Compact laser light-scattering instrument for microgravity research

Patchy colloidal gels under the influence of gravity

In this contribution, gravitational effects in gel-forming patchy colloidal systems are studied. We focus on how the gel structure is modified by gravity. Through Monte Carlo computer simulations of gel-like states recently identified by the rigidity percolation criterion [J. A. S. Gallegos et al., Phys. Rev. E 104, 064606 (2021)], the influence of the gravitational field, characterized by the gravitational Péclet number, Pe, on patchy colloids is studied in terms of the patchy coverage, χ. Our findings point out that there exists a threshold Péclet number, Pe_g, that depends on χ above which the gravitational field enhances the particle bonding and, in consequence, promotes the aggregation or clustering of particles; the smaller the χ value, the higher the Pe_g. Interestingly, when χ ∼ 1 (near the isotropic limit), our results are consistent with an experimentally determined threshold Pe value where gravity affects the gel formation in short-range attractive colloids. In addition, our results show that the cluster size distribution and the density profile undergo variations that lead to changes in the percolating cluster, i.e., gravity is able to modify the structure of the gel-like states. These changes have an important impact on the structural rigidity of the patchy colloidal dispersion; the percolating cluster goes from a uniform spatially network to a heterogeneous percolated structure, where an interesting structural scenario emerges, namely, depending on the Pe value, the new heterogeneous gel-like states can coexist with both diluted and dense phases or they simply reach a crystalline-like state. In the isotropic case, the increase in the Pe number can shift the critical temperature to higher temperatures; however, when Pe > 0.01, the binodal disappears and the particles fully sediment at the bottom of the sample cell. Furthermore, gravity moves the rigidity percolation threshold to lower densities. Finally, we also note that within the values of the Péclet number here explored, the cluster morphology is barely altered.

Microgravity Removes Reaction Limits from Nonpolar Nanoparticle Agglomeration

Gravity can affect the agglomeration of nanoparticles by changing convection and sedimentation. The temperature-induced agglomeration of hexadecanethiol-capped gold nanoparticles in microgravity (µ g) is studied at the ZARM (Center of Applied Space Technology and Microgravity) drop tower and compared to their agglomeration on the ground (1 g). Nonpolar nanoparticles with a hydrodynamic diameter of 13 nm are dispersed in tetradecane, rapidly cooled from 70 to 10 °C to induce agglomeration, and observed by dynamic light scattering at a time resolution of 1 s. The mean hydrodynamic diameters of the agglomerates formed after 8 s in microgravity are 3 times (for low initial concentrations) to 5 times (at high initial concentrations) larger than on the ground. The observations are consistent with an agglomeration process that is closer to the reaction limit on thground and closer to the diffusion limit in microgravity.

Gel transition in adhesive hard-sphere colloidal dispersions: the role of gravitational effects

The role of gravity in gelation of adhesive hard spheres is studied and a critical criterion developed for homogeneous gelation within the gas-liquid binodal. We hypothesize that gelation by Brownian diffusion competes with phase separation enhanced by gravitational settling. This competition is characterized by the gravitational Péclet number Pe(g), which is a function of particle size, volume fraction, and gravitational acceleration. Through a systematic variation of the parameters, we observe the critical Pe(g) of ∼ 0.01 can predict the stability of gels composed of adhesive hard spheres.

Miniaturized dynamic light scattering instrumentation for use in microfluidic applications

Five designs for a miniaturized dynamic light scattering (DLS) instrument are described that incorporate microfluidic flow of the sample volume and fiber optic probes directly embedded into the sample. These instruments were demonstrated to accurately determine the size of 10-100 nm particles dispersed in organic and aqueous solvents with most sample sizes less than 150 microl. Small stir bars were incorporated directly into the instruments, and enabled blending of different solutions immediately prior to DLS measurements. Demonstration of the instruments' capabilities include high throughput measurements of the micelle to unimer transition for poly(styrene-b-isoprene) in mixed toluene/hexadecane solvent, obtained by systematically blending toluene-rich and hexadecane-rich polymer solutions. The critical solvent composition was quickly identified with less than 20 mg of polymer. Further capabilities include temperature control, demonstrated by identification of a critical micelle temperature of poly(ethylene oxide-b-propylene oxide-b-ethylene oxide), as well as multiangle DLS measurements.

Tissue phantom for optical diagnostics based on a suspension of microspheres with a fractal size distribution

We demonstrate experimentally the possibility of reproducing the phase function, absorption, and scattering coefficients of a real biological tissue (adult brain white matter and liver) using a suspension of polystyrene microspheres with a fractal size distribution. The design of a light scattering goniometer with a cylindrical cell in air is discussed, and phase function measurements using the device are described. The scattering coefficient is measured using transmission spectrophotometry and the absorption and reduced scattering coefficients are measured using a time-resolved method. A good match between real tissue and phantom parameters is demonstrated.

Homodyne optical fiber dynamic light scattering

Optical fiber homodyne dynamic light scattering employs both the advantage of a sensitivity improvement over the standard self-beating technique and the inherent self-aligning simplicity of the optics. Ultralow concentrations of approximately nanometer-sized particles become accessible by dynamic light-scattering techniques.