How do particles move in a liquid
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Particle Motion in Liquids: Mechanisms and Dynamics
Understanding how particles move in a liquid is crucial for various scientific and industrial applications. This article synthesizes findings from multiple research studies to provide a comprehensive overview of the mechanisms and dynamics governing particle motion in liquids.
Particle Orientation in Viscous Liquids
Small Spheroidal Particles
Small spheroidal particles in a sheared viscous liquid tend to adopt preferred orientations over time, independent of their initial release conditions. This behavior cannot be fully explained by the linearized Navier-Stokes equations. Instead, it is suggested that the non-Newtonian properties of the liquid play a significant role, causing the particles to orient in a way that minimizes energy dissipation .
Dispersion and Attraction on Fluid-Liquid Interfaces
Initial Dispersion and Oscillation
When particles come into contact with a fluid-liquid interface, they initially disperse due to capillary forces pulling them into the interface. This causes them to accelerate and oscillate about an equilibrium position before stopping under viscous drag. The spontaneous dispersion is driven by repulsive hydrodynamic forces arising from these oscillations .
Formation of Monolayers
After the initial dispersion, particles may cluster into monolayers due to attractive lateral forces, especially if they are larger than 10 µm. The spacing and defect elimination in these monolayers can be controlled by applying an electric field, which induces attractive lateral capillary forces .
Electrokinetic Motion at Liquid-Fluid Interfaces
Influence of Electric Fields
Spherical polystyrene particles at various liquid-fluid interfaces move in the opposite direction of an applied electric field. The velocity of these particles increases linearly with the electric field's strength. Smaller particles exhibit higher electrokinetic velocities compared to larger ones, attributed to surface charges at the interfaces .
Single-Particle Dynamics in Liquid Water
Relaxing Cage Concept
In liquid water, single-particle motion can be described using the relaxing cage concept. A water molecule is surrounded by a solid-like cage exhibiting normal modes similar to those in ice. The decay of these resonance modes, due to coupling with other normal modes, differentiates the frequency spectra of ice and liquid water. This model helps explain the velocity autocorrelation function of water molecules .
Numerical Simulations of Particulate Flow
Discrete Element Method (DEM)
Numerical simulations using DEM reveal that a small amount of liquid significantly affects particle motion. In a centrifugal tumbling granulator, the formation of liquid bridges between particles influences their velocities, which can be validated through particle tracking velocimetry (PTV) techniques .
Direct Simulation of Fluid-Particle Motions
Simulations using the Navier-Stokes equations for liquids and Newton's equations for solid particles show the effects of vortex shedding on particle motion. This method reproduces phenomena like drafting, kissing, and tumbling in two-phase flows, providing insights into the rearrangement mechanisms in particle beds .
Liquid Transport in Sheared Particle Beds
Convective and Collisional Transport
Liquid transport in moving particle beds occurs due to convective transport and liquid transfer upon particle collisions. Simulations of homogeneous shear flow involving soft, frictional spheres help build regime maps of effective liquid flux, drawing analogies to thermal energy transport in these systems .
Conclusion
The motion of particles in liquids is influenced by a variety of factors, including the properties of the liquid, external forces, and particle interactions. From the orientation of spheroidal particles in viscous liquids to the electrokinetic motion at fluid interfaces, each mechanism provides unique insights into the complex dynamics of particulate flow. Numerical simulations and experimental studies continue to enhance our understanding, paving the way for more accurate models and applications in various fields.
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