Diffusing Wave Spectroscopy Measurements of Colloidal Suspension Dynamics.

Diffusing wave spectroscopy (DWS) is used to measure the dynamics of charged silica particles between the volume fractions 0.065 ≤ ϕ ≤ 0.352 (weight percentages from 12.7 to 55.8 wt %). The short-time diffusivity averaged over the scattering vectors sampled by DWS D¯(ϕ) decreases with an increasing concentration. An effective hard-sphere model that accounts for hydrodynamic interactions and a double-layer repulsion fits the values up to an effective volume fraction ϕeff=ϕb^3≈0.6, where b^ is the excluded shell radius normalized by the particle radius b^ = b/a = 1.3. While DWS measurements of diffusivity are sensitive to repulsive interactions, we show that they are relatively insensitive to attraction, such as those due to secondary minima in the interaction potential or weak depletion interaction.

Combined Effects of Pressure and Ionic Strength on Protein-Protein Interactions: An Empirical Approach.

Proteins are exposed to hydrostatic pressure (HP) in a variety of ecosystems as well as in processing steps such as freeze-thaw, cell disruption, sterilization, and homogenization, yet pressure effects on protein-protein interactions (PPIs) remain underexplored. With the goal of contributing toward the expanded use of HP as a fundamental control parameter in protein research, processing, and engineering, small-angle X-ray scattering was used to examine the effects of HP and ionic strength on ovalbumin, a model protein. Based on an extensive data set, we develop an empirical method for scaling PPIs to a master curve by combining HP and osmotic effects. We define an effective pressure parameter that has been shown to successfully apply to other model protein data available in the literature, with deviations evident for proteins that do not follow the apparent Hofmeister series. The limitations of the empirical scaling are discussed in the context of the hypothesized underlying mechanisms.

Antibodies Adsorbed to the Air-Water Interface Form Soft Glasses.

When monoclonal antibodies are exposed to an air-water interface, they form aggregates, which negatively impacts their performance. Until now, the detection and characterization of interfacial aggregation have been difficult. Here, we exploit the mechanical response imparted by interfacial adsorption by measuring the interfacial shear rheology of a model antibody, anti-streptavidin immunoglobulin-1 (AS-IgG1), at the air-water interface. Strong viscoelastic layers of AS-IgG1 form when the protein is adsorbed from the bulk solution. Creep experiments correlate the compliance of the interfacial protein layer with the subphase solution pH and bulk concentration. These, along with oscillatory strain amplitude and frequency sweeps, show that the viscoelastic behavior of the adsorbed layers is that of a soft glass with interfacial shear moduli on the order of 10-3 Pa m. Shifting the creep compliance curves under different applied stresses forms master curves consistent with stress-time superposition of soft interfacial glasses. The interfacial rheology results are discussed in the context of the interface-mediated aggregation of AS-IgG1.

Yield Stress Aging in Attractive Colloidal Suspensions.

We investigate the origin of yield stress aging in semidense, saline, and turbid suspensions in which structural evolution is rapidly arrested by the formation of thermally irreversible roll-resisting interparticle contacts. By performing optical tweezer three-point bending tests on particle rods, we show that these contacts yield by overcoming a rolling threshold, the critical bending moment of which grows logarithmically with time. We demonstrate that this time-dependent contact-scale rolling threshold controls the suspension yield stress and its aging kinetics. We identify a simple constitutive relation between the contact-scale flexural rigidity and rolling threshold, which transfers to macroscopic scales. This leads us to establishing a constitutive relation between macroscopic shear modulus and yield stress that is generic for an array of colloidal systems.

Quantization of Acoustic Modes in Dumbbell Nanoparticles.

The vibrational eigenmodes of dumbbell-shaped polystyrene nanoparticles are recorded by Brillouin light spectroscopy (BLS), and the full experimental spectra are calculated theoretically. Different from spheres with a degeneracy of (2l+1), with l being the angular momentum quantum number, the eigenmodes of dumbbells are either singly or doubly degenerate owing to their axial symmetry. The BLS spectrum reveals a new, low-frequency peak, which is attributed to the out-of-phase vibration of the two lobes of the dumbbell. The quantization of acoustic modes in these molecule-shaped dumbbell particles evolves from the primary colloidal spheres as the separation between the two lobes increases.

Contact and macroscopic ageing in colloidal suspensions.

The ageing behaviour of dense suspensions or pastes at rest is almost exclusively attributed to structural dynamics. Here, we identify another ageing process, contact-controlled ageing, consisting of the progressive stiffening of solid-solid contacts of an arrested colloidal suspension. By combining rheometry, confocal microscopy and particle-scale mechanical tests using laser tweezers, we demonstrate that this process governs the shear-modulus ageing of dense aqueous silica and polymer latex suspensions at moderate ionic strengths. We further show that contact-controlled ageing becomes relevant as soon as Coulombic interactions are sufficiently screened out that the formation of solid-solid contacts is not limited by activation barriers. Given that this condition only requires moderate ion concentrations, contact-controlled ageing should be generic in a wide class of materials, such as cements, soils or three-dimensional inks, thus questioning our understanding of ageing dynamics in these systems.

Temperature Dependence of Protein Solution Viscosity and Protein-Protein Interactions: Insights into the Origins of High-Viscosity Protein Solutions.

Protein solution viscosity (η) as a function of temperature was measured at a series of protein concentrations under a range of formulation conditions for two monoclonal antibodies (MAbs) and a globular protein (aCgn). Based on theoretical arguments, a strong temperature dependence for protein-protein interactions (PPI) indicates highly anisotropic, short-ranged attractions that could lead to higher solution viscosities. The semi-empirical Ross-Minton model was used to determine the apparent intrinsic viscosity, shape, and "crowding" factors for each protein as a function of temperature and formulation conditions. The apparent intrinsic viscosity was independent of temperature for aCgn, while a slight decrease with increasing temperature was observed for the MAbs. The temperature dependence of solution viscosity was analyzed using the Andrade-Eyring equation to determine the effective activation energy of viscous flow (Ea,η). While Ea,η values were different for each protein, they were independent of formulation conditions for a given protein. PPI were quantified via the osmotic second virial coefficient (B22) and the protein diffusion interaction parameter (kD) as a function of temperature under the same formulation conditions as the viscosity measurements. Net interactions ranged from strongly attractive to repulsive by changing formulation pH and ionic strength for each protein. Overall, larger activation energies for PPI corresponded to larger activation energies for η, and those were predictive of the highest η values at higher protein concentrations.

An Expanded State Diagram for the Directed Self-Assembly of Colloidal Suspensions in Toggled Fields.

The suspension structure and assembly kinetics of micrometer-diameter paramagnetic spheres in toggled magnetic fields are investigated at a constant field strength H = 1750A·m-1 while toggling the field on and off over the frequency range 0.3

Magnetite nanoparticles program the assembly, response, and reconfiguration of structured emulsions.

Endoskeletal droplets-non-spherical emulsion droplets that respond to external stimuli with shape change-are modified with ferromagnetic iron oxide nanoparticles to make them susceptible to magnetic fields. The resulting droplets can be manipulated using static or oscillating magnetic fields, each producing a different response. Static fields control the orientation and position of the droplets, important in driving assembly into organized structures. Oscillating fields are shown to cause magnetic hyperthermia in ferrofluid nanoparticles, leading to droplet heating and forcing droplet reconfiguration. We demonstrate the use of static and dynamic fields to affect the organization and stability of endoskeletal droplets and their assemblies, producing highly-tunable programmable colloids that adapt to changing environmental conditions.

Viscosities and Protein Interactions of Bispecific Antibodies and Their Monospecific Mixtures.

Solution viscosities (η) and protein-protein interactions (PPI) of three monoclonal antibodies (mAb-A, mAb-B, mAb-C), two bispecific antibodies (BsAb-A/B, BsAb-A/C), and two 1:1 binary mixtures (mAb-A + mAb-B and mAb-A + mAb-C) were measured. mAb-A and mAb-C have similar isoelectric point (pI) values but significantly different η versus protein concentration ( c2) profiles. The viscosity of the mAb-A + mAb-C mixture followed an Arrhenius mixing rule and was identical to viscosity of the bispecific BsAb-A/C. In contrast, mAb-A and mAb-B had similar η versus c2 profiles, but the Arrhenius mixing rule failed to predict the higher viscosities of their mixtures. The viscosity of the bispecific BsAb-A/B followed the 1:1 mAb-A + mAb-B mixture at all concentrations. The nature of the interactions for mAb-A, mAb-B, the BsAb-A/B bispecific, and the 1:1 mAb-A + mAb-B mixture were characterized by static and dynamic light scattering (SLS and DLS). mAb-A and mAb-B exhibited net-attractive and -repulsive electrostatic interactions, respectively. The bispecific antibody (BsAb-A/B) had short-ranged attractive interactions, suggesting that the increase in viscosity for this molecule and the mAb-A + mAb-B mixture was due to cross-interactions between Fab regions. At high and low ionic strengths and protein concentrations, the Rayleigh scattering profile, the collective diffusion coefficient, and viscosity for the mixture closely followed that for the bispecific antibody. These results highlight the possible anomalous viscosity increases of bispecific antibodies constructed from relatively low-viscosity mAbs but demonstrates a potentially fruitful approach of using mAb mixtures to predict the viscosity of candidate bispecific constructs.

Model of Structured Emulsion Droplet Stability and Reconfigurability.

A model of internally structured emulsion droplets is presented that accounts for the traction forces generated by interfacial tension and the von Mises yield criterion of the internal supporting network. For symmetric droplets, the method calculates the total stress acting on a droplet locally, allowing droplet stability and location of failure to be predicted. It is not regions of high interfacial curvature that prompt droplet reconfiguration, rather regions transitioning from high to low curvature. The model enables the design of emulsion droplet response and reconfigurability to external triggers such as changes in surface tension (surfactant concentration) and temperature.

Crystallization Kinetics of Partially Crystalline Emulsion Droplets in a Microfluidic Device.

We measure the crystallization kinetics of petrolatum-hexadecane emulsion droplets as they are produced in a microfluidic device. After droplets form, they are cooled, causing an interior network of wax crystallites to grow. Polarized light microscopy is used to quantify the droplet crystallinity as a function of residence time in the device. Two wavelengths and two polarization orientations are used to decouple the wavelength dependence of the optical retardation, the crystallite orientation, and the crystallite number density. The droplet crystallinity follows the Avrami kinetic model with parameter values in agreement with the theoretically expected values. These results provide a means to engineer the crystallization kinetics, stability, and arrested coalescence of partially crystalline emulsion droplets.

Suppressing the Rayleigh-Plateau Instability in Field-Directed Colloidal Assembly.

Suspensions of superparamagnetic colloids that equilibrate in a toggled magnetic field undergo a Rayleigh-Plateau instability with a characteristic wavelength λ = 600 µm for the toggle frequency ν = 0.66 Hz. The instability is suppressed when the chamber length L in the field direction is less than 2λ. The final size of the magnetic domains perpendicular to the field, D, follows a power law relation of D ∼ L(0.71±0.07). These results demonstrate the structural differences of field-directed suspensions when confined to lengths scale set by the phase separation process and can potentially be used to create self-assembled colloidal crystals with well-defined size and shape.

Temperature-Induced Collapse, and Arrested Collapse, of Anisotropic Endoskeleton Droplets.

Micron-scale rod-shaped droplets with a range of aspect ratios are produced using extrusion of oil containing a soft wax crystal network to permit shape customization. A physical model of the droplet shape stability is developed based on balancing interfacial stresses with the internal crystal network yield stress. The model predicts the mechanical properties required for particular droplet size stability, in a given physicochemical environment, and is tested by microscopic observations of droplets over a range of relevant applied temperatures. The time-dependent response to temperature of individual rods is monitored and used to identify the collapse temperature based on structural yielding. Precise temperature control allows variation of the droplet endoskeleton yield stress and direct determination of the droplet stability as a function of size, by observing the onset of collapse by interfacial compression, and enables validation of the model predictions. Mapping the regions of droplet stability and instability for various-sized droplets yields a basis for designing droplet shapes for multiple applications using easily measured physical variables. The phenomenon of arrested collapse is also explored as a means of transforming simple rod-shaped starting materials into more complex shapes and enhancing adhesion to targeted solid surfaces, enabling exploitation of the hybrid solid-liquid nature of these droplets.

Lateral capillary interactions between colloids beneath an oil-water interface that are driven by out-of-plane electrostatic double-layer interactions.

We study the lateral capillary interactions between colloids beneath an oil-water interface that lead to closely packed two-dimensional self-assembled colloidal crystals. These capillary forces are caused by the overlap of deformed interfaces above colloids on a solid substrate. The interface deformation is due to the electrostatic disjoining pressure between the charged particles and the charged oil-water interface. It is notable that the short-range (i.e., on the nanometer scale) and out-of-plane electrostatic double-layer interactions, which occur through an aqueous phase, can generate the long-range lateral capillary attraction (i.e., on the micrometer scale).

Coarsening mechanics of a colloidal suspension in toggled fields.

Suspensions of paramagnetic colloids are driven to phase separate and self-assemble in toggled magnetic fields. At field strengths above 575 A/m and toggle frequencies between 0.66 and 2 Hz, an initial gel-like, arrested network collapses into condensed, ellipsoidal aggregates. The evolution to this equilibrium structure occurs via a Rayleigh-Plateau instability. The toggle frequency ν determines the fluidity of the breakup process. At frequencies between 0.66 and 1.5 Hz, the suspension breaks up similar to a viscous, Newtonian fluid. At frequencies ν > 1.5 Hz, the network ruptures like a viscoplastic material. The field strength alters the onset time of the instability. A power law relationship emerges as the scaled frequency and field strength can be used to predict the onset of breakup. These results further aid in understanding the mechanics and dynamics of the phase separation process of suspensions of polarizable colloids in toggled external fields.

Multi-scale kinetics of a field-directed colloidal phase transition.

Polarizable colloids are expected to form crystalline equilibrium phases when exposed to a steady, uniform field. However, when colloids become localized this field-induced phase transition arrests and the suspension persists indefinitely as a kinetically trapped, percolated structure. We anneal such gels formed from magneto-rheological fluids by toggling the field strength at varied frequencies. This processing allows the arrested structure to relax periodically to equilibrium--colloid-rich, cylindrical columns. Two distinct growth regimes are observed: one in which particle domains ripen through diffusive relaxation of the gel, and the other where the system-spanning structure collapses and columnar domains coalesce apparently through field-driven interactions. There is a stark boundary as a function of magnetic field strength and toggle frequency distinguishing the two regimes. These results demonstrate how kinetic barriers to a colloidal phase transition are subverted through measured, periodic variation of driving forces. Such directed assembly may be harnessed to create unique materials from dispersions of colloids.

Buckling instability of self-assembled colloidal columns.

Suspended, slender self-assembled domains of magnetically responsive colloids are observed to buckle in microgravity. Upon cessation of the magnetic field that drives their assembly, these columns expand axially and buckle laterally. This phenomenon resembles the buckling of long beams due to thermal expansion; however, linear stability analysis predicts that the colloidal columns are inherently susceptible to buckling because they are freely suspended in a Newtonian fluid. The dominant buckling wavelength increases linearly with column thickness and is quantitatively described using an elastohydrodynamic model and the suspension thermodynamic equation of state.

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. These results reveal the potential for powerful control of the hypersonic phononic band diagram by combining anisotropic particles and self-assembly.

Effects of coating on the optical trapping efficiency of microspheres via geometrical optics approximation.

We present the optical trapping forces that are generated when a single laser beam strongly focuses on a coated dielectric microsphere. On the basis of geometrical optics approximation (GOA), in which a particle intercepts all of the rays that make up a single laser beam, we calculate the trapping forces with varying coating thickness and refractive index values. To increase the optical trapping efficiency, the refractive index (n(b)) of the coating is selected such that n(a) < n(b) < n(c), where na and nc are the refractive indices of the medium and the core material, respectively. The thickness of the coating also increases trapping efficiency. Importantly, we find that trapping forces for the coated particles are predominantly determined by two rays: the incident ray and the first refracted ray to the medium.