Evaluating the impact of simulated microgravity of a random positioning machine on the stability of emulsions applying scaling analysis <i>via</i> dimensionless numbers.

Abstract

Random positioning machines (RPM) are commonly used to simulate microgravity for plant growth and cell culturing experiments, but not properly in multi-phase flow studies, <i>e.g.</i>, emulsions. The implications of fluid motion induced by RPM movement patterns have only been studied for one-phase system using computational fluid dynamics (CFD). This study investigates the impact of fluid motion of 5 different RPM motion modes (0 g, 0.4 g, clinostat of different frame rates) on dispersed droplets (<i>d</i>₃₂ = 0.1-70 μm) applying scaling analysis. These computations are based on well-established fluid-dynamic laws and correlations, thereby giving microgravity researchers easier tool to evaluate potential deficiencies in their study design compared to CFD. We found that the clinostat modes (80 deg s^-1; 100 deg s^-1; 120 deg s^-1) induce a transitional flow regime in the continuous phase, and considerate shear rates acting on the dispersed droplets. Under certain conditions, the shear rates might even impact the average particle size, representing a major corruption in study design, which must not be mistaken as an effect of simulated microgravity. On the other hand, the 0 g and 0.4 g motion modes lead to a laminar flow in the continuous phase, low shear forces, Stokes flow surrounding the dispersed droplets, little relative droplet movement, as well as neglectable forced convection and gravitational force, thus resembling a state similar to true microgravity (0 g motion mode) and partial gravity (0.4 g motion mode).