Session N24: Gallery of Soft Matter

Exotic liquids crystals from colloidal banana self-assembly

The curvature of elongated microscopic building blocks, such as proteins or bacteria, plays an essential role in their self-assembly and functional properties. In the biological world, for instance, cell division relies on the ability of “banana-shaped” BAR proteins to generate curvature in cell membrane. At the molecular scale, banana-shaped molecules exhibit a range of fascinating liquid crystalline (LC) phases with supramolecular chirality and polarity, which play a role in the development of faster sensors and LC displays [1]. Experimental studies of how and why these phases form at the molecular level have remained challenging until very recently, as the time and length scales involved in these systems are extremely fast and small.

 

In this talk, I will present our recently developed system of colloidal banana-shaped particles, which has allowed the direct study of the structure and dynamics of exotic colloidal LC phases using confocal microscopy [2].  By carefully tuning the shape of the particles [2,3], we studied the effect of curvature in LC assembly, and elucidated the full phase behaviour of the particles. We found that while highly curved bananas only form isotropic phases,  less curved bananas exhibit a very rich phase behaviour including the elusive splay-bend nematic phase, whose very existence is finally experimentally established more than 40 years after its theoretical prediction. Finally, I will also discuss the effect of polydispersity in these systems [4], and show how this is a key ingredient for the formation of hierarchical liquid crystal phases, known as the colloidal vortices.

[1]   A. Jakli, et al., Rev. Mod. Phys., 90, 45004 (2018).

[2]   C. Fernández-Rico, et. al, Science, 369, 950-955 (2020).

[3]   C. Fernández-Rico, et. al, Adv. Mat., 31, 1807514 (2019).

[4]   C. Fernández-Rico, et. al, PNAS, 118, 33 (2021).   

Interactions and Pattern Formation in a Macroscopic Magnetocapillary SALR System

Interaction potentials defined by Short-range Attraction and Long-range Repulsion (SALR), frequently arise in the modeling of colloidal systems. Here, we introduce a new SALR potential that combines capillary attraction and repulsive magnetic forces and can be realized in an accessible tabletop experiment. We develop a simplified model of the pairwise interaction potential that predicts a variety of regimes, including the possibility of a local minimum energy configuration for certain parameters. Experiments confirm that a minimum energy configuration corresponding to a finite equilibrium spacing is possible for a pair of millimetric magnetic floating disks. 2D experiments and simulations show that beyond a critical packing fraction, the uniform lattice state becomes unstable and localized clusters spontaneously form which is reminiscent of microscopic colloidal systems. Finally, we show that the state of the system can be controlled by tuning the capillary force via an external magnetic force.   

What's (Not) Popping? The Long Life and Quick Death of a Marangoni Bubble

Free-standing films are inherently fragile, unless stabilized with surfactants. Similarly, air bubbles on the surface of a pure liquid are ephemeral, which leads to the intriguing question: Can air bubbles be stabilized in the absence of surfactants? In pursuit of an answer, we discovered a compelling scenario – bubbles at the surface of a pure yet hot oil pool exhibit surprisingly extended lifespans, lasting for tens of minutes or more. I discuss here the nature and origin of this longevity as well as some curious facets of these strangely stable objects.   

Photoelastic precursors to clogging events

Clogging affects countless systems, from microscopic agglomerations in inkjet printers and clots in blood vessels, to large-scale sewer blockages and clogs in silos of grains. Although clogging can be expensive and dangerous, our understanding of the particle scale stress and velocity signatures preceding a clog is currently limited due to experimental challenges.

Here, we study clogs at a fundamental level by closely observing the behaviour of two-dimensional photoelastic particles as they fall through a constriction. In this scenario, a clog occurs when a stable arch of particles forms at the orifice. We use a hopper setup that allows us to tune parameters such as the orifice angle and width. We examine how hopper geometry affects clogging mechanisms and flow regimes. High-resolution imaging techniques and advanced processing methods enable us to quantitatively extract inter-particle forces in the flow before, during, and after a clogging event. Experimental observations indicate the existence of arches that form and break spontaneously, preceding the formation of a stable clogging arch. Our photoelastic force data is used to discern the difference between these different arches. The distribution of forces gives us insight into why stable arches appear and provides a step toward predicting when a clogging event occurs.   

Different Modes of Stress Release in Drying Drops of Colloidal Suspensions

Sessile droplets of nanoparticle suspensions dry into solid deposits subject to large drying stresses. We study two suspensions that release these stresses in distinct ways: droplets of polystyrene (PS) nanoparticle suspensions form complex crack patterns while droplets of silica (Si) nanoparticle suspensions bend into shapes resembling blooming flowers.

As water evaporates from a freshly deposited drop, a thin close-packed particle deposit forms at the edge of the drop and grows inward. Water evaporation from this particle deposit combined with adhesion of the deposit to the substrate causes tensile stresses leading to the formation of regular radial cracks. As the radial cracks form, we observe a critical difference between the two systems: the PS particle deposit remains attached to the substrate while the Si particle deposit delaminates and separates from the substrate. Stresses in the attached PS particle deposit are released by the formation of orthoradial cracks that bridge the radial cracks, creating a complex crack pattern. Conversely, no additional cracks form in the delaminated Si particle deposit. Instead, the deposit curves and bends out of plane to relieve drying stresses. We show how the combination of poroelasticity with fracture mechanics and non-Euclidian plate mechanics captures both the complex crack pattern formation and the out-of-plane deformation as distinct responses to a similar drying stress.