Colloids On Lipid Bilayers: Deformations, Interactions And Migration

In this thesis, I focus on studying interaction between colloidal particles and lipid bilayers. Inspired by proteins that generate membrane curvature, sense the underlying membrane geometry, and migrate driven by curvature gradients, we explore the question: Can colloids, adhered to lipid bilayers, also sense and respond to membrane geometry? In the first part of the thesis, I report experimental results of homogeneous nanoparticles and microparticles on lipid bilayers. Charged nanoparticles were used to study the dependence on tension of particle wrapping by bilayer membranes. The particle wrapping process is a competition between adhesion energy on the particle/lipid interface, and the energy cost to deform the membrane. I found that when membrane tension was below 0.27 mN/m, the apparent area of an aspirated giant unilamellar vesicle (GUV) decreased during nanoparticles binding, likely due to wrapping of particles by the membrane. This area decrease was eliminated by increasing the membrane tension. I also report results on pair interactions between streptavidin-coated microparticles bound to biotinylated GUVs. A preferred separation distance was found between pairs of particles, and an interaction potential energy on the order of thermal fluctuations was found. To control the degree of wrapping systematically, I used Janus microparticles with two different surface properties on each of the hemisphere. I report the migration of Janus microparticles adhered to giant unilamellar vesicles elongated to present spatially varying principal curvatures. In experiments, colloids migrated on these vesicles toward sites of high deviatoric curvature. This migration occurred only when the membranes were tense, suggesting that they migrate to minimize membrane area. By determining the energy dissipated along a trajectory, the energy field was inferred to depend linearly on the local deviatoric curvature, like curvature driven capillary migration on interfaces between immiscible fluids. In this latter system, energy gradients were larger, so colloids move deterministically, whereas the paths traced by colloids on vesicles had significant fluctuations. By addressing the role of Brownian motion, I show that the observed migration is analogous to curvature driven capillary migration, with membrane tension playing the role of interfacial tension. Since this motion is mediated by membrane shape, it can be turned on and off by dynamically deforming the vesicle. While particle-particle interactions on lipid membranes have been considered in many contributions, I report here an exciting and previously unexplored modality to actively direct the migration of colloids to desired locations on lipid bilayers

Lassoing saddle splay and the geometrical control of topological defects

Systems with holes, such as colloidal handlebodies and toroidal droplets, have been studied in the nematic liquid crystal (NLC) 4-cyano-4'-pentylbiphenyl (5CB): both point and ring topological defects can occur within each hole and around the system, while conserving the system's overall topological charge. However, what has not been fully appreciated is the ability to manipulate the hole geometry with homeotropic (perpendicular) anchoring conditions to induce complex, saddle-like deformations. We exploit this by creating an array of holes suspended in an NLC cell with oriented planar (parallel) anchoring at the cell boundaries. We study both 5CB and a binary mixture of bicyclohexane derivatives (CCN-47 and CCN-55). Through simulations and experiments, we study how the bulk saddle deformations of each hole interact to create novel defect structures, including an array of disclination lines, reminiscent of those found in liquid crystal blue phases. The line locations are tunable via the NLC elastic constants, the cell geometry, and the size and spacing of holes in the array. This research lays the groundwork for the control of complex elastic deformations of varying length scales via geometrical cues in materials that are renowned in the display industry for their stability and easy manipulability.Comment: 9 pages, 7 figures, 1 supplementary figur

Shaping nanoparticle fingerprints at the interface of cholesteric droplets

The ordering of nanoparticles into predetermined configurations is of importance to the design of advanced technologies. In this work, we moderate the surface anchoring against the bulk elasticity of liquid crystals to dynamically shape nanoparticle assemblies at a fluid interface. By tuning the degree of nanoparticle hydrophobicity with surfactants that alter the molecular anchoring of liquid crystals, we pattern nanoparticles at the interface of cholesteric liquid crystal emulsions. Adjusting the particle hydrophobicity more finely further modifies the rigidity of assemblies. We establish that patterns are tunable by varying both surfactant and chiral dopant concentrations. Since particle assembly occurs at the interface with the desired structures exposed to the surrounding phase, we demonstrate that particles can be readily crosslinked and manipulated, forming structures that retain their shape under external perturbations. This study establishes the templating of nanomaterials into reconfigurable arrangements. Interfacial assembly is tempered by elastic patterns that arise from the geometric frustration of confined cholesterics. This work serves as a basis for creating materials with chemical heterogeneity and with linear, periodic structures, essential for optical and energy applications.Comment: 16 pages with 5 figures, 4 page supplementary with 5 supplementary figure

Biomechanical microenvironment regulates fusogenicity of breast cancer cells

Fusion of cancer cells is thought to contribute to tumor development and drug resistance. The low frequency of cell fusion events and the instability of fused cells have hindered our ability to understand the molecular mechanisms that govern cell fusion. We have demonstrated that several breast cancer cell lines can fuse into multinucleated giant cells in vitro, and the initiation and longevity of fused cells can be regulated solely by biophysical factors. Dynamically tuning the adhesive area of the patterned substrates, reducing cytoskeletal tensions pharmacologically, altering matrix stiffness, and modulating pattern curvature all supported the spontaneous fusion and stability of these multinucleated giant cells. These observations highlight that the biomechanical microenvironment of cancer cells, including the matrix rigidity and interfacial curvature, can directly modulate their fusogenicity, an unexplored mechanism through which biophysical cues regulate tumor progression

Change in Stripes for Cholesteric Shells via Anchoring in Moderation

Chirality, ubiquitous in complex biological systems, can be controlled and quantified in synthetic materials such as cholesteric liquid crystal (CLC) systems. In this work, we study spherical shells of CLC under weak anchoring conditions. We induce anchoring transitions at the inner and outer boundaries using two independent methods: by changing the surfactant concentration or by raising the temperature close to the clearing point. The shell confinement leads to new states and associated surface structures: a state where large stripes on the shell can be filled with smaller, perpendicular substripes, and a focal conic domain (FCD) state, where thin stripes wrap into at least two, topologically required, double spirals. Focusing on the latter state, we use a Landau–de Gennes model of the CLC to simulate its detailed configurations as a function of anchoring strength. By abruptly changing the topological constraints on the shell, we are able to study the interconversion between director defects and pitch defects, a phenomenon usually restricted by the complexity of the cholesteric phase. This work extends the knowledge of cholesteric patterns, structures that not only have potential for use as intricate, self-assembly blueprints but are also pervasive in biological systems

Removal of Strontium Ions in Aqueous Solution by Polymer Enhanced Ultra-filtration

It is shown in many laboratories and pilot plants that membrane techniques are more effective than other traditional treatments for low level radioactive liquid waste. By applying membrane techniques, energy consumption can be reduced and better removal rate can be achieved. In this project, research on a polymer enhanced ultra-filtration process for strontium ions removal was conducted. A dead end ultra-filtration system was used. Operating parameters such as feed pH values and polymer loading ratio were varied in order to optimize the system. Up to 10% of strontium ions in the feed were removed and the system can be further improved

Reduce, Reuse, and Replace: A Study on Solutions to Plastic Wastes

This project aimed to examine the quantities of plastic wastes discarded and the fate of various plastics in the environment. Our Interactive Qualifying Project was concerned with researching the possible alternatives to plastics, mainly biodegradable plastics that have been claimed to biodegrade and help environment. Moreover, it was also required to evaluate the best methods to deal with our plastic wastes that are environmentally, economically, and energy-wise viable

Multiple Stimuli-Responsive Fluorescence Behavior of Novel Polyamic Acid Bearing Oligoaniline, Triphenylamine, and Fluorene Groups

Multiple stimuli-responsive fluorescent materials have gained increasing attention for their fundamental investigation and intelligent applications. In this work, we report design and synthesis of a novel polyamic acid bearing oligoaniline, triphenylamine, and fluorene groups, which served as sensitive units and fluorescence emission unit, respectively. The resulting polymer exhibits multiple stimuli-responsive fluorescence switching behavior triggered by redox species, pH, electrochemical, and pressure stimuli. Every fluorescence switching mechanism upon each stimulus was studied in detail. The interactions and energy transfer between sensitive units and emission unit are largely responsible for this fascinating fluorescent switching behavior. This work provides a deep understanding of the optical switching essence upon these stimuli, opening the way for the development of new fluorescent sensing applications