Active emulsions, i.e., emulsions whose droplets perform self-propelled motion, are of tremendous interest for mimicking collective phenomena in biological populations such as phytoplankton and bacterial colonies, but also for experimentally studying rheology, pattern formation, and phase transitions in systems far from thermal equilibrium. For fuelling such systems, molecular processes involving the surfactants which stabilize the emulsions are a straightforward concept. We outline and compare two different types of reactions, one which chemically modifies the surfactant molecules, the other which transfers them into a different colloidal state. While in the first case symmetry breaking follows a standard linear instability, the second case turns out to be more complex. Depending on the dissolution pathway, there is either an intrinsically nonlinear instability, or no symmetry breaking at all (and hence no locomotion).

During drying, colloidal dispersions undergo processes such as solidification, cracking, and the draining of interstitial pores. Here we show that the solidification of polystyrene and silica dispersions, during directional drying, occurs in two separate stages. These correspond to the initial ordering, and subsequent aggregation, of the colloidal particles. Transitions between these stages are observed as changes in transparency and color that propagate as distinct fronts along the drying layer. The dynamics of these fronts are shown to arise from a balance between compressive capillary forces, and the electrostatic and van der Waals forces described by DLVO theory. This suggests a simple method by which the maximum inter-particle repulsion between particles can be measured through the optical inspection of the dynamics of a drying dispersion, under a microscope.

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Aqueous colloidal dispersions of silica particles become anisotropic when they are dried through evaporation. This anisotropy is generated by a uniaxial strain of the liquid dispersions as they are compressed by the flow of water toward a solidification front. Part of the strain produced by the compression is relaxed, and part of it is stored and transferred to the solid. This stored elastic strain has consequences for the properties of the solid, where it may facilitate the growth of shear bands, and generate birefringence.

PACS. 45.70.Qj -pattern formation. PACS. 62.20.Mk -Fatigue, brittleness, fracture, and cracks.Abstract. -Columnar joints are three-dimensional fracture networks that form in cooling basalt and several other media. The network organizes itself into ordered, mostly hexagonal columns. The same pattern can be observed on a smaller scale in desiccating starch. We show how surface boundary conditions in the desiccation of starch affect the formation of columnar joints. Under constant drying power conditions, we find a power law dependence of columnar cross-sectional area with depth, while under constant drying rate conditions this coarsening is eventually halted. Discontinuous transitions in pattern scale can be observed under constant external conditions, which may prompt a reinterpretation of similar transitions found in basalt. Starch patterns are statistically similar to those found in basalt, suggesting that mature columnar jointing patterns contain inherent residual disorder, but are statistically scale invariant.

Columnar jointing is a fracture pattern common in igneous rocks in which cracks self-organize into a roughly hexagonal arrangement, leaving behind an ordered colonnade. We report observations of columnar jointing in a laboratory analog system, desiccated corn starch slurries. Using measurements of moisture density, evaporation rates, and fracture advance rates as evidence, we suggest that an advective-diffusive system is responsible for the rough scaling behavior of columnar joints. This theory explains the order of magnitude difference in scales between jointing in lavas and in starches. We investigated the scaling of average columnar cross-sectional areas due to the evaporation rate, the analog of the cooling rate of igneous columnar joints. We measured column areas in experiments where the evaporation rate depended on lamp height and time, in experiments where the evaporation rate was fixed using feedback methods, and in experiments where gelatin was added to vary the rheology of the starch. Our results suggest that the column area at a particular depth is related to both the current conditions, and hysteretically to the geometry of the pattern at previous depths. We argue that there exists a range of stable column scales allowed for any particular evaporation rate.

[1] We describe field work, analysis, and modeling of columnar joints from the Columbia River Basalt Group. This work is focused on the regions around the Grand Coulee, Snake River, and Columbia Gorge, which form parts of this unusually homogeneous and very large sample of columnar basalt. We examine in detail the scaling relationship between the column width and the size of the striae and relate these quantitatively to thermal and fracture models. We found that the column radius and stria size are proportional to each other and inversely proportional to the cooling rate of the lava. Near a flow margin, our results put observational constraints on diffusive thermal models of joint formation. Deeper than a few meters into a colonnade, our measurements are consistent with a simple advection-diffusion model of two-phase convective cooling within the joints, regardless of the direction of cooling. This model allows an accurate comparison of igneous columnar jointing and joints due to desiccation in laboratory analog systems. We also identify a new length scale in which wavy columns can appear in some colonnades. The mechanisms leading to the wavy columns are likely related to those underlying similar wavy cracks in 2-D analog systems.

Crack patterns in laboratory experiments on thick samples of drying cornstarch are geometrically similar to columnar joints in cooling lava found at geological sites such as the Giant's Causeway. We present measurements of the crack spacing from both laboratory and geological investigations of columnar jointing, and show how these data can be collapsed onto a single master scaling curve. This is due to the underlying mathematical similarity between theories for the cracking of solids induced by differential drying or by cooling. We use this theory to give a simple quantitative explanation of how these geometrically similar crack patterns arise from a single dynamical law rooted in the nonequilibrium nature of the phenomena. We also give scaling relations for the characteristic crack spacing in other limits consistent with our experiments and observations, and discuss the implications of our results for the control of crack patterns in thin and thick solid films.pattern formation | fracture | geomorphology | volcanology | faulting D rying solids lose moisture from their exposed surfaces and shrink as a consequence. Similarly, cooling solids lose heat from their exposed surfaces and shrink as a consequence. In either case, this differential shrinkage of one part of the solid relative to another leads to stresses that can eventually lead to cracking (1-3). Although much is known about the nucleation, growth, dynamics, and stability of a single crack in an elastic solid, most questions associated with the patterns of multiple cracks due to stresses that arise from nonequilibrium processes such as drying and cooling (4-7) remain wide open. The resulting polygonal planform patterns can arise in a variety of situations, from the mundane cracks in drying mud, to the deliberately artistic cracks in ceramics and pottery, to the spectacular columnar joint formations of the Giant's Causeway in Northern Ireland, Fingal's Cave on Staffa, in Scotland, and the Devil's Postpile in California. The latter formations have fascinated casual observers, artists, and scientists for centuries (8-10), but no comprehensive physical theory for their form or scale exists. Indeed, it is only in the past decade or so that careful laboratory experiments have started to address the dependence of any of these crack patterns on such quantities as the rate of drying or cooling, the thickness of the layers, and their mechanical properties (4-7, 11-13). For example, recent experiments show that crack formation and propagation in drying thin films leads to length scales and patterns that can be strongly timedependent; cracks in directionally drying films grow diffusively at short times, and can advance intermittently via stick-slip-like motion over longer times (11,12). The patterns formed by these cracks depend in detail on the spatiotemporal dynamics of drying, substrate adhesion, and thickness variations (4,6,13,14). This immediately suggests a nonequilibrium origin to these crack patterns, one that couples the heterogeneous elastic stresses in th...