Soft Matter最新文献

Active matter systems are strongly driven to assume non-equilibrium distributions owing to their self-propulsion, e.g., flocking and clustering. Controlling the active matter systems' spatiotemporal distributions offers exciting applications such as directed assembly, programmable materials, and microfluidic actuation. However, these applications involve environments with coupled dynamics and complex tasks, making intuitive control strategies insufficient. This necessitates the development of an automatic feedback control framework, where an algorithm determines appropriate actions based on the system's current state. In this work, we control the distribution of active Brownian particles by applying model predictive control (MPC), a model-based control algorithm that predicts future states and optimizes the control inputs to drive the system along a user-defined objective. The MPC model is based on the Smoluchowski equation with a self-propulsive convective term and an actuated spatiotemporal-varying external field that aligns particles with the applied direction, similar to a magnetic field. We apply the MPC framework to control a Brownian dynamics simulation of non-interacting active particles and illustrate the controller capabilities with two objectives: splitting and juggling sub-populations, and polar order flocking control.

This work proposes an analytical model considering the effects of hydrodynamic drag and kinetic barriers induced by liquid solvation forces to predict the translational diffusivity of a nanoparticle on an adsorbing surface. Small nanoparticles physically adsorbed to a well-wetted surface can retain significant in-plane mobility through thermally activated stick-slip motion, which can result in surface diffusivities comparable to the bulk diffusivity due to free-space Brownian motion. Theoretical analysis and molecular dynamics simulations in this work show that the surface diffusivity is enhanced when (i) the Hamaker constant is smaller than a critical value prescribed by the interfacial surface energy and particle dimensions, and (ii) the nanoparticle is adsorbed at specific metastable separations of molecular dimensions away from the wall. Understanding and controlling this phenomenon can have significant implications for technical applications involving mass, charge, or energy transport by nanomaterials dispersed in liquids under micro/nanoscale confinement, such as membrane-based separation and ultrafiltration, surface electrochemistry and catalysis, and interfacial self-assembly.

The phase behavior of polyelectrolyte complexes and coacervates (PECs) at low salt concentrations has been well characterized, but their behavior at concentrations well above the binodal is not well understood. Here, we investigate the phase behavior of stoichiometric poly(styrene sulfonate)/poly(diallyldimethylammonium) mixtures at high salt and high polymer concentrations. Samples were prepared by direct mixing of PSS/PDADMA PECs, water, and salt (KBr). Phase separation was observed at salt concentrations approximately 1 M above the binodal. Characterization by thermogravimetric analysis, FTIR, and NMR revealed that both phases contained significant amounts of polymer, and that the polymer-rich phase was enriched in PSS, while the polymer-poor phase was enriched in PDADMA. These results suggest that high salt concentrations drive salting out of the more hydrophobic polyelectrolyte (PSS), consistent with behavior observed in weak polyelectrolyte systems. Interestingly, at the highest salt and polymer concentrations studied, the polymer-rich phase contained both PSS and PDADMA, suggesting that high salt concentrations can drive salting out of partially-neutralized complexes as well. Characterization of the behavior of PECs in the high concentration limit appears to be a fruitful avenue for deepening fundamental understanding of the molecular-scale factors driving phase separation in these systems.

Colloidal model systems are successful in rationalizing emergent phenomena like aggregation, rheology and phase behaviour of protein solutions. Colloidal theory in conjunction with isotropic interaction models is often employed to estimate the stability of such solutions. In particular, a universal criterion for the reduced second virial coefficient at the critical point is frequently invoked which is based on the behavior of short-range attractive fluids (Noro–Frenkel rule, ). However, if anisotropic models for the protein–protein interaction are considered, e.g. the Kern–Frenkel (KF) patchy particle model, the value of the criterion is shifted to lower values and explicitly depends on the number of patches. If an explicit shape anisotropy is considered, as e.g. in a coarse-grained protein model, the normalization of becomes ambiguous to some extent, as no unique exclusion volume can be defined anymore. Here, we investigate a low-resolution, coarse-grained model for the globular protein bovine serum albumin (BSA) and study effects of charge-anisotropy on the phase diagram (determined by simulations) at the isoelectric point. We present methods of assigning an “effective patchiness” to our protein model by comparing its critical properties to the KF model. We find that doubling the native charges increases the critical temperature T_c by ≈14% and that our BSA model can be compared to a 3 to 5 patch KF model. Finally, we argue that applying existing criteria from colloidal theory should be done with care, due to multiple, physically plausible ways of how to assign effective diameters to shape-anisotropic models.

Nanoparticles (NPs) that contain both organic molecules and inorganic metal or metal oxide colloids in the same NP core are “composite nanoparticles” which are of interest in many applications, particularly in biomedicine as “theranostics” for the combined delivery of colloidal diagnostic imaging agents with therapeutic drugs. The rapid precipitation technique Flash NanoPrecipitation (FNP) enables continuous and scalable production of composite nanoparticles with hydrodynamic diameters between 40–200 nanometers (nm) that contain hydrophobic superparamagnetic iron oxide primary colloids. Composite NPs co-encapsulate these primary colloids (diameters of 6 nm, 15 nm, or 29 nm), a fluorescent dye (600 Daltons), and poly(styrene) homopolymer (1800, 50 000, or 200 000 Daltons) with NPs stabilized by a poly(styrene)-block-poly(ethylene glycol) (1600 Da-b-5000 Da) block copolymer. Nanoparticle assembly in FNP occurs by diffusion limited aggregation of the hydrophobic core components followed by adsorption of the hydrophobic block of the stabilizing polymer. The hydrodynamic diameter mismatch between the collapsed organic species and the primary colloids (0.5–5 nm versus 6–29 nm) creates a diffusion-aggregation time scale mismatch between components that can lead to nonstoichiometric co-encapsulation in the final nanoparticles; some nanoparticles are composites with primary colloids co-encapsulated alongside organics while others are devoid of the primary colloids and contain only organic species. We use a magnetic capture process to separate magnetic composite nanoparticles from organic-only nanoparticles and quantify the amount of iron oxide colloids and hydrophobic fluorescent dye (as a proxy for total hydrophobic polymer content) in the magnetic and nonmagnetic fractions of each formulation. Analysis of the microstructure in over 1100 individual nanoparticles by TEM imaging and composition measurements identifies the conditions that produce nonstoichiometric composite NP populations without co-encapsulated magnetic iron oxide colloids. Stoichiometric magnetically responsive composite NPs are produced when the ratio of characteristic diffusion-aggregation time scales between the inorganic primary colloid and the organic core component is less than 30 and all NPs in a dispersion contain organic and inorganic species in approximately the same ratio. These rules for assembly of colloids and organic components into homogeneous composite nanoparticles are broadly applicable.

Many biological microswimmers are capable of chemotaxis, i.e., they can sense an ambient chemical gradient and adjust their mechanism of motility to move towards or away from the source of the gradient. Synthetic active colloids endowed with chemotactic behavior hold considerable promise for targeted drug delivery and the realization of programmable and reconfigurable materials. Here, we study the chemotactic behavior of a Janus particle, which converts “fuel” molecules, released at an axisymmetric chemical patch located on a planar wall, into “product” molecules at its catalytic cap and moves by self-phoresis induced by the product. The chemotactic behavior is characterized as a function of the interplay between the rates of release (at the patch) and the consumption (at the particle) of fuel, as well as of details of the phoretic response of the particle (i.e., its phoretic mobility). Among other results, we find that, under certain conditions, the particle is attracted to a stable “hovering state” in which it aligns its axis normal to the wall and rests (positions itself) at an activity-dependent distance above the center of the patch.

Common neutral polymer microgels exhibit an inhomogeneous density profile with a gradual decay that is commonly described using the fuzzy sphere model. The model is based on the idea of convolving the collapsed solid sphere profile with a Gaussian to describe inhomogeneous swelling of the microgel in a good solvent. Here we show that the corresponding density profile in real space used in several recent works – such as in super-resolution microscopy – is different from the fuzzy sphere model, and we explain how to correctly transition between reciprocal space modelling to real space. Our work aims to clarify the application of the model so that errors can be avoided in the future. Our discussion is also crucial when comparing alternative real-space models for the density profile with the established fuzzy sphere model.

Polydimethylsiloxane is an important polymeric material with a wide range of applications. However, environmental effects like low temperature can induce crystallization in this material with resulting changes in its structural and dynamic properties. The incorporation of phenyl-siloxane components, e.g., as in a poly(dimethyl-co-diphenyl)siloxane random copolymer, is known to suppress such crystallization. Molecular dynamics (MD) simulations can be a powerful tool to understand such effects in atomistic detail. Unfortunately, all-atomistic molecular dynamics (AAMD) is limited in both spatial dimensions and simulation times it can probe. To overcome such constraints and to extend to more useful length- and time-scales, we systematically develop a coarse-grained molecular dynamics (CGMD) model for the poly(dimethyl-co-diphenyl)siloxane system with bonded and non-bonded interactions determined from all-atomistic simulations by the iterative Boltzmann inversion (IBI) method. Additionally, we propose a lever rule that can be useful to generate non-bonded potentials for such systems without reference to the all-atomistic ground truth. Our model captures the structural and dynamic properties of the copolymer material with quantitative accuracy and is useful to study long-time dynamics of highly-entangled systems, sequence-dependent properties, phase behaviour, etc.

Six different flexible stereocontrolled oligo(triazole-urethane)s substituted by precise sequences of nucleobases or analogs are synthesized. Molecular dynamics simulations indicate that the flexibility of the backbone leads to unspecific complexation of pairs of oligomers, irrespective of the complementarity of their sequences. This is ascribed to the existence of other interactions between pairs of oligomers, as well as to the spatial blurring of the sequence order encoded in the chemical structure of the chain due to its flexibility. The same conclusions are drawn when investigating the irreversible adsorption of different probe oligomers onto a layer of target oligomers grafted by click chemistry in a mushroom configuration on a silicon substrate. In contrast, when the target oligomers are grafted in denser brush configurations, irreversible adsorption becomes more specific, with it being twice as probable that probe chains of complementary sequence would be irreversibly-bound to the layer of target chains than those of non-complementary sequence. This is ascribed to lateral excluded volume interactions between chains in the brush, leading to partial chain stretching and increased spatial preservation of the information contained in the monomer sequence of the chains. At even higher grafting densities, however, the penetration of the probe chains in the brush becomes increasingly difficult, resulting in a loss of binding efficiency. Our work thus demonstrates the adverse role of chain flexibility in the specificity of complexation between nucleobase-functionalized oligomers and provides directions for an improvement of specificity by tuning the grafting density of target chains on a substrate.

Compared to bulk water, the effect of ions in confined environments or heterogeneous aqueous solutions is less understood. In this study, we characterize the influence of ions on hydrogen bond populations and dynamics within minimally hydrated polyethylene glycol diacrylate (PEGDA) solutions using Fourier-transform infrared (FTIR) and two-dimensional infrared (2D IR) spectroscopies. We demonstrate that hydrogen bond populations and lifetimes are directly related to ion size and hydration levels within the polymer matrix. Specifically, larger monovalent cation sizes (Li⁺, Na⁺, K⁺) as well as anion sizes (F⁻, Cl⁻, Br⁻) increase hydrogen bond populations and accelerate hydrogen bond dynamics, with anions having more pronounced effects compared to cations. These effects can be attributed to the complex interplay between ion hydration shells and the polymer matrix, where larger ions with diffuse charge distributions are less efficiently solvated, leading to a more pronounced disruption of the local hydrogen bonding network. Additionally, increased overall water content results in a significant slowdown of dynamics. Increased water content enhances the hydrogen bonding network, yet simultaneously provides greater ionic mobility, resulting in a delicate balance between stabilization and dynamic restructuring of hydrogen bonds. These results contribute to the understanding of ion-specific effects in complex partially-hydrated polymer systems, highlighting the complex interplay between ion concentration, water structuring, and polymer hydration state. The study provides a framework for designing polymer membrane compositions with ion-specific properties.