Impact of single-particle compressibility on the fluid-solid phase transition for ionic microgel suspensions

TL;DR

This study investigates how single-particle compressibility influences the fluid-solid phase transition in ionic microgel suspensions, revealing that particle softness affects phase coexistence width and particle compression at high volume fractions.

Contribution

It introduces a method to determine volume fraction from osmotic pressure and links particle compressibility to phase transition behavior in ionic microgels.

Findings

Phase coexistence width varies with particle stiffness.

Softer microgels exhibit fluid-like behavior at high volume fractions.

Particle compression correlates with phase transition characteristics.

Abstract

We study ionic microgel suspensions composed of swollen particles for various single-particle stiffnesses. We measure the osmotic pressure $\pi$ of these suspensions and show that it is dominated by the contribution of free ions in solution. As this ionic osmotic pressure depends on the volume fraction of the suspension $\phi$, we can determine $\phi$ from $\pi$, even at volume fractions so high that the microgel particles are compressed. We find that the width of the fluid-solid phase coexistence, measured using $\phi$, is larger than its hard-sphere value for the stiffer microgels that we study and progressively decreases for softer microgels. For sufficiently soft microgels, the suspensions are fluid-like, irrespective of volume fraction. By calculating the dependence on $\phi$ of the mean volume of a microgel particle, we show that the behavior of the phase-coexistence width correlates with whether or not the microgel particles are compressed at the volume fractions corresponding to fluid-solid coexistence.