Hydrodynamic and brownian fluctuations in sedimenting suspensions

Area of Science:

• Soft Matter Physics
• Computational Fluid Dynamics
• Colloidal Science

Background:

• Sedimentation of particles is influenced by both fluid flow (hydrodynamics) and random motion (Brownian forces).
• Understanding the interplay between these forces is crucial for predicting particle behavior in suspensions.
• Previous studies often focused on regimes where one force dominates over the other.

Purpose of the Study:

• To investigate the combined effects of hydrodynamic and Brownian forces on particle sedimentation.
• To quantify the influence of hydrodynamic interactions on sedimentation velocity and fluctuations.
• To determine the critical Peclet number (Pe) at which non-equilibrium hydrodynamic effects emerge.

Main Methods:

• Mesoscopic computer simulations were employed to model hard sphere particle sedimentation.
• Simulations covered a range of Peclet numbers (Pe) from 0.1 to 15.
• Analysis focused on average sedimentation velocity and velocity fluctuations.

Main Results:

• Hydrodynamic interactions, even when weaker than Brownian forces, induce backflow, significantly reducing sedimentation velocity.
• This backflow effect becomes the dominant factor in velocity reduction as particle packing fraction increases.
• Velocity fluctuations exhibit non-equilibrium hydrodynamic characteristics for Peclet numbers greater than 1.

Conclusions:

• Hydrodynamic interactions play a critical role in particle sedimentation dynamics, even at low strengths.
• The transition to non-equilibrium hydrodynamic behavior is observable in velocity fluctuations at moderate Pe.
• Mesoscopic simulations provide valuable insights into complex fluid-particle interactions in colloidal systems.