We report a simple correlation between microstructure and straindependent elasticity in colloidal gels by visualizing the evolution of cluster structure in high strain-rate flows. We control the initial gel microstructure by inducing different levels of isotropic depletion attraction between particles suspended in refractive index matched solvents. Contrary to previous ideas from mode coupling and micromechanical treatments, our studies show that bond breakage occurs mainly due to the erosion of rigid clusters that persist far beyond the yield strain. This rigidity contributes to gel elasticity even when the sample is fully fluidized; the origin of the elasticity is the slow Brownian relaxation of rigid, hydrodynamically interacting clusters. We find a power-law scaling of the elastic modulus with the stress-bearing volume fraction that is valid over a range of volume fractions and gelation conditions. These results provide a conceptual framework to quantitatively connect the flow-induced microstructure of soft materials to their nonlinear rheology.colloids | confocal microscopy | suspensions | shear flow C olloidal gels form sample-spanning networks (1) and are used to generate solid-like properties in a broad range of materials such as direct-write inks (2), nanoemulsions (3), tissue scaffolds (4), and membranes (5). When gels undergo large deformations, their network ruptures into clusters via a complex process. The ability to connect structural changes to suspension rheology is critical in understanding the mechanism of yielding. The network of clusters that makes up colloidal gels arises due to percolation, dynamic arrest, and phase separation, where the volume fraction and pair potential play an important part in their structure and rheology (6-8). Interparticle bonds rupture under a sufficiently large stress; the result is a flow-induced fluidization transition accompanied by the formation of voids and aggregates that continuously break and reform along the principal axes of flow (9, 10). A complex two-step yielding process has been observed in gels at intermediate volume fractions (0.05 ≤ ϕ ≤ 0.30) (11,12). Within this regime, a small number of bonds are broken in gels undergoing steady shear yielding (9, 10). Methods that track the evolution of ensemble-averaged structure, such as mode coupling theory (6) and light scattering, lack sensitivity to these subpopulations. On the other hand, micromechanical treatments directly model the contributions of local microstructure to the macroscopic elasticity of the material (13-15). However, experiments to connect the yield stress to different interparticle potentials, volume fractions, and particle sizes show little agreement. Presently, these theories can only provide estimates of colloidal rheology under specific conditions.We demonstrate that a simple, general correlation between microstructure and strain-dependent rheology exists for colloidal depletion gels undergoing large deformations at high shear rates. Our experiments harness confocal laser scanning mi...

Self-assembly of individual building blocks into highly ordered structures, analogous to spontaneous growth of crystals from atoms, is a promising approach to realize the collective properties of nanocrystals. Yet the ability to reliably produce macroscopic assemblies is unavailable and key factors determining assembly quality/yield are not understood. Here we report the formation of highly ordered superlattice films, with single crystalline domains of up to half a millimetre in two dimensions and thickness of up to several microns from nanocrystals with tens of nanometres in diameter. Combining experimental and computational results for gold nanocrystals in the shapes of spheres, cubes, octahedra and rhombic dodecahedra, we investigate the entire self-assembly process from disordered suspensions to large-scale ordered superlattices induced by nanocrystal sedimentation and eventual solvent evaporation. Our findings reveal that the ultimate coherence length of superlattices strongly depends on nanocrystal shape. Factors inhibiting the formation of high-quality large-scale superlattices are explored in detail.

Solid-solid phase transitions are the most ubiquitous in nature, and many technologies rely on them. However, studying them in detail is difficult because of the extreme conditions (high pressure/temperature) under which many such transitions occur and the high-resolution equipment needed to capture the intermediate states of the transformations. These difficulties mean that basic questions remain unanswered, such as whether so-called diffusionless solid-solid transitions, which have only local particle rearrangement, require thermal activation. Here, we introduce a family of minimal model systems that exhibits solid-solid phase transitions that are driven by changes in the shape of colloidal particles. By using particle shape as the control variable, we entropically reshape the coordination polyhedra of the particles in the system, a change that occurs indirectly in atomic solid-solid phase transitions via changes in temperature, pressure, or density. We carry out a detailed investigation of the thermodynamics of a series of isochoric, diffusionless solid-solid phase transitions within a single shape family and find both transitions that require thermal activation or are "discontinuous" and transitions that occur without thermal activation or are "continuous." In the discontinuous case, we find that sufficiently large shape changes can drive reconfiguration on timescales comparable with those for self-assembly and without an intermediate fluid phase, and in the continuous case, solid-solid reconfiguration happens on shorter timescales than self-assembly, providing guidance for developing means of generating reconfigurable colloidal materials.colloids | self-assembly | phase transitions | nanoparticles D espite wide-ranging implications for metallurgy (1), ceramics (2), earth sciences (3, 4), reconfigurable materials (5, 6), and colloidal matter (7), fundamental questions remain about basic physical mechanisms of solid-solid phase transitions. One major class of solid-solid transitions is diffusionless transformations. Although in diffusionless transformations, particles undergo only local rearrangement, the thermodynamic nature of diffusionless transitions is unclear (8). This gap in our understanding arises from technical details that limit what we can learn about solid-solid transitions from standard laboratory techniques, such as X-ray diffraction or EM (9). The use of a broader array of experimental, theoretical, and computational techniques could provide better understanding of solidsolid transitions if an amenable class of models could be developed (10). To develop minimal models, it is important to note that solid-solid transitions are accompanied by a change in shape of the coordination polyhedra in the structure (11). Coordination polyhedra reflect the bonding of atoms in a crystal, which suggests that minimal models of solid-solid transitions could be provided by systems in which the shape of coordination polyhedra is directly manipulated. Direct manipulation of coordination polyhedra may be achieved ...

We report the formation of a binary crystal of hard polyhedra due solely to entropic forces. Although the alternating arrangement of octahedra and tetrahedra is a known space-tessellation, it had not previously been observed in self-assembly simulations. Both known one-component phases - the dodecagonal quasicrystal of tetrahedra and the densest-packing of octahedra in the Minkowski lattice - are found to coexist with the binary phase. Apart from an alternative, monoclinic packing of octahedra, no additional crystalline phases were observed.