Osmotic force resisting chain insertion in a colloidal suspension

TL;DR

This paper investigates the force resisting the insertion of a stiff chain into a colloidal suspension, combining scaled particle theory and scaling arguments to estimate forces relevant to biophysical processes like DNA ejection.

Contribution

It introduces a combined theoretical approach to quantify insertion forces in colloidal suspensions, applicable to biophysical systems.

Findings

Insertion forces are on the order of tens of pN for nanometer-scale chains and colloids.

The work provides estimates relevant to DNA ejection from viral capsids.

The methods bridge hard sphere fluids and flexible polymer solutions.

Abstract

We consider the problem of inserting a stiff chain into a colloidal suspension of particles that interact with it through excluded volume forces. The free energy of insertion is associated with the work of creating a cavity devoid of colloid and sufficiently large to accomodate the chain. The corresponding work per unit length is the force that resists the entry of the chain into the colloidal suspension. In the case of a hard sphere fluid, this work can be calculated straightforwardly within the scaled particle theory; for solutions of flexible polymers, on the other hand, we employ simple scaling arguments. The forces computed in these ways are shown, for nanometer chain and colloid diameters, to be of the order of tens of pN for solution volume fraction for biophysical processes such as the ejection of DNA from viral capsids into the cell cytoplasm.