UC Santa Barbara

A gel is a soft material that is made when a three-dimensional polymer or colloidal network is hydrated in water or suspended in some other solvent. Gels can be found in nature (e.g., inside cells, lining the stomach, etc.) and are synthesized for many different consumer and medical applications. Their versatility and utility stem from a large array of accessible phase behaviors (e.g., sol-gel, liquid-liquid, etc.) and viscoelastic properties, both of which can be controlled at the polymer/colloidal level. Recent theoretical works predict that the phase behavior and viscoelasticity of a colloidal/molecular gel network is strongly modulated by the connectivity of the colloid in solution (e.g., valence). However, in practice, engineering particles of well-defined valence is difficult, making experimental insight hard to come by. Here, I take advantage of DNA programmability to self-assemble, via base-pairing, transiently bonded DNA particles of designed valence, called DNA nanostars (NSs). I measure NS phase diagrams and network viscoelasticity as a function of NS valence, z. My measurements show that increasing z results in a larger coexistence regime for phase separation and a stiffer, more brittle NS network. In particular, I find that: (i) the valence effect on phase behavior is largely in line with theoretical expectations and (ii) NS viscoelasticity is controlled by an interplay between entropic elasticity of network chains and NS valence imposing junction constraints (e.g., approaching an isostatic threshold). I also make a NS with two types of bonds on it, where one bond is short-lived and the other is much longer-lived. I find that such NSs make networks with reproducible power-law stress-relaxation between the two bond lifetimes and the power-law exponent depends on the valence of the stronger-bond. I further show that the power-law exponent during stress-relaxation is explained by a model that considers how the strong-bonded network relaxes in an effective medium with a viscosity defined by the weak-bonds. Overall, the work here provides insight into how valence modulates the phase behavior and viscoelasticity of transient gel networks.