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Flow dichroism in critical colloidal fluids
1Van't Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
Summary
Shear flow distorts near-critical fluid microstructure, causing dichroism. Experiments show unexpected dichroism decrease near the critical point, unlike turbidity, challenging current theories.
Area of Science:
- Soft Matter Physics
- Fluid Dynamics
- Colloid Science
Background:
- Near-critical fluids exhibit long-range correlations and slow dynamics.
- Shear flow induces anisotropy in fluid microstructure, leading to observable dichroism.
- Dichroic behavior serves as a probe for microstructural ordering under shear.
Purpose of the Study:
- To investigate the dichroic behavior of a colloid-polymer mixture under shear flow.
- To compare experimental results with theoretical predictions in different regions relative to the critical point.
- To explore phenomena beyond the mean-field region where existing theories are insufficient.
Main Methods:
- Static and dynamic dichroism measurements.
- Turbidity measurements.
- Utilized a colloid-polymer mixture (silica spheres and polydimethylsiloxane).
Main Results:
- Experimental data agreed with theory in the mean-field region, far from the critical point.
- Observed an unexpected decrease in dichroism upon approaching the critical point, in the non-mean-field region.
- Critical slowing down of shear-induced dichroism was not observed, contrasting with turbidity measurements.
Conclusions:
- Current theories adequately describe dichroism in the mean-field region.
- The behavior of dichroism very close to the critical point deviates from predictions and requires new theoretical frameworks.
- The absence of critical slowing down in shear-induced dichroism highlights unique dynamics near the critical point.