Light from within: Illuminating the complexity of co-assembly from the inside out

The assembly of well-defined nanostructures from smaller (macro)molecular building blocks relies on a delicate balance between attractive and repulsive interactions between the basic building blocks. To avoid kinetic trapping and structural polymorphism, the balance between antagonistic and synergistic interactions must be carefully tailored. Controlling this complex process is challenging, but we can look to Nature for inspiration. As it turns out, biology often use a combination of molecular templating and allostery to control self-assembly processes, resulting in an unusual degree of fidelity of the final nanostructures formed. Please click Additional Files below to see the full abstract

Anomalous dynamics of interstitial dopants in soft crystals

The dynamics of interstitial dopants governs the properties of a wide variety of doped crystalline materials. To describe the hopping dynamics of such interstitial impurities, classical approaches often assume that dopant particles do not interact and travel through a static potential energy landscape. Here we show, using computer simulations, how these assumptions and the resulting predictions from classical Eyring-type theories break down in entropically-stabilised BCC crystals due to the thermal excitations of the crystalline matrix. Deviations are particularly severe close to melting where the lattice becomes weak and dopant dynamics exhibit strongly localised and heterogeneous dynamics. We attribute these anomalies to the failure of both assumptions underlying the classical description: i) the instantaneous potential field experienced by dopants becomes largely disordered due to thermal fluctuations and ii) elastic interactions cause strong dopant-dopant interactions even at low doping fractions. These results illustrate how describing non-classical dopant dynamics requires taking the effective disordered potential energy landscape of strongly excited crystals and dopant-dopant interactions into account.Comment: 16 pages, 14 figures. Includes Supplementary Informatio

Complex coacervation and metal-ligand bonding as synergistic design elements for aqueous viscoelastic materials

The application of complex coacervates in promising areas such as coatings and surgical glues requires a tight control of their viscous and elastic behaviour, and a keen understanding of the corresponding microscopic mechanisms. While the viscous, or dissipative, aspect is crucial at pre-setting times and in preventing detachment, elasticity at long waiting times and low strain rates is crucial to sustain a load-bearing joints. The independent tailoring of dissipative and elastic properties proves to be a major challenge that can not be addressed adequately by the complex coacervate motif by itself. We propose a versatile model of complex coacervates with customizable rheological fates by functionalization of polyelectrolytes with terpyridines, which provide transient crosslinks through complexation with metals. We show that the rheology of the hybrid complexes shows distinct footprints of both metal-ligand and coacervate dynamics, the former as a contribution very close to pure Maxwell viscoelasticity, the latter approaching a sticky Rouse fluid. Strikingly, when the contribution of metal-ligand bonds is dominant at long times, the relaxation of the overall complex is much slower than either the "native"coacervate relaxation time or the dissociation time of a comparable non-coacervate polyelectrolyte-metal-ligand complex. We recognize this slowing-down of transient bonds as a synergistic effect that has important implications for the use of complementary transient bonding in coacervate complexes.</p

Strand plasticity governs fatigue in colloidal gels

Repeated loading of a solid leads to microstructural damage that ultimately results in catastrophic material failure. While posing a major threat to the stability of virtually all materials, the microscopic origins of fatigue, especially for soft solids, remain elusive. Here we explore fatigue in colloidal gels as prototypical inhomogeneous soft solids by combining experiments and computer simulations. Our results reveal how mechanical loading leads to irreversible strand stretching, which builds slack into the network that softens the solid at small strains and causes strain hardening at larger deformations. We thus find that microscopic plasticity governs fatigue at much larger scales. This gives rise to a new picture of fatigue in soft thermal solids and calls for new theoretical descriptions of soft gel mechanics in which local plasticity is taken into account.Comment: 5 pages, 4 figure

Linking particle dynamics to local connectivity in colloidal gels

Colloidal gels are a prototypical example of a heterogeneous network solid whose complex properties are governed by thermally-activated dynamics. In this Letter we experimentally establish the connection between the intermittent dynamics of individual particles and their local connectivity. We interpret our experiments with a model that describes single-particle dynamics based on highly cooperative thermal debonding. The model, in quantitative agreement with experiments, provides a microscopic picture for the structural origin of dynamical heterogeneity in colloidal gels and sheds new light on the link between structure and the complex mechanics of these heterogeneous solids.Comment: 5 pages, 5 figure

Chemical Stability of α-Tocopherol in Colloidal Lipid Particles with Various Morphologies

Colloidal lipid particles (CLPs) are promising encapsulation systems for lipophilic bioactives, such as oil-soluble antioxidants that are applied in food and pharmaceutical formulations. Currently, there is no clear consensus regarding the relation between particle structure and the chemical stability of such bioactives. Using α-tocopherol as a model antioxidant, it is shown that emulsifier type (Tween 20 or 40, or sodium caseinate) and lipid composition (tripalmitin, tricaprylin, or combinations thereof) modulated particle morphology and antioxidant stability. The emulsifier affects particle shape, with the polysorbates facilitating tripalmitin crystallization into highly ordered lath-like particles, and sodium caseinate resulting in less ordered spherical particles. The fastest degradation of α-tocopherol is observed in tripalmitin-based CLPs, which may be attributed to its expulsion to the particle surface induced by lipid crystallization. This effect is stronger in CLPs stabilized by Tween 40, which may act as a template for crystallization. This work not only shows how the architecture of CLPs can be controlled through the type of lipid and emulsifier used, but also gives evidence that lipid crystallization does not necessarily protect entrapped lipophilic bioactives, which is an important clue for encapsulation system design. Practical Applications: Interest in enriching food and pharmaceutical products with lipophilic bioactives such as antioxidants through encapsulation in lipid particles is growing rapidly. This research suggests that for efficient encapsulation, the particle architecture plays an important role; to tailor this, the contribution of both the lipid carrier and the emulsifier needs to be considered.</p

Precise colloids with tunable interactions for confocal microscopy

Model colloidal systems studied with confocal microscopy have led to numerous insights into the physics of condensed matter. Though confocal microscopy is an extremely powerful tool, it requires a careful choice and preparation of the colloid. Uncontrolled or unknown variations in the size, density, and composition of the individual particles and interactions between particles, often influenced by the synthetic route taken to form them, lead to difficulties in interpreting the behavior of the dispersion. Here we describe the straightforward synthesis of copolymer particles which can be refractive index- and density-matched simultaneously to a non-plasticizing mixture of high dielectric solvents. The interactions between particles are accurately tuned by surface grafting of polymer brushes using Atom Transfer Radical Polymerization (ATRP), from hard-sphere-like to long-ranged electrostatic repulsion or mixed charge attraction. We also modify the buoyant density of the particles by altering the copolymer ratio while maintaining their refractive index match to the suspending solution resulting in well controlled sedimentation. The tunability of the inter-particle interactions, the low volatility of the solvents, and the capacity to simultaneously match both the refractive index and density of the particles to the fluid opens up new possibilities for exploring the physics of colloidal systems

Buckling of self-assembled colloidal structures

Although buckling is a prime route to achieve functionalization and synthesis of single colloids, buckling of colloidal structures---made up of multiple colloids---remains poorly studied. Here, we investigate the buckling of the simplest form of a colloidal structure, a colloidal chain that is self-assembled through critical Casimir forces. We demonstrate that the mechanical instability of such a chain is strikingly reminiscent of that of classical Euler buckling but with thermal fluctuations and plastic effects playing a significant role. Namely, we find that fluctuations tend to diverge close to the onset of buckling and that plasticity controls the buckling dynamics at large deformations. Our work provides insight into the effect of geometrical, thermal and plastic interactions on the nonlinear mechanics of self-assembled structures, of relevance for the rheology of complex and living matter and the rational design of colloidal architectures