Category Archives: liquid crystals

Colloidal aggregation in liquid crystal

Colloidal aggregation in anisotropic liquid crystal solvent

Devika G. Sudha, Jocelyn Ochoa and Linda S. Hirst, Soft Matter (2021) 

The mutual attraction between colloidal particles in an anisotropic fluid, such as the nematic liquid crystal phase, leads to the formation of hierarchical aggregate morphologies distinct from those that tend to form in isotropic fluids. Previously it was difficult to study this aggregation process for a large number of colloids due to the difficulty of achieving a well dispersed initial colloid distribution under good imaging conditions. In this paper, we report the use of a recently developed self-assembling colloidal system to investigate this process. Hollow, micron-scale colloids are formed in situ in the nematic phase and subsequently aggregate to produce fractal structures and colloidal gels, the structures of which are determined by colloid concentration and temperature quench depth through the isotropic to nematic phase transition point. This self-assembling colloidal system provides a unique method to study particle aggregation in liquid crystal over large length scales. We use fluorescence microscopy over a range of length scales to measure aggregate structure as a function of temperature quench depth, observe ageing mechanisms and explore the driving mechanisms in this unique system. Our analyses suggest that aggregate dynamics depend on a combination of Frank elasticity relaxation, spontaneous defect line annihilation and internal aggregate fracturing.

Topological Chaos in active nematics

Topological chaos in active nematics, Amanda J. Tan, Eric Roberts, Spencer Smith, Ulyses Alvarado, Jorge Arteaga, Sam Fortini, Kevin Mitchell, and Linda S. Hirst, Aug 5th, NATURE PHYSICS (2019) Link

Abstract: Active nematics are out-of-equilibrium fluids composed of rod-like subunits, which can generate large-scale, self-driven flows. We examine a microtubule-kinesin-based active nematic confined to two dimensions, exhibiting chaotic flows with moving topological defects. Applying tools from chaos theory, we investigate self-driven advection and mixing on different length scales. Local fluid stretching is quantified by the Lyapunov exponent. Global mixing is quantified by the topological entropy, calculated from both defect braiding and curve extension rates. We find excellent agreement between these independent mea-sures of chaos, demonstrating that the extensile stretching between microtubules directly translates into macroscopic braiding of positive defects. Remarkably, increasing extensile activity (through ATP concentration) does not increase the dimensionless topological entropy. This study represents an application of chaotic advection to the emerging field of active nematics and quantification of the collective motion of an ensemble of defects (through topological entropy) in a liquid crystal.

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Fluorescence microscope image of the active nematic phase

The full text of the paper can be found here

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E-tec braiding algorithm used on experimentally collected defect trajectory data

Nanofoams – Nature communications

Nanoparticle-based hollow microstructures formed by two-stage nematic nucleation and phase separation

Sheida T. Riahinasab, Amir Keshavarz, Charles N. Melton, Ahmed Elbaradei, Gabrielle I. Warren, Robin L. B. Selinger, Benjamin J. Stokes & Linda S. Hirst 

Nature Communications volume 10, Article number: 894 (2019)

read the paper here

Rapid bulk assembly of nanoparticles into microstructures is challenging, but highly desirable for applications in controlled release, catalysis, and sensing. We report a method to form hollow microstructures via a two-stage nematic nucleation process, generating size-tunable closed-cell foams, spherical shells, and tubular networks composed of closely packed nanoparticles. Mesogen-modified nanoparticles are dispersed in liquid crystal above the nematic-isotropic transition temperature (TNI). On cooling through TNI, nanoparticles first segregate into shrinking isotropic domains where they locally depress the transition temperature. On further cooling, nematic domains nucleate inside the nanoparticle-rich isotropic domains, driving formation of hollow nanoparticle assemblies. Structural differentiation is controlled by nanoparticle density and cooling rate. Cahn-Hilliard simulations of phase separation in liquid crystal demonstrate qualitatively that partitioning of nanoparticles into isolated domains is strongly affected by cooling rate, supporting experimental observations that cooling rate controls aggregate size. Microscopy suggests the number and size of internal voids is controlled by second-stage nucleation.

Congratulations Dr Melton!

Nathan Melton, our most recent graduate walked at UC Merced commencement this month. Nathan’s recent work, focused on topological defects in liquid crystals with a new paper just out last month in the journal Nanomaterials.

“Phase-transition-driven nanoparticle assembly in liquid crystal droplets” Charles N. Melton, Sheida T. Riahinasab, Amir Keshavarz, Benjamin J. Stokes and Linda S. Hirst, Nanomaterials, 8, 146 (2018). Link

Dr Melton and Dr Hirst

Nathan has already started work as a postdoctoral scientist at Lawrence Berkeley National Lab at the Advanced Light Source!

Kyle Kabasaras, CAMP summer student, presents research at UC Merced Symposium

This summer Kyle Kabasaras presented his original research project at the UC Merced undergrad research symposium.

Investigating quantum dot assembly in a cholesteric liquid crystal

An ongoing goal in condensed matter physics is directly controlling the self-assembly of quantum dots (QDs) into specific structures while maintaining their original electronic and optical properties. One method of controlling the self-assembly of QDs is to disperse them within a liquid crystal (LC) medium and apply a variety of thermal stimulations. Recently, our lab developed a method of creating spherical, vesicle-shaped QDs within a nematic LCVesicle formation depends on the QD concentration in the LC as well as the LC’s intermolecular dispersion forces and thermal properties. In this project, we investigate the dispersion of CdSe/ZnS (core/shell) QDs in a cholesteric LC (CLC) medium and predict the QD aggregations to cluster near the LC defects. By varying parameters such as QD concentration and temperature, we exploit the CLC’s sensitive optical and thermal propertiesTo observe these effects, we apply spectrophotometry, polarized optical microscopy, and fluorescence microscopy. These techniques highlight the aggregation of QDs within the host CLC and identify how LC phase transitions determine where QDaggregates form. This work illustrates the possibility of new LC-based QD devices, and we will continue by exploring the lasing potential of our sample.

CAMP is a National Science Foundation funded project to promote undergraduate research participation.




A plasmon induced liquid crystal device

“All optical switching of nematic liquid crystal films driven by localized surface plasmons” M.T. Quint, S. Delgado, Z.S. Nuno, L.S. Hirst and S. Ghosh, OPTICS EXPRESS, 23,5, 6888 (2015) Link

We have demonstrated an all-optical technique for reversible in-plane and out-of-plane switching of nematic liquid crystal molecules in few micron thick films. Our method leverages the highly localized electric fields (“hot spots”) and plasmonic heating that are generated in the near-field region of densely packed gold nanoparticle layers optically excited on-resonance with the localized surface plasmon absorption. Using polarized microscopy and transmission measurements, we observe this switching from homeotropic to planar over a temperature range starting at room temperature to just below the isotropic transition, and at on-resonance excitation intensity less than 0.03 W/cm2. In addition, we controllably vary the in-plane directionality of the liquid crystal molecules in the planar state by altering the linear polarization of the incident excitation. Using discrete dipole simulations and control measurements, we establish spectral selectivity in this new and interesting perspective for photonic application using low light power.

Quantum dot micro-shells

Self-assembled nanoparticle micro-shells templated by liquid crystal sorting”A. R. Rodarte, B.H. Cao, H. Panesar, R.J. Pandolfi, M. Quint, L. Edwards, S. Ghosh, J.E. Hein and L.S. Hirst, Soft Matter, 10.1039/C4SM02326A (2015) Link

A current goal in nanotechnology focuses on the assembly of different nanoparticle types into 3D organized structures. In this paper we report the use of a liquid crystal host phase in a new process for the generation of micron-scale vesicle-like nanoparticle shells stabilized by ligand–ligand interactions. The constructs formed consist of a robust, thin spherical layer, composed of closely packed quantum dots (QDs) and stabilized by local crystallization of the mesogenic ligands. Ligand structure can be tuned to vary QD packing within the shell and made UV cross-linkable to allow for intact shell extraction into toluene. The assembly method we describe could be extended to other nanoparticle types (metallic, magnetic etc.), where hollow shell formation is controlled by thermally sorting mesogen-functionalized nanoparticles in a liquid crystalline host material at the isotropic to nematic transition. This process represents a versatile method for making non-planar 3D nano-assemblies.

Magnetic field induced quantum dot brightening

Magnetic field induced quantum dot brightening in liquid crystal synergized magnetic and semiconducting nanoparticle composite assemblies” 
Jose Jussi Amaral,   Jacky Wan,   Andrea L. Rodarte,  Christopher Ferri,   Makiko T. Quint,  Ronald J. Pandolfi, Michael Scheibner,  Linda S. Hirst and   Sayantani Ghosh, Soft Matter (2014)

The design and development of multifunctional composite materials from artificial nano-constituents is one of the most compelling current research areas. This drive to improve over nature and produce ‘meta-materials’ has met with some success, but results have proven limited with regards to both the demonstration of synergistic functionalities and in the ability to manipulate the material properties post-fabrication and in situ. Here, magnetic nanoparticles (MNPs) and semiconducting quantum dots (QDs) are co-assembled in a nematic liquid crystalline (LC) matrix, forming composite structures in which the emission intensity of the quantum dots is systematically and reversibly controlled with a small applied magnetic field (<100 mT). This magnetic field-driven brightening, ranging between a two- to three-fold peak intensity increase, is a truly cooperative effect: the LC phase transition creates the co-assemblies, the clustering of the MNPs produces LC re-orientation at atypical low external field, and this re-arrangement produces compaction of the clusters, resulting in the detection of increased QD emission. These results demonstrate a synergistic, reversible, and an all-optical process to detect magnetic fields and additionally, as the clusters are self-assembled in a fluid medium, they offer the possibility for these sensors to be used in broad ranging fluid-based applications.