Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C16: Pattern Formation in Soft MaterialsInvited
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Sponsoring Units: GSOFT Chair: Irmgard Bischofberger, Massachusetts Institute of Technology-MIT Room: LACC 305 |
Monday, March 5, 2018 2:30PM - 3:06PM |
C16.00001: Pattern formation in suspensions Invited Speaker: Sungyon Lee In this talk, we examine the spontaneous emergence of particle clusters inside the suspension that is radially displacing air inside a Hele-Shaw cell. This pattern formation is driven by the interfacial instability that follows the particle accumulation at the suspension-air interface. We observe that the key characteristics of the pattern and its growth (i.e. shape and wavenumber) are directly affected by the nature of particle accumulation leading up to the instability. In particular, we focus on the regime in which particles form a discrete band of packing on the interface; the subsequent break-up of the particle band causes the formation of the particle islands inside the suspension that evolve over time. We also discuss the balance of the stabilizing and destabilizing effects inside the particle band which rationalize the observed wavelength. |
Monday, March 5, 2018 3:06PM - 3:42PM |
C16.00002: Pattern formation in confined colloidal liquid crystals Invited Speaker: Dirk Aarts We report on the confinement of colloidal liquid crystals in three dimensional chambers, where the confinement lengthscale is comparable to the length of the rods. We present results for both virus particles and silica rods, where in the latter case we exploit silica's large density difference with respect to the dispersing solvent to study isotropic, nematic and smectic phases confined into a single chamber. Combining laser scanning confocal microscopy and soft-lithography techniques enables us to characterize the configurations down to the single particle level. We compare to simulations and theory. Finally, by modifying the silica rods we are able to produce novel low-density colloidal liquid crystals with anti-nematic order. This shows how colloidal liquid crystals can both shine light on more traditional liquid crystal problems and enable us to design novel tunable liquid crystalline phases. |
Monday, March 5, 2018 3:42PM - 4:18PM |
C16.00003: Critters: stable clusters born from an unstable front Invited Speaker: Michelle Driscoll Rotating colloidal particles near a floor creates strong advective flows, which lead to a rich variety of collective effects. Using a model experimental system in tandem with large-scale 3D simulations, we have shown that driving a compact group of microrollers leads to a flow instability, which produces fingers of a well-defined wavelength. Under the right conditions, stable structures we term "critters" can emerge from these fingers. We first identified these new structures in simulations, and have recently made promising steps towards producing them in the lab. Our simulations and experiments suggest that these critters are a stable state of the system, move much faster than individual rollers, and quickly respond to a changing drive. We believe that critters are unique in that they are clusters which form only with hydrodynamic interactions; no interparticle potentials are needed to create these structures. Furthermore, as compact, self-assembled clusters which can easily be remotely guided, critters may offer a promising tool for microscopic transport. |
Monday, March 5, 2018 4:18PM - 4:54PM |
C16.00004: Geometry and Mechanics of Thin Elastic Ribbons Invited Speaker: Julien Chopin Thin elastic structures such as rods and plates can undergo large displacements without breaking due to the inexpensive energy cost of bending compared to stretching as the thickness vanishes. This ability to dramatically change shape due to confinement, external loads and stimuli is of great interest for functionalization and improving mechanical properties based on geometry rather than chemistry. Ribbons are examples of elongated elastic structures with a strongly anisotropic cross-section making them intermediate between flat flexible filament and narrow thin plate. Due to this specific geometry, ribbons offer an interesting experimental and theoretical playground to explore elastic instabilities under large displacement, near and far from threshold [J. Elast. 119, 137 (2015)]. When stretched and twisted quasi-statically around its axis, elastic ribbons exhibit a wealth of morphologies including single buckle and wrinkles, curled and looped configurations [Phys. Rev. Lett. 111, 174302 (2013)], as well as triangular facets [Soft Matter, 12, 4457 (2016)]. We characterized experimentally the various phases observed varying the twist, tension and the geometry using X-ray imaging and laser profilometry. I will present minimal models incorporating strong geometric non linearities and finite rotation to explain quantitatively the scaling observed for the measured threshold and shapes. Finally, I will present experimental results on the dynamic wrinkling instability of ribbon compressed at a finite rate in a viscous fluid [Phys. Rev. Lett, 119, 088001, (2017)] . The drag induces a dynamic lateral reinforcement of the filament leading to growth of wrinkles that coarsen over time . I will discuss a new dynamical regime where the pattern selection mechanism involves a non-trivial dependence with the loading rate. |
Monday, March 5, 2018 4:54PM - 5:30PM |
C16.00005: Topological patterns in active liquid crystals Invited Speaker: M Cristina Marchetti Active materials are composed of interacting entities that are individually powered and exhibit rich emergent behavior at larges scales. For instance, a dense swarm of bacteria can behave collectively as a living liquid crystal, self-organize in complex regular patterns, exhibit turbulent-like motion, or ‘freeze’ into a solid-like biofilm. In this talk I will discuss theoretical work on active liquid crystals confined to two-dimensional planar and curved interfaces. I will show how the topology of the environment can create new types of collective behaviors in these systems, including complex dynamical patterns, self-sustained oscillations and topologically protected sound waves. |
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