Reversible-to-irreversible transition of colloidal polycrystals under cyclic athermal quasistatic deformation.

Cyclic loading on granular packings and amorphous media exhibits a transition from reversible elastic behavior to irreversible plasticity. The present study compares the irreversibility transition and microscopic details of colloidal polycrystals under oscillatory tensile-compressive and shear strain. Under both modes, the systems exhibit a reversible to irreversible transition. However, the strain amplitude at which the transition is observed is larger in the shear strain than in the tensile-compressive mode. The threshold strain amplitude is confirmed by analyzing the dynamical properties, such as mobility and atomic strain (von Mises shear strain and the volumetric strain). The structural changes are quantified using a hexatic order parameter. Under both modes of deformation, dislocations and grain boundaries in polycrystals disappear, and monocrystals are formed. We also recognize the dislocation motion through grains. The key difference is that strain accumulates diagonally in oscillatory tensile-compressive deformation, whereas in shear deformation, strain accumulation is along the x or y axis.

Impurity-induced nematic-isotropic transition of liquid crystals.

Complex fluids made of liquid crystals (LCs) and small molecules, surfactants, nanoparticles or 1D/2D nanomaterials show novel and interesting features, making them suitable materials for various applications starting from optoelectronics to biosensing. While these additives (impurities) introduce new features in the complex fluids, they may also alter the phase transition behaviour of LCs depending on the physiochemical properties of the added impurity. This article reports on the phase transition of 4-cyano-4'-alkylbiphenyl (nCB) LCs in the presence of an associative impurity, i.e., water and a non-associative impurity, i.e., hexane employing computational methods and experiments. In particular, all-atom (AA) simulations and coarse-grained (CG) models were designed for two complex systems, i.e., 6CB + water and 6CB + hexane and corresponding spectrophotometry experiments were performed using a homologous LC, i.e., 5CB. Results from the simulations and experiments elucidate that the phase transition of LCs depends on the mixing/demixing phenomenon of the impurity in the LC. While associative liquids like water which do not mix with LCs do not influence the nematic-to-isotropic phase transition of LCs, hexane, being a non-associative liquid, mixes well with LCs and induces a sharp impurity-induced nematic-to-isotropic phase transition. Upon application of both AA and CG simulations, we could reach the conclusion that the mixing/demixing phenomenon in an LC + impurity system influences the entropy of the system and hence the observed phase transitions are entropy-driven.

Adaptable DNA interactions regulate surface triggered self assembly.

DNA-mediated multivalent interactions between colloidal particles have been extensively applied for their ability to program bulk phase behaviour and dynamic processes. Exploiting the competition between different types of DNA-DNA bonds, here we experimentally demonstrate the selective triggering of colloidal self-assembly in the presence of a functionalised surface, which induces changes in particle-particle interactions. Besides its relevance to the manufacturing of layered materials with controlled thickness, the intrinsic signal-amplification features of the proposed interaction scheme make it valuable for biosensing applications.

Self-assembly of finite-sized colloidal aggregates.

One of the challenges of self-assembling finite-sized colloidal aggregates with a sought morphology is the necessity of precisely sorting the position of the colloids at the microscopic scale to avoid the formation of off-target structures. Microfluidic platforms address this problem by loading into single droplets the exact amount of colloids entering the targeted aggregate. Using theory and simulations, in this paper, we validate a more versatile design allowing us to fabricate different types of finite-sized aggregates, including colloidal molecules or core-shell clusters, starting from finite density suspensions of isotropic colloids in bulk. In our model, interactions between particles are mediated by DNA linkers with mobile tethering points, as found in experiments using DNA oligomers tagged with hydrophobic complexes immersed into supported bilayers. By fine-tuning the strength and number of the different types of linkers, we prove the possibility of controlling the morphology of the aggregates, in particular, the valency of the molecules and the size of the core-shell clusters. In general, our design shows how multivalent interactions can lead to microphase separation under equilibrium conditions.

Viscoelastic Properties of Polyelectrolyte Multilayers Swollen with Ionic Liquid Solutions.

Polyelectrolyte multilayers (PEM) obtained by layer-by-layer assembly can be doped with ionic liquid (IL) via the swelling of the films with IL solutions. In order to examine the mechanical properties of IL-containing PEM, we implement a Kelvin-Voigt model to obtain thickness, viscosity and elastic modulus from the frequency and dissipation shifts determined by a dissipative quartz crystal microbalance (QCM-D). We analyze the changes in the modeled thickness and viscoelasticity of PEI(PSS/PADMAC)4PSS and PEI(PSS/PAH)4PSS multilayers upon swelling by increasing the concentration of either 1-Ethyl-3-methylimidazolium chloride or 1-Hexyl-3-methylimidazolium chloride, which are water soluble ILs. The results show that the thickness of the multilayers changes monotonically up to a certain IL concentration, whereas the viscosity and elasticity change in a non-monotonic fashion with an increasing IL concentration. The changes in the modeled parameters can be divided into three concentration regimes of IL, a behavior specific to ILs (organic salts), which does not occur with swelling by simple inorganic salts such as NaCl. The existence of the regimes is attributed to a competition of the hydrophobic interactions of large hydrophobic ions, which enhance the layer stability at a low salt content, with the electrostatic screening, which dominates at a higher salt content and causes a film softening.

Surface-triggered cascade reactions between DNA linkers direct the self-assembly of colloidal crystals of controllable thickness.

Functionalizing colloids with reactive DNA linkers is a versatile way of programming self-assembly. DNA selectivity provides direct control over colloid-colloid interactions allowing the engineering of structures such as complex crystals or gels. However, the self-assembly of localized and finite structures remains an open problem with many potential applications. In this work, we present a system in which functionalized surfaces initiate a cascade reaction between linkers leading to the self-assembly of crystals with a controllable number of layers. Specifically, we consider colloidal particles functionalized by two families of complementary DNA linkers with mobile anchoring points, as found in experiments using emulsions or lipid bilayers. In bulk, intra-particle linkages formed by pairs of complementary linkers prevent the formation of inter-particle bridges and therefore colloid-colloid aggregation. However, colloids interact strongly with the surface given that the latter can destabilize intra-particle linkages. When in direct contact with the surface, colloids are activated, meaning that they feature more unpaired DNA linkers ready to react. Activated colloids can then capture and activate other colloids from the bulk through the formation of inter-particle linkages. Using simulations and theory, validated by existing experiments, we clarify the thermodynamics of the activation and binding process and explain how particle-particle interactions, within the adsorbed phase, weaken as a function of the distance from the surface. The latter observation underlies the possibility of self-assembling finite aggregates with controllable thickness and flat solid-gas interfaces. Our design suggests a new avenue to fabricate heterogeneous and finite structures.

Kinetics of Nanoparticle-Membrane Adhesion Mediated by Multivalent Interactions.

Multivalent adhesive interactions mediated by a large number of ligands and receptors underpin many biological processes, including cell adhesion and the uptake of particles, viruses, parasites, and nanomedical vectors. In materials science, multivalent interactions between colloidal particles have enabled unprecedented control over the phase behavior of self-assembled materials. Theoretical and experimental studies have pinpointed the relationship between equilibrium states and microscopic system parameters such as the ligand-receptor binding strength and their density. In regimes of strong interactions, however, kinetic factors are expected to slow down equilibration and lead to the emergence of long-lived out-of-equilibrium states that may significantly influence the outcome of self-assembly experiments and the adhesion of particles to biological membranes. Here we experimentally investigate the kinetics of adhesion of nanoparticles to biomimetic lipid membranes. Multivalent interactions are reproduced by strongly interacting DNA constructs, playing the role of both ligands and receptors. The rate of nanoparticle adhesion is investigated as a function of the surface density of membrane-anchored receptors and the bulk concentration of nanoparticles and is observed to decrease substantially in regimes where the number of available receptors is limited compared to the overall number of ligands. We attribute such peculiar behavior to the rapid sequestration of available receptors after initial nanoparticle adsorption. The experimental trends and the proposed interpretation are supported by numerical simulations.

Translational and rotational dynamics of colloidal particles interacting through reacting linkers.

Much work has studied effective interactions between micron-sized particles carrying linkers forming reversible, interparticle linkages. These studies allowed understanding the equilibrium properties of colloids interacting through ligand-receptor interactions. Nevertheless, understanding the kinetics of multivalent interactions remains an open problem. Here, we study how molecular details of the linkers, such as the reaction rates at which interparticle linkages form or break, affect the relative dynamics of pairs of cross-linked colloids. Using a simulation method tracking single binding and unbinding events between complementary linkers, we rationalize recent experiments and prove that particles' interfaces can move across each other while being cross-linked. We clarify how, starting from diffusing colloids, the dynamics become arrested when increasing the number of interparticle linkages or decreasing the reaction rates. Before getting arrested, particles diffuse through rolling motion. The ability to detect rolling motion will be useful to shed new light on host-pathogen interactions.

Nanoscale liquid crystal lubrication controlled by surface structure and film composition.

Liquid crystals have emerged as potential candidates for next-generation lubricants due to their tendency to exhibit long-range ordering. Here, we construct a full atomistic model of 4-cyano-4-hexylbiphenyl (6CB) nematic liquid crystal lubricants mixed with hexane and confined by mica surfaces. We explore the effect of the surface structure of mica, as well as lubricant composition and thickness, on the nanoscale friction in the system. Our results demonstrate the key role of the structure of the mica surfaces, specifically the positions of potassium (K+) ions, in determining the nature of sliding friction with monolayer lubricants, including the presence or absence of stick-slip dynamics. With the commensurate setup of confining surfaces, when the grooves created between the periodic K+ ions are parallel to the sliding direction we observe a lower friction force as compared to the perpendicular situation. Random positions of ions exhibit even smaller friction forces with respect to the previous two cases. For thicker lubrication layers the surface structure becomes less important and we observe a good agreement with the experimental data on bulk viscosity of 6CB and the additive hexane. In case of thicker lubrication layers, friction may still be controlled by tuning the relative concentrations of 6CB and hexane in the mixture.

Irreversibility transition of colloidal polycrystals under cyclic deformation.

Cyclically loaded disordered particle systems, such as granular packings and amorphous media, display a non-equilibrium phase transition towards irreversibility. Here, we investigate numerically the cyclic deformation of a colloidal polycrystal with impurities and reveal a transition to irreversible behavior driven by the displacement of dislocations. At the phase transition we observe enhanced particle diffusion, system size effects and broadly distributed strain bursts. In addition to provide an analogy between the deformation of amorphous and polycrystalline materials, our results allow to reinterpret Zener pinning of grain boundaries as a way to prevent the onset of irreversible crystal ordering.

Anomalous approach to thermodynamic equilibrium: Structure formation of molecules after vapor deposition.

We describe experiments and computer simulations of molecular deposition on a substrate in which the molecules (substituted adenine derivatives) self-assemble into ordered structures. The resulting structures depend strongly on the deposition rate (flux). In particular, there are two competing surface morphologies (α and ß), which differ by their topology (interdigitated vs lamellar structure). Experimentally, the α phase dominates at both low and high flux, with the ß phase being most important in the intermediate regime. A similar nonmonotonic behavior is observed on varying the substrate temperature. To understand these effects from a theoretical perspective, a lattice model is devised which reproduces qualitatively the topological features of both phases. Via extensive Monte Carlo studies we can, on the one hand, reproduce the experimental results and, on the other hand, obtain a microscopic understanding of the mechanisms behind this anomalous behavior. The results are discussed in terms of an interplay between kinetic trapping and temporal exploration of configuration space.

Controlling two-phase self-assembly of an adenine derivative on HOPG via kinetic effects.

Large-area self-assembled structures of a nucleobase adenine derivative were successfully realized through vacuum deposition. STM images reveal two types of structures, which could be regulated by substrate temperature and the evaporation rate, indicating the relevance of kinetic effects. The results are supported by computer simulations.

Pattern formation of anisotropic molecules on surfaces under non-equilibrium conditions as described by a minimum model.

The self-organization of lipophilic chain molecules on surfaces in vacuum deposition experiments has been recently studied by Monte Carlo simulations of a coarse grained microscopic model system. Surprisingly, the final potential energy depends in a non-monotonous way on the chosen flux and the surface temperature. Here we introduce a schematic model which contains the relevant physical ingredients of the microscopic model and which elucidates the origin of this anomalous non-equilibrium effect. Intra-cluster effects, reflecting the chain arrangement within one cluster, and inter-cluster effects, based on the distribution of chains among the different formed clusters, are taken into account. This schematic model is solved numerically as well as via analytical means. From the analytical solutions, it is possible to understand quantitatively for which interaction parameters the observed anomalies can indeed be observed. The generality of the observed phenomena is stressed. It is related to the concept of kinetic trapping, which often occurs during self-assembly.

Deposition of model chains on surfaces: anomalous relation between flux and stability.

Model chains are studied via Monte Carlo simulations which are deposited with a fixed flux on a substrate. They may represent, e.g., stiff lipophilic chains with an head group and tail groups mimicking the alkyl chain. After some subsequent fixed simulation time we determine the final energy as a function of flux and temperature. Surprisingly, in some range of temperature and flux the final energy increases with decreasing flux. The physical origin of this counterintuitive observation is elucidated. In contrast, when performing equivalent cooling experiments no such anomaly is observed. Furthermore, it is elaborated whether flux experiments give rise to configurations with lower energies as compared to cooling experiments. These results are related to recent experiments by the Ediger group where very stable configurations of glass-forming systems have been generated via flux experiments.