Gelation and Re-entrance in Mixtures of Soft Colloids and Linear Polymers of Equal Size

TL;DR

This study reveals a novel liquid-gel-re-entrant liquid transition in mixtures of soft colloids and linear polymers of equal size, driven by complex depletion and osmotic effects, with implications for designing flow properties of soft materials.

Contribution

It demonstrates the unexpected re-entrant behavior in soft colloid-polymer mixtures and explains it through a detailed interplay of depletion forces and osmotic effects, expanding understanding of soft matter phase behavior.

Findings

Gelation occurs upon adding linear polymers to soft colloids.

Further addition of polymers causes re-entrant liquefaction.

The behavior is explained by depletion interactions and osmotic pressure effects.

Abstract

Liquid mixtures composed of colloidal particles and much smaller non-adsorbing linear homopolymers can undergo a gelation transition due to polymer-mediated depletion forces. We now show that the addition of linear polymers to suspensions of soft colloids having the same hydrodynamic size yields a liquid-to-gel-to-re-entrant liquid transition. In particular, the dynamic state diagram of 1,4-polybutadiene star-linear polymer mixtures was determined with the help of linear viscoelastic and small angle X-ray scattering experiments. While keeping the star polymers below their nominal overlap concentration, a gel was formed upon increasing the linear polymer content. Further addition of linear chains yielded a re-entrant liquid. This unexpected behavior was rationalized by the interplay of three possible phenomena: (i) depletion interactions, driven by the size disparity between the stars and the polymer length scale which is the mesh size of this entanglement network; (ii) colloidal deswelling due to the increased osmotic pressure exerted onto the stars; and (iii) a concomitant progressive suppression of depletion efficiency on increasing polymer concentration due to reduced mesh size, hence a smaller range of attraction. Our results unveil an exciting new way to tailor the flow of soft colloids and highlight a largely unexplored path to engineer soft colloidal mixtures.