The Paradoxical Behavior of Rough Colloids at Fluid Interfaces.

Colloidal particles adsorb and remain trapped at immiscible fluid interfaces due to strong interfacial adsorption energy with a contact angle defined by the chemistry of the particle and fluid phases. An undulated contact line may appear due to either particle surface roughness or shape anisotropy, which results in a quadrupolar interfacial deformation and strong long-range capillary interaction between neighboring particles. While each effect has been observed separately, here we report the paradoxical impact of surface roughness on spherical and anisotropic ellipsoidal polymer colloids.

Effects of Ion Concentration and Headgroup Chemistry on Thin Lipid Film Drainage.

While the use of lipid nanoparticles in drug delivery applications has grown over the past few decades, much work remains to be done toward the characterization and rational design of the drug carriers. A key feature of delivery is the interaction of the exterior leaflet of the LNP with the outer leaflet of the cell membrane, which relies in part on the fusogenicity of the lipids and the ionic environment. In this paper, we study the interactions between two lipid monolayers using a thin film balance to create lipid thin films and interferometry to measure film evolution.

Effects of crowding on the diffusivity of membrane adhered particles.

The lateral diffusion of cell membrane inclusions, such as integral membrane proteins and bound receptors, drives critical biological processes, including the formation of complexes, cell-cell signaling, and membrane trafficking. These diffusive processes are complicated by how concentrated, or "crowded", the inclusions are, which can occupy between 30-50% of the area fraction of the membrane. In this work, we elucidate the effects of increasing concentration of model membrane inclusions in a free-standing artificial cell membrane on inclusion diffusivity and the apparent viscosity of the membrane.

Engineering the surface patchiness and topography of polystyrene colloids: From spheres to ellipsoids.

Hypothesis: Colloidal surface morphology determines suspension properties and applications. While existing methods are effective at generating specific features on spherical particles, an approach extending this to non-spherical particles is currently missing. Synthesizing un-crosslinked polymer microspheres with controlled chemical patchiness would allow subsequent thermomechanical stretching to translate surface topographical features to ellipsoidal particles.

Nanoscale Porosity in Microellipsoids Cloaks Interparticle Capillary Attraction at Fluid Interfaces.

Anisotropic particles pinned at fluid interfaces tend toward disordered multiparticle configurations due to large, orientationally dependent, capillary forces, which is a significant barrier to exploiting these particles to create functional self-assembled materials. Therefore, current interfacial assembly methods typically focus on isotropic spheres, which have minimal capillary attraction and no dependence on orientation in the plane of the interface. In order to create long-range ordered structures with complex configurations interfacially trapped anisotropic particles, control over the interparticle interaction energy external fields and/or particle engineering is necessary.

Determining the Bending Rigidity of Free-Standing Planar Phospholipid Bilayers.

We describe a method to determine membrane bending rigidity from capacitance measurements on large area, free-standing, planar, biomembranes. The bending rigidity of lipid membranes is an important biological mechanical property that is commonly optically measured in vesicles, but difficult to quantify in a planar, unsupported system. To accomplish this, we simultaneously image and apply an electric potential to free-standing, millimeter area, planar lipid bilayers composed of DOPC and DOPG phospholipids to measure the membrane Young's (elasticity) modulus.

Domain Size Regulation in Phospholipid Model Membranes Using Oil Molecules and Hybrid Lipids.

The formation of domains in multicomponent lipid mixtures has been suggested to play a role in moderating signal transduction in cells. Understanding how domain size may be regulated by both hybrid lipid molecules and impurities is important for understanding real biological processes; at the same time, developing model systems where domain size can be regulated is crucial to enable systematic studies of domain formation kinetics and thermodynamics. Here, we perform a model study of the effects of oil molecules, which swell the bilayer, and line-active hybrid phospholipids using a thermally induced liquid-solid phase separation in planar, free-standing lipid bilayers consisting of DOPC and DPPC (1,2-dioleoyl--glycero-3-phosphocholine and 1,2-dipalmitoyl--glycero-3-phosphocholine, respectively).

Fabrication and electromechanical characterization of freestanding asymmetric membranes.

All biological cell membranes maintain an electric transmembrane potential of around 100 mV, due in part to an asymmetric distribution of charged phospholipids across the membrane. This asymmetry is crucial to cell health and physiological processes such as intracell signaling, receptor-mediated endocytosis, and membrane protein function. Experimental artificial membrane systems incorporate essential cell membrane structures, such as the phospholipid bilayer, in a controllable manner in which specific properties and processes can be isolated and examined.

Gradient stretching to produce variable aspect ratio colloidal ellipsoids.

Developing reliable synthetic methods for producing shape-anisotropic polymer colloids is essential for their use in novel functional materials. In designing such materials from ellipsoidal particles, it is often necessary to screen a wide range of particle sizes and aspect ratios to appropriately understand how microscopic particle characteristics dictate macroscopic material response. Here, we describe a technique to simultaneously produce a broad range of aspect ratio polymer ellipsoid samples from a single synthetic step.

Toward Realistic Large-Area Cell Membrane Mimics: Excluding Oil, Controlling Composition, and Including Ion Channels.

Capacitance measurements provide unique insights into the thickness, compressibility, and composition of large-area membrane bilayers and are used here in addition to demonstrate the successful incorporation of model ion channels. The simultaneous ability to control the bilayer size, manipulate tension, and optically monitor and electrically stimulate freestanding membranes enables precise determination of their specific capacitance and thickness across a wide range of areas. We confirm that membranes formed by this recently developed technique have capacitive properties similar to those formed by existing protocols, including solvent-free approaches, and discuss the effect using either hexadecane or squalene as the oil solvent.

Arresting dissolution by interfacial rheology design.

A strategy to halt dissolution of particle-coated air bubbles in water based on interfacial rheology design is presented. Whereas previously a dense monolayer was believed to be required for such an "armored bubble" to resist dissolution, in fact engineering a 2D yield stress interface suffices to achieve such performance at submonolayer particle coverages. We use a suite of interfacial rheology techniques to characterize spherical and ellipsoidal particles at an air-water interface as a function of surface coverage.

Molecular, Local, and Network-Level Basis for the Enhanced Stiffness of Hydrogel Networks Formed from Coassembled Racemic Peptides: Predictions from Pauling and Corey.

Hydrogels prepared from self-assembling peptides are promising materials for medical applications, and using both l- and d-peptide isomers in a gel's formulation provides an intuitive way to control the proteolytic degradation of an implanted material. In the course of developing gels for delivery applications, we discovered that a racemic mixture of the mirror-image β-hairpin peptides, named MAX1 and DMAX1, provides a fibrillar hydrogel that is four times more rigid than gels formed by either peptide alone-a puzzling observation. Herein, we use transmission electron microscopy, small angle neutron scattering, solid state NMR, diffusing wave, infrared, and fluorescence spectroscopies, and modeling to determine the molecular basis for the increased mechanical rigidity of the racemic gel.

Colloidal Switches by Electric and Magnetic Fields.

External electric and magnetic fields have already been proven to be a versatile tool to control the particle assembly; however, the degree of control of the dynamics and versatility of the produced structures is expected to increase if both can be implemented simultaneously. For example, while micromagnets can rapidly assemble superparamagnetic particles, repeated, rapid disassembly or reassembly is not trivial because of the remanence and coercivity of metals used in such applications. Here, an interdigitated design of micromagnet and microfabricated electrodes enables rapid switching of colloids between their magnetic and electric potential minima.

Simple Optical Imaging of Nanoscale Features in Free-Standing Films.

Measuring thicknesses in thin films with high spatial and temporal resolution is of prime importance for understanding the structure and dynamics in thin films and membranes. In the present work, we introduce fluorescence-interferometry, a method that combines standard reflected light thin film interferometry with simultaneous fluorescence measurements. We apply this method to the thinning dynamics and phase separation in free-standing inverse phospholipid bilayer films.

Millimeter-area, free standing, phospholipid bilayers.

Minimal model biomembrane studies have the potential to unlock the fundamental mechanisms of cellular function that govern the processes upon which life relies. However, existing methods to fabricate free-standing model membranes currently have significant limitations. Bilayer sizes are often tens of micrometers, decoupling curvature or substrate effects, orthogonal control over tension, and solvent exchange combined with microscopy techniques is not possible, which restricts the studies that can be performed.

Anisotropic hypersonic phonon propagation in films of aligned ellipsoids.

A material with anisotropic elastic mechanical properties and a direction-dependent hypersonic band gap is fabricated using ac electric field-directed convective self-assembly of colloidal ellipsoids. The frequency of the gap, which is detected in the direction perpendicular to particle alignment and entirely absent parallel to alignment, and the effective sound velocities can be tuned by the particle aspect ratio. We hypothesize that the band gap originates from the primary eigenmode peak, the m-splitted (s,1,2) mode, of the particle resonating with the effective medium.

Dielectric spectroscopy of concentrated colloidal suspensions.

A comparison between experimental measurements and theoretical calculations of the permittivity and conductivity of concentrated colloidal suspensions is presented. Dielectric spectroscopy measurements for 100nm and 200nm diameter polystyrene spheres at volume fractions between ϕ=0.01-0.

Predicting the disorder-order transition of dielectrophoretic colloidal assembly with dielectric spectroscopy.

The dielectrophoretic assembly of colloidal suspensions into crystalline arrays is described by a master scaling that collapses the disorder-order transition as a function of field strength, frequency, and particle size. This master scaling has been verified for particle diameters ranging from 2a = 200 nm to 3 μm by light scattering (Lumsdon et al., Langmuir 2004, 20, 2108-2116; McMullan and Wagner, Langmuir 2012, 28, 4123-4130), optical laser tweezer measurements (Mittal et al.

Transition from dilute to concentrated electrokinetic behavior in the dielectric spectra of a colloidal suspension.

Dielectric spectroscopy is used to measure the complex permittivity of 200 and 100 nm diameter polystyrene latex suspended in potassium chloride (KCl) solutions over the frequency range 10(4)-10(7) Hz as a function of particle volume fraction (ϕ) and ionic strength. Dilute suspension dielectric spectra are in excellent agreement with electrokinetic theory. A volume fraction dependence of the dielectric increment is observed for low electrolyte concentrations (0.

Dielectric spectroscopy of bidisperse colloidal suspensions.

Dielectric spectroscopy is used to measure the complex permittivity of bidisperse colloidal suspensions over the frequency range 2.5 kHz ≤ ω/2π ≤ 10 MHz using the spectrometer design of Hollingsworth and Saville (A.D.

Adsorption of homopolypeptides on gold investigated using atomistic molecular dynamics.

We investigate the role of dynamics on adsorption of peptides to gold surfaces using all-atom molecular dynamics simulations in explicit solvent. We choose six homopolypeptides [Ala(10), Ser(10), Thr(10), Arg(10), Lys(10), and Gln(10)], for which experimental surface coverages are not correlated with amino acid level affinities for gold, with the idea that dynamic properties may also play a role. To assess dynamics we determine both conformational movement and flexibility of the peptide within a given conformation.