Session N12: Gallery of Soft Matter

Constructive Frustration : From a flower to auto-morphing architecture

In recent years, architects and engineers are seeking for alternative formation processes. A growing architectural urge to create complex shapes with high degree of variation and intricacy, stands in contradiction to the rising awareness to energy consumption, carbon footprint and material circular economy. Where we designers, architects and builders invest tremendous efforts in shaping matter to achieve morphological complexity, Nature demonstrates the beauty and efficiency of geometrically complex surfaces generated by self-shaping processes. Echoing the new understanding of material formation processes in physics, architecture too no longer sees matter as passive or ideally inert, but rather as a generative source for the development of form and structure. The theory of incompatible sheets describes the emergence of form and motion in matter, resulting from intrinsic material properties through stress-induced processes. Geometrical incompatibilities are prescribed in the material, making a flat ‘Frustrated Material’ that spontaneously configures itself into a predictable complex 3D shape. In search of robust materials suitable for architectural scale and environment, we have developed two material systems:  Frustrated Ceramics and Frustrated Composites. Going against ‘best practice’ which attentionally avoids the building of internal stresses in the fabrication process, we intentionally introduce incompatibilities; we do so by joining  materials of different shrinkage rate for the ceramics, and by fibre orientations and unbalanced laminates in the composites. Demonstrating the application of self-shaping principles in materials adapted to the architectural world- large scale, robust and durable - opens a new path towards sustainable materialisation of complex shapes, through auto-morphing architecture.     

Behind the Interface and Beyond

Fluid fingers form when a lower viscosity fluid penetrates a higher viscosity one within a confined geometry such as in the gap of a Hele-Shaw cell. Experiments using a novel flow-tracking technique make observable formerly hidden structures in the pattern growth of this iconic instability. By alternately injecting fluid with varying opacity from the center and through the gap between two parallel circular glass plates, we can measure the local velocities of propagating rings of dyed fluid. As inner fluid approaches the interface, and as the outer fluid is approached by the interface, we observe local variation in the fluid velocity along a finger protrusion and in an adjacent valley resulting in the deformation of dyed fluid rings. We relate this structure in the flow far from the interface to local pressure gradients which decay exponentially with distance from the interface. The associated decay length represents a new length scale for the flow which increases with time from finger onset. It is present in both the inner and outer fluids but differs in magnitude and growth rate.   

Morphomechanical Rods: From Fabrication to Function

From the DNA that encodes our genetics to the giant redwood forests, slender structures are ubiquitous across length scales in nature. Likewise, the canonical rod (i.e., objects with two dimensions much smaller than a third) provides the basic unit of engineered structures around us (e.g., thread in our clothes, scaffolds of our homes). However, using deformable rods with controllable shapes and strength (e.g., octopus arms, human fingers) to accomplish complex tasks primarily remains the handiwork of biology. Here we take lessons from thin-film manufacturing, insect wings, and traditional bead-weaving to fabricate rods with reversible shape, size, and stiffness. We study the critical ingredients that enable programming these rods' properties. We then explore how to use these structures - independently and in unison - to create soft robotics, deployable structures, and functional materials.

    

Bending Plateau's laws: Equilibrium shape of an elastic ribbon in a 2D bubble column

A foam is a set of gas bubbles trapped in a continuous liquid or solid material. In the low-density limit, liquid foams usually exhibit tetrahedral structures guided by capillarity and Plateau’s laws. Despite recent progress in “liquid foam templating” to produce solid foams in a controlled and self-assembled manner from liquid precursors, we still lack methods to tune explicitly the geometry and topology of foams (shape, size and connection of the bubbles). To reach a better customization of such structures, we integrate deformable solids to modify the structure of foams in the liquid state, using an elastocapillarity approach. The competition between elasticity and capillarity has indeed already proven to be an efficient way to assemble, orient or spontaneously bend slender elastic structures.

We consider a model experiment, consisting of elastomeric ribbons which deform under the action of capillary forces in a quasi-2D foam column. Confining bubbles into square section tubes leads to periodically ordered structures. We focus on the so-called “staircase” structure where bubbles of equal volume are rearranged in a staggered pattern. This structure has central soap films connected with 120° angles, in which we introduce elastic ribbons of different bending rigidities. We establish the key parameters leading to the deformation of the ribbon (capillarity dominated) or to the deformation of the foam (rigidity dominated) in the extreme cases. Using X-ray micro-tomography, we quantify the equilibrium shapes of the quasi-2D foam/ribbon systems: we present a detailed analysis of the ribbon profile, that compares well with theoretical predictions in the whole range of bending rigidities. We also provide a proof-of-concept that such setup can be used as a method to mold materials with characteristic shapes and curves imprinted by the foam structure. Finally, we discuss the extension of this approach to 3D complex materials.   

Keeping an eye on flowing liquid crystals

Flow instabilities lead to the formation of complex structures. We have established novel strategies to control the growth morphology of the viscous-fingering instability, which occurs as a less viscous fluid displaces a more viscous one in the thin gap between two parallel plates. We discuss how we can induce a morphology transition from the generic dense-branching growth characterized by repeated tip-splitting of the growing fingers to dendritic growth characterized by stable fingertips by exploiting the shear-enhanced anisotropy of lyotropic chromonic liquid crystal (LCLC) solutions in the nematic phase. We will further discuss chiral structures that emerge as the LCLC solutions are pushed out of equilibrium by a pressure-driven flow. The formation of these structures is remarkable as the LCLC itself is achiral. The chirality results from a periodic double-twist deformation of the liquid crystal and leads to striking stripe patterns vertical to the flow direction. We discuss the mechanism of this unique pathway to spontaneous mirror symmetry breaking and rationalize the selection of a well-defined period of the chiral domains.