[Paper Review] Interfacial colloidal monolayers under steady shear: structure and flow profiles

Ivo Buttinoni, Zachary A. Zell+2 more|arXiv (Cornell University)|Apr 1, 2015

Material Dynamics and Properties1 references16 citations

TL;DR

This study investigates shear-induced structuring in two-dimensional binary colloidal monolayers at water/oil interfaces using a rotating microdisk to apply steady shear. It reveals that particles organize into alternating rings of large and small particles due to position-dependent shear rates, with distinct dynamical regimes—continuous flow near the disk and intermittent hopping farther out—linked to local viscosity and mechanical response, demonstrating a direct coupling between microscopic dynamics, structure, and macroscopic rheology in 2D soft matter systems.

ABSTRACT

We study the coupling between the structural dynamics and rheological response of charged colloidal monolayers at water/oil interfaces, driven into steady shear by a microdisk rotating at a controlled angular velocity. The flow causes particles to layer into rotating concentric rings linked to the local, position-dependent shear rate, which triggers two distinct dynamical regimes: particles move continuously "Flowing") close to the microdisk, or exhibit intermittent "Hopping" between local energy minima farther away. The shear-rate dependent surface viscosity of a monolayer can be extracted from an interfacial stress balance, giving "macroscopic" flow curves whose behavior corresponds to the distinct microscopic regimes of particle motion. Hopping Regions correspond to a surface yield stress $η\sim τ_S^Y \dotγ^{-1}$, whereas Flowing Regions exhibit surface viscosities with power-law shear-thinning characteristics.

Motivation & Objective

• To understand how binary colloidal monolayers at fluid interfaces restructure under steady shear.
• To investigate the coupling between particle-scale dynamics, local structure, and macroscopic mechanical response in 2D soft materials.
• To determine how interfacial viscosity emerges from local particle motion and spatial organization under shear.
• To explore the transition between continuous flow and intermittent hopping regimes in sheared colloidal monolayers.
• To establish a connection between microscopic dynamics and macroscopic rheological behavior in 2D colloidal systems.

Proposed method

• Experiments use a microdisk rotated at controlled angular velocities to apply steady shear to colloidal monolayers at water/decane interfaces.
• Binary mixtures of polystyrene spheres (diameter 4 µm and 1 µm) are spread at the interface to form loosely packed, non-equilibrium monolayers.
• Confocal microscopy enables real-time visualization of particle positions and dynamics across the interface.
• Interfacial velocity profiles are measured to extract local shear rates and calculate surface shear viscosity via a local interfacial stress balance.
• The system is analyzed across varying area fractions and rotation speeds to map structural, dynamical, and rheological transitions.
• A minimal theoretical model is used to interpret the transition between flow and hopping regimes based on local energy landscapes and shear rate.

Experimental results

Research questions

• RQ1 How do binary colloidal monolayers at fluid interfaces structurally reorganize under steady shear?
• RQ2 What is the relationship between particle-scale dynamics (flow vs. hopping) and local shear rate in 2D monolayers?
• RQ3 How does the mechanical response (surface viscosity) of the monolayer vary with position and shear rate?
• RQ4 What is the connection between structural ordering (ring formation) and dissipation in sheared colloidal monolayers?
• RQ5 How do the distinct dynamical regimes (flowing vs. hopping) correspond to macroscopic flow curves and interfacial rheology?

Key findings

• Particles self-organize into alternating concentric rings of large and small colloids due to position-dependent shear rates, forming a radial structure around the rotating microdisk.
• Near the microdisk, particles exhibit continuous flow with reduced dissipation, corresponding to a shear-thinning behavior in the surface viscosity.
• Farther from the disk, particles transition to intermittent hopping between local energy minima, indicating a higher resistance to flow and increased dissipation.
• The surface viscosity extracted from velocity profiles collapses onto area fraction-dependent master flow curves, confirming a macroscopic rheological response linked to microscopic dynamics.
• The transition between flowing and hopping regimes occurs at critical shear rates consistent with intrinsic material properties, not external control parameters.
• The study demonstrates a direct, quantifiable link between local particle dynamics, interfacial structure, and macroscopic rheology in 2D colloidal systems.