Boundary flow on end-grafted PEG brushes.

Area of Science:

• Soft matter physics
• Surface science
• Fluid dynamics

Background:

• Understanding liquid flow at interfaces is crucial for various applications.
• Polymer brushes create complex interfacial environments that can alter fluid behavior.
• Previous simulations suggested liquid penetration into polymer brushes, but experimental validation was needed.

Purpose of the Study:

• To experimentally determine the boundary conditions for Newtonian liquid flow over soft polymer brush interfaces.
• To quantify the slip length and interpret it in terms of liquid penetration.
• To validate previous simulation findings on liquid-polymer brush interactions.

Main Methods:

• Utilized colloid probe atomic force microscopy (AFM) to measure hydrodynamic drainage forces.
• Employed end-grafted thiolated poly(ethylene glycol) (PEG) brushes (1k and 30k molecular weight) on gold surfaces.
• Optimized brush preparation for reproducible and uniform surface properties essential for surface force measurements.

Main Results:

• Experimental data fitted to a slip model yielded a slip length of 16 nm for the PEG 1k brush.
• A slip length of 25 nm was determined for the PEG 30k brush.
• Slip lengths were interpreted as penetration depths, with longer chains showing greater penetration.

Conclusions:

• Experimental results confirm that liquids can penetrate polymer brush layers.
• The slip length provides a quantitative measure of liquid-surface interaction and penetration depth.
• Findings align with and experimentally validate prior theoretical and simulation studies on fluid flow in polymer brushes.