Evaluating the impact of simulated microgravity of a Random Positioning Machine on the stability of emulsions applying scaling analysis via dimensionless numbers

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, e.g., 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 (d32 = 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/sec; 100 deg/sec; 120 deg/sec) 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).