Scientific Articles

Colloidal gelation induced by ring polymers

We provide unambiguous experimental evidence that ring polymers are stronger depleting agents in colloidal suspensions than their linear counterparts. We use an intermediate volume fraction (φ_c = 0.44) colloidal gel based on the classic poly(methyl methacrylate) (PMMA) hard spheres, in which the polystyrene depletant is either linear or ring of the same molar mass or the same size. We systematically increase the depletant concentration from zero (no attraction) to well above the gelation point and find that in the presence of rings, gels are formed at smaller concentrations and possess a larger storage modulus in comparison to those induced by the linear chains. Consequently, the yield stress is enhanced; however, the yield strain (gel deformability) remains concomitantly unaffected. Our experimental findings agree with theoretical calculations based on effective interaction potentials. Hence, polymer architecture is a powerful entropic tool to tailor the strength of colloidal gels.

(Fig.) Representative theoretical phase for a colloid-ring mixture (black lines) and a colloid-linear mixture (red lines) of similar size ratio. The gray- (red-)shaded regions denote phase coexistence between the pure phases in the white area below (for the linear depletants, part of that white area appears gray as it is occupied by the phasecoexistence region of the colloid-ring mixtures). The pure phases are the colloidal fcc crystal (S), the colloidal liquid (L), and the colloidal vapor (V). The think black (red) lines denote the three-phase triangle between the V, L, and S at the triple points, and the black (red) square the critical point. The filled triangle and circle denote the location of the L and S phases at triple coexistence with the V phase, for which φ_c ∼= 0. The vertical blue line represents a path of fixed colloid packing fraction φ_c = 0.44, as in the experiments.

Engineering Ultrasoft Interactions in Stiff All-DNA Dendrimers by Site-Specific Control of Scaffold Flexibility

Nataša Adžic Clemens Jochum, Christos N. Likos, and Emmanuel Stiakakis, Small 2023, 2308763.

A combined experimental and theoretical study of the structural correlations in moderately concentrated suspensions of all-DNA dendrimers of the second generation (G2) with controlled scaffold rigidity is reported here. Small-angle X-ray scattering experiments in concentrated aqueous saline solutions of stiff all-DNA G2 dendritic constructs reveal a novel anomalous liquid-like phase behavior which is reflected in the calculated structure factors as a two-step increase at low scattering wave vectors. By developing a new design strategy for adjusting the particle’s internal flexibility based on site-selective incorporation of single-stranded DNA linkers into the dendritic scaffold, it is shown that this unconventional type of self-organization is strongly contingent on the dendrimer’s stiffness. A comprehensive computer simulation study employing dendritic models with different levels of coarse-graining, and two theoretical approaches based on effective, pair-potential interactions, remarkably confirmed the origin of this unusual liquid-like behavior. The results demonstrate that the precise control of the internal structure of the dendritic scaffold conferred by the DNA can be potentially used to engineer a rich palette of novel ultrasoft interaction potentials that could offer a route for directed self-assembly of intriguing soft matter phases and experimental realizations of a host of unusual phenomena theoretically predicted for ultrasoft interacting systems.

(Fig.) Effective pair interactions between two G2-stiff (solid lines) and between two G2-flex (dashed lines), calculated using Widom insertion method within oxDNA model and shown as a function of center-of-mass-to-center-of-mass separation r, at two different salt concentrations c.

Interfacial Fluid Rheology of Soft Particles

Maximilian M. Schmidt, José Ruiz-Franco, Steffen Bochenek, Fabrizio Camerin, Emanuela Zaccarelli and Andrea Scotti, Physical Review Letters 131, 258202 (2023).

In situ interfacial rheology and numerical simulations are used to investigate microgel monolayers in a wide range of packing fractions, ζ_2D. The heterogeneous particle compressibility determines two flow regimes characterized by distinct master curves. To mimic the microgel architecture and reproduce experiments, an interaction potential combining a soft shoulder with the Hertzian model is introduced. In contrast to bulk conditions, the elastic moduli vary nonmonotonically with ζ_2D at the interface, confirming long-sought predictions of reentrant behavior for Hertzian-like systems.

(Fig.) Compression isotherms reporting the surface pressure π (squares) and plateau of the elastic modulus G_p (circles), normalized by k_B T=ξ^2, as a function of generalized packing fraction ζ_2D. Lines are guides to the eye. Different colors identify different regimes of the compression isotherms.

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