A qualitative confocal microscopy study on a range of colloidal processes by simulating microgravity conditions through slow rotations

Abstract

We report qualitatively on the differences between colloidal systems left to evolve in the Earth's gravitational field and the same systems for which a slow continuous rotation averaged out the effects of particle sedimentation on a distance scale small compared to the particle size. Several systems of micron-sized colloidal particles were studied: a hard sphere fluid, colloids interacting via long-range electrostatic repulsion above the freezing volume fraction, an oppositely charged colloidal system close to either gelation and/or crystallization, colloids with a competing short-range depletion attraction and a long-range electrostatic repulsion, colloidal dipolar chains, and colloidal gold platelets under conditions where they formed stacks. Important differences in structure formation were observed between the experiments where the particles were allowed to sediment and those where sedimentation was averaged out. For instance, in the case of colloids interacting via long-range electrostatic repulsion, an unusual sequence of dilute-fluid-dilute-crystal-dense-fluid-dense-crystal phases was observed throughout the suspension under the effect of gravity. This was related to the volume fraction dependence of the colloidal interactions, whereas the system stayed homogeneously crystallized with rotation. For the oppositely charged colloids, a gel-like structure was found to collapse under the influence of gravity with a few crystalline layers grown on top of the sediment, whereas when the colloidal sedimentation was averaged out, the gel completely transformed into crystallites that were oriented randomly throughout the sample. Rotational averaging out of gravitational sedimentation is an effective and cheap way to estimate the importance of gravity for colloidal self-assembly processes.

Numerical investigation of channel blockage by flowing microparticles

“…Such colloidal dispersion can be obtained by dispersing micrometric particle in a solvent having low dielectric constant and low salt concentration; Debye length and particle size can then stand in the micrometric range [37].…”

Spatial inhomogeneities in ionic liquids, charged proteins, and charge stabilized colloids from collective variables theory

“…In experiments a fluid-bcc crystal-fluid phase coexistence was found with an increase of the colloid volume fraction [12,13]. The dilute fluid -dilute crystal -dense fluiddense crystal transitions, for increasing ζ, with the re-entrant melting for ζ ∼ 0.1 were also seen [11]. In the latter experiment room temperature in our reduced units is T * ≈ 0.39.…”

“…Charged nanoparticles or colloidal particles with a diameter ∼ 10 ÷ 1000 nm are several orders of magnitude larger than the microscopic counterions in a solution, and their charge can be orders of magnitude larger than the charge of the counterion. In these systems, the experiments indicate the formation of colloidal crystals with large interparticle distance for small volume fractions, and a re-entrant melting for larger volume fractions [10,11,13].…”

“…On the other hand, recent experimental, simulation and theoretical studies of room temperature ionic liquids (RTILs) [7], charged globular proteins [8,9] or colloidal particles [8,[10][11][12][13] reveal structural inhomogeneities of different type and spatial extent, and it becomes evident that the ordering in systems dominated by Coulomb interactions has a very rich and complex nature, and is far from being understood. Consequently, ordering of ions or charged particles on different length scales attracts increasing attention.…”

Efficient preservation of sprouting vegetables under simulated microgravity conditions

“…To be precise, the created conditions are different from the microgravity conditions found in outer space, so the machine is suitably referred to as a simulated microgravity device. Several studies published investigating the applications of this device include the homogenization of crystals [17], the attenuation of rat myogenic differentiation [18], and the effects of microgravity on epidermal stem cell metabolism [19]. Experiments utilizing plants have reported microRNAs in tomato that respond to gravity [20] and the delay of rice flowering in a simulated microgravity environment [21].…”