Exploring the dynamics of phase separation in colloid-polymer mixtures with long range attraction.

We have studied the kinetics of phase separation and gel formation in a low-dispersity colloid - non-adsorbing polymer system with long range attraction using small-angle light scattering. This system exhibits two-phase and three-phase coexistence of gas, liquid and crystal phases when the strength of attraction is between 2 and 4kBT and gel phases when the strength of attraction is increased. For those samples that undergo macroscopic phase separation, whether to gas-crystal, gas-liquid or gas-liquid-crystal coexistence, we observe dynamic scaling of the structure factor and growth of a characteristic length scale that behaves as expected for phase separation in fluids. In samples that gel, the power law associated with the growth of the dominant length scale is not equal to 1/3, but appears to depend mainly on the strength of attraction, decreasing from 1/3 for samples near the coexistence region to 1/27 at 8kBT, over a wide range of colloid and polymer concentrations.

Vessel orientation-dependent sensitivity of optoacoustic imaging using a linear array transducer.

For clinical optoacoustic imaging, linear probes are preferably used because they allow versatile imaging of the human body with real-time display and free-hand probe guidance. The two-dimensional (2-D) optoacoustic image obtained with this type of probe is generally interpreted as a 2-D cross-section of the tissue just as is common in echo ultrasound. We demonstrate in three-dimensional simulations, phantom experiments, and in vivo mouse experiments that for vascular imaging this interpretation is often inaccurate. The cylindrical blood vessels emit anisotropic acoustic transients, which can be sensitively detected only if the direction of acoustic radiation coincides with the probe aperture. Our results reveal for this reason that the signal amplitude of different blood vessels may differ even if the vessels have the same diameter and initial pressure distribution but different orientation relative to the imaging plane. This has important implications for the image interpretation, for the probe guidance technique, and especially in cases when a quantitative reconstruction of the optical tissue properties is required.

Crystal-arrested phase separation.

We have studied the interplay between phase separation and crystallization in a colloid-polymer mixture along one kinetic pathway in samples which exhibit three-phase equilibrium coexistence. In analogy with atomic systems, the range of the effective attractive interaction between colloids is sufficiently long to allow for a stable liquid phase. By direct imaging in microgravity on the International Space Station, we observe a unique structure, a "crystal gel," that occurs when gas-liquid phase separation arrests due to crystallites within the liquid domain spanning the cell. From the initial onset of spinodal decomposition until arrest caused by this structure, the kinetics of phase separation remain largely unaffected by the formation of the third phase. This dynamic arrest appears to result from the stiffness of the crystalline strands exceeding the liquid-gas interfacial tension.

Investigating the microstructure of a yield-stress fluid by light scattering.

We report the results of an experimental study of the microstructure of dispersions of Carbopol ETD 2050, a model yield-stress fluid. Using two different light scattering instruments, measurements were made over three decades in scattering wave vector, from 0.02 to 25 µm⁻¹. These measurements reveal microstructure characterized by two length scales: a longer length scale, 6 µm and larger, that depends on Carbopol concentration and the pH of the dispersion and a shorter length scale of approximately 400 nm that is independent of both sample concentration and pH. We relate these results to shear rheology measurement of the yield stress of these materials.

Interaction of a charged polymer with zwitterionic lipid vesicles.

The interaction between polyethylenimine (PEI) and phospholipid bilayers plays an important role in several biophysical applications such as DNA transfection of target cells. Despite considerable investigation into the nature of the interaction between PEI and phospholipid bilayers, the physical process remains poorly understood. In this paper, we study the impact of PEI on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles as a function of salt concentration using several techniques including dynamic (DLS) and static (SLS) light scattering, differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR). At low salt concentration, vesicles aggregate, leading to the formation of stable clusters whose final size depends on the PEI concentration. At high salt concentration the system does not aggregate; DSC and NMR data reveal that the PEI penetrates into the bilayer, and SLS measurements are consistent with PEI crossing the bilayer. The transfectional ability of PEI is discussed in terms of these results.

Microrheology and structure of a yield-stress polymer gel.

The small-scale rheology of Carbopol ETD 2050, a polymer gel with a yield stress, is studied as a function of polymer concentration by measuring the diffusion of submicron-sized spherical fluorescent particles suspended in gel. Dynamic light scattering is used to determine the mean-squared displacement (of the particles as a function of lag time t. Fluorescence microscopy is used to track the particle trajectories directly, from which and the van Hove correlation function are determined. From our results we calculate the microrheological viscous and elastic moduli of the material. The two techniques cover complementary ranges of tau and and give results that agree well. The microrheological moduli are substantially smaller than the bulk values as determined by conventional shear rheometry. The bulk viscoelastic behavior is dominated by the elastic modulus, while at low enough concentrations and high enough frequencies the microrheological response is predominantly viscous. These results will be discussed in the context of the gel structure.