Spatial Crossover Between Far-From-Equilibrium and Near-Equilibrium Dynamics in Locally Driven Suspensions

Area of Science:

• Soft Matter Physics
• Colloidal Science
• Non-equilibrium Systems

Background:

• Colloidal suspensions exhibit complex behaviors under external forces.
• Understanding particle dynamics in non-equilibrium systems is crucial.
• Optical tweezers offer precise control for studying microscopic systems.

Purpose of the Study:

• To investigate the response of a quasi-two-dimensional colloidal suspension to localized optical driving.
• To explore particle migration and density profile formation across a wide range of Péclet numbers.
• To identify emergent length scales in driven colloidal systems.

Main Methods:

• Utilizing optical tweezers to apply localized circular driving to a colloidal suspension.
• Resolving particle behavior over three orders of magnitude of the Péclet number (Pe).
• Analyzing steady-state density profiles and emergent length scales.

Main Results:

• Observed a sharp spatial crossover from far- to near-thermal-equilibrium regions.
• Demonstrated particle migration from high to low Pe regions.
• Identified an emergent length scale independent of particle density, determined by Pe≈1.
• Validated results with a phenomenological two-phase fluid constitutive model.

Conclusions:

• Localized driving in colloidal suspensions leads to significant spatial inhomogeneity.
• An emergent length scale, independent of particle density, governs the system's steady state.
• The Péclet number is a critical parameter controlling particle distribution and system behavior.