaethylene glycol monododecyl ether (C12E5) surfactant on silica NPs. We find an increase in the amount of surfactant adsorbed onto the NPs with increasing temperature and salinity. Based on SANS measurements and corresponding analysis using computational reverse-engineering analysis of scattering experiments (CREASE), we show that the increase in salinity and temperature results in the aggregation of silica NPs. We further demonstrate the non-monotonic changes in viscosity for the C12E5-silica NP mixture with increasing temperature and salinity and correlate the observations to the aggregated state of NPs. The study provides a fundamental understanding of the configuration and phase transition of the surfactant-coated NPs and presents a strategy to manipulate the viscosity of such dispersion using temperature as a stimulus.

Computational Reverse-Engineering Analysis for Scattering Experiments for Form Factor and Structure Factor Determination ("P(q) and S(q) CREASE").

In this paper, we present an open-source machine learning (ML)-accelerated computational method to analyze small-angle scattering profiles [I(q) vs q] from concentrated macromolecular solutions to simultaneously obtain the form factor P(q) (e.g., dimensions of a micelle) and the structure factor S(q) (e.g., spatial arrangement of the micelles) without relying on analytical models. This method builds on our recent work on Computational Reverse-Engineering Analysis for Scattering Experiments (CREASE) that has either been applied to obtain P(q) from dilute macromolecular solutions (where S(q) ∼1) or to obtain S(q) from concentrated particle solutions when P(q) is known (e.g., sphere form factor). This paper's newly developed CREASE that calculates P(q) and S(q), termed as "P(q) and S(q) CREASE", is validated by taking as input I(q) vs q from in silico structures of known polydisperse core(A)-shell(B) micelles in solutions at varying concentrations and micelle-micelle aggregation. We demonstrate how "P(q) and S(q) CREASE" performs if given two or three of the relevant scattering profiles-I total(q), I A(q), and I B(q)-as inputs; this demonstration is meant to guide experimentalists who may choose to do small-angle X-ray scattering (for total scattering from the micelles) and/or small-angle neutron scattering with appropriate contrast matching to get scattering solely from one or the other component (A or B). After validation of "P(q) and S(q) CREASE" on in silico structures, we present our results analyzing small-angle neutron scattering profiles from a solution of core-shell type surfactant-coated nanoparticles with varying extents of aggregation.

Active Colloids as Models, Materials, and Machines.

Active colloids use energy input at the particle level to propel persistent motion and direct dynamic assemblies. We consider three types of colloids animated by chemical reactions, time-varying magnetic fields, and electric currents. For each type, we review the basic propulsion mechanisms at the particle level and discuss their consequences for collective behaviors in particle ensembles. These microscopic systems provide useful experimental models of nonequilibrium many-body physics in which dissipative currents break time-reversal symmetry. Freed from the constraints of thermodynamic equilibrium, active colloids assemble to form materials that move, reconfigure, heal, and adapt. Colloidal machines based on engineered particles and their assemblies provide a basis for mobile robots with increasing levels of autonomy. This review provides a conceptual framework for understanding and applying active colloids to create material systems that mimic the functions of living matter. We highlight opportunities for chemical engineers to contribute to this growing field.

Effects of Weathering on Microplastic Dispersibility and Pollutant Uptake Capacity.

Microplastics are ubiquitous in the environment, leading to a new form of plastic pollution crisis, which has reached an alarming level worldwide. Micron and nanoscale plastics may get integrated into ecological cycles with detrimental effects on various ecosystems. Commodity plastics are widely considered to be chemically inert, and alterations in their surface properties due to environmental weathering are often overlooked. This lack of knowledge on the dynamic changes in the surface chemistry and properties of (micro)plastics has impeded their life-cycle analysis and prediction of their fate in the environment. Through simulated weathering experiments, we delineate the role of sunlight in modifying the physicochemical properties of microplastics. Within 10 days of accelerated weathering, microplastics become dramatically more dispersible in the water column and can more than double the surface uptake of common chemical pollutants, such as malachite green and lead ions. The study provides the basis for identifying the elusive link between the surface properties of microplastics and their fate in the environment.

Topologically Precise and Discrete Bottlebrush Polymers: Synthesis, Characterization, and Structure-Property Relationships.

As the complexity of polymer structure grows, so do the challenges for developing an accurate understanding of their structure-property relationships. Here, the synthesis of bottlebrush polymers with topologically precise and fully discrete structures is reported. A key feature of the strategy is the synthesis of discrete macromonomer libraries for their polymerization into topologically precise bottlebrushes that can be separated into discrete bottlebrushes (D = 1.0). As the system becomes more discrete, packing efficiency increases, distinct three-phase Langmuir-Blodgett isotherms are observed, and its glass transition temperature becomes responsive to side-chain sequence. Overall, this work presents a versatile strategy to access a range of precision bottlebrush polymers and unravels the impact of side-chain topology on their macroscopic properties. Precise control over side chains opens a pathway for tailoring polymer properties without changing their chemical makeup.

Field-Induced Assembly and Propulsion of Colloids.

Electric and magnetic fields have enabled both technological applications and fundamental discoveries in the areas of bottom-up material synthesis, dynamic phase transitions, and biophysics of living matter. Electric and magnetic fields are versatile external sources of energy that power the assembly and self-propulsion of colloidal particles. In this Invited Feature Article, we classify the mechanisms by which external fields impact the structure and dynamics in colloidal dispersions and augment their nonequilibrium behavior. The paper is purposely intended to highlight the similarities between electrically and magnetically actuated phenomena, providing a brief treatment of the origin of the two fields to understand the intrinsic analogies and differences. We survey the progress made in the static and dynamic assembly of colloids and the self-propulsion of active particles. Recent reports of assembly-driven propulsion and propulsion-driven assembly have blurred the conceptual boundaries and suggest an evolution in the research of nonequilibrium colloidal materials. We highlight the emergence of colloids powered by external fields as model systems to understand living matter and provide a perspective on future challenges in the area of field-induced colloidal phenomena.

Controlled adhesion, membrane pinning and vesicle transport by Janus particles.

The interactions between biomembranes and particles are key to many applications, but the lack of controllable model systems to study them limits the progress in their research. Here, we describe how Janus polystyrene microparticles, half coated with iron, can be partially engulfed by artificial cells, namely giant vesicles, with the goals to control and investigate their adhesion and degree of encapsulation. The interaction between the Janus particles and these model cell membrane systems is mediated by electrostatic charge, offering a further mode of modulation in addition to the iron patches. The ferromagnetic particle coatings also enable manipulation and transport of the vesicles by magnetic fields.

Adsorption and Catalytic Activity of Gold Nanoparticles in Mesoporous Silica: Effect of Pore Size and Dispersion Salinity.

The assembled state of nanoparticles (NPs) within porous matrices plays a governing role in directing their biological, electronic, and catalytic properties. However, the effects of the spatial confinement and environmental factors, such as salinity, on the NP assemblies within the pores are poorly understood. In this study, we use adsorption isotherms, spectrophotometry, and small-angle neutron scattering to develop a better understanding of the effect of spatial confinement on the assembled state and catalytic performance of gold (Au) NPs in propylamine-functionalized SBA-15 and MCM-41 mesoporous silica materials (mSiO2). We carry out a detailed investigation of the effect of pore diameter and ionic strength on the packing and spatial distribution of AuNPs within mSiO2 to get a comprehensive insight into the structure, functioning, and activity of these NPs. We demonstrate the ability of the adsorbed AuNPs to withstand aggregation under high salinity conditions. We attribute the observed preservation of the adsorbed state of AuNPs to the strong electrostatic attraction between oppositely charged pore walls and AuNPs. The preservation of the structure allows the AuNPs to retain their catalytic activity for a model reaction in high salinity aqueous solution, here, the reduction of p-nitrophenol to p-aminophenol, which otherwise is significantly diminished due to bulk aggregation of the AuNPs. This fundamental study demonstrates the critical role of confinement and dispersion salinity on the adsorption and catalytic performance of NPs.

Dual nature of magnetic nanoparticle dispersions enables control over short-range attraction and long-range repulsion interactions.

Competition between attractive and repulsive interactions drives the formation of complex phases in colloidal suspensions. A major experimental challenge lies in decoupling independent roles of attractive and repulsive forces in governing the equilibrium morphology and long-range spatial distribution of assemblies. Here, we uncover the 'dual nature' of magnetic nanoparticle dispersions, particulate and continuous, enabling control of the short-range attraction and long-range repulsion (SALR) between suspended microparticles. We show that non-magnetic microparticles suspended in an aqueous magnetic nanoparticle dispersion simultaneously experience a short-range depletion attraction due to the particulate nature of the fluid in competition with an in situ tunable long-range magnetic dipolar repulsion attributed to the continuous nature of the fluid. The study presents an experimental platform for achieving in situ control over SALR between colloids leading to the formation of reconfigurable structures of unusual morphologies, which are not obtained using external fields or depletion interactions alone.

Microplastics through the Lens of Colloid Science.

Microplastics are sub-millimeter-sized fragments of plastics and a relatively new class of pollutant increasingly found in the environment. Due to their size and surface area to volume ratio, the physicochemical characteristics of microplastics can diverge from those of their macroscopic counterparts. This is partly why it is challenging to understand their origin, analyze their behavior, and predict their fates in the environment compared to large pollutants. We believe that adopting a view of microplastics as a colloid provides a holistic framework that connects their physical properties and surface chemistries with observations of their dynamics in the environment. In particular, we discuss the role of fundamental principles of colloid science in interpreting phenomena of wetting, adsorption, aggregation, and transport of microplastics. Colloid and interface science can provide the tools to couple or decouple the physicochemical behaviors of microplastics, which may aid in understanding the environmental challenge both from a fundamental perspective and with respect to practical remediation methods.