Depletion layer dynamics of polyelectrolyte solutions under Poiseuille flow

Area of Science:

• Physical Chemistry
• Soft Matter Physics
• Fluid Dynamics

Background:

• Complex liquids exhibit faster-than-expected flow in channels, attributed to low-viscosity depletion layers at boundaries.
• Direct experimental characterization of depletion layer length scale, concentration, and dynamics has been challenging due to limitations in real-space imaging techniques.

Purpose of the Study:

• To overcome limitations in imaging short length scales associated with depletion layers.
• To directly measure the concentration profile of polymer solutions at a nonadsorbing wall under Poiseuille flow.

Main Methods:

• Adaptations of stimulated emission depletion (STED) microscopy were employed to overcome traditional imaging limitations.
• Direct real-space imaging was used to measure the concentration profile of polymer solutions.

Main Results:

• The study confirmed theoretically predicted concentration profiles governed by entropically driven depletion.
• Depletion layer narrowing was observed at low to intermediate shear rates.
• Depletion layer composition was found to approach pure solvent at unexpectedly low shear rates.

Conclusions:

• Direct imaging of depletion layers is achievable with adapted STED microscopy.
• Experimental results align with theoretical predictions for depletion-driven concentration profiles.
• Shear rate significantly influences depletion layer characteristics, including narrowing and composition.