title: Re-entrant solidification in polymer-colloid mixtures as a consequence of competing entropic and enthalpic attractions.
abstract: In polymer-colloid mixtures, non-adsorbing polymers dispersed with much larger colloids provide a universal yet specific entropic attraction between the colloids. Such so-called depletion interaction arises from an osmotic-pressure imbalance caused by the polymers and is considered to be independent of temperature. Here we show that, for the most commonly used polymer-colloid depletion systems, the polymer undergoes a crossover from non-adsorbing to adsorbing and that, consequently, the effective colloidal interactions depend on temperature. We also find that a combination of the enthalpic (polymer bridging) and entropic (polymer exclusion) interactions, both attractive, leads to a re-entrant regime where the colloids are dispersed and form solids both on heating and on cooling. We provide a simple model to explain the observed transitions and to fill the theoretical gap at the polymer-adsorption crossover. Our findings open possibilities for colloidal self-assembly, the formation of colloidal crystals and glasses, and the behaviour of temperature-controlled viscoelastic materials.
authors.affiliation: Center for Soft Matter Research, Physics Department, New York University, 4 Washington Place, New York New York 10003, USA.
authors.forename: Lang
authors.initials: L
authors.lastname: Feng
authors.affiliation: Center for Soft Matter Research, Physics Department, New York University, 4 Washington Place, New York New York 10003, USA.
authors.forename: Bezia
authors.initials: B
authors.lastname: Laderman
authors.affiliation: 1] Center for Soft Matter Research, Physics Department, New York University, 4 Washington Place, New York New York 10003, USA [2] Chemistry Department, New York University, 100 Washington Square East, New York New York 10003, USA.
authors.forename: Stefano
authors.initials: S
authors.lastname: Sacanna
authors.affiliation: Center for Soft Matter Research, Physics Department, New York University, 4 Washington Place, New York New York 10003, USA.
authors.forename: Paul
authors.initials: P
authors.lastname: Chaikin