Alberto Fernandez-Nieves

Alginate is a negatively charged polymer that is often cross-linked in the presence of Ca2+ ions to form alginate gels. Their swelling is often induced by the addition of NaCl via what is believed to be the exchange of the calcium ions responsible for cross-linking and the added Na+ ions. Our experiments, however, indicate that the swelling of the gels with NaCl is followed by deswelling at higher concentrations, a fact that cannot be explained through cross-link degradation. Dialyzing these gels against large volumes of ultrapure water confirms the stability of the cross-links. Assuming that the gel is an ionic gel with some permanent cross-links, we develop a model that shows that the key to our observations is the distribution of mobile ions inside and outside the gel, together with electroneutrality and the valence difference between Na+ and Ca2+ ions. Changing the suspension pH, which allows for changing the charging state of alginate, confirms that the observed swelling behavior is consistent with that of ionic gels. Hence, our results, which we also show to be independent of gel morphology, unavoidably indicate that the conventional way to think about alginate swelling must be revised.

We observe the formation of stable, magnetic field induced, alignment inversion walls, also called Helfrich walls, in nematic liquid crystals (NLCs) confined to a toroidal droplet with tangential anchoring at the boundaries. Walls are rare in NLCs due to their high energy compared to lower-dimensional defect structures. However, we find that Helfrich walls naturally form, transform, and migrate in this unconventional toroidal setting, generating intricate director-field configurations due to the interplay of confining geometry and external fields. Interestingly, the presence and number of point defects in the zero field nematic configuration of the toroid influences the types of walls that arise. In the absence of point defects, splay-bend walls emerge. By contrast, when a point defect pair exists on the toroidal surface, a wall not previously classified by Helfrich, and that involves both twist and splay-bend distortions, forms, and transforms. Our experiments and numerical simulations enable the quantification and elucidation of the physics behind the formation and transformation of these wall structures, extending the class of walls that were believed to occur in NLCs.

Viscosity describes how easily a liquid can flow under an applied stress. Dimensionally, it is the product of an elastic modulus and a timescale. The significance of these quantities in determining the viscosity of a liquid can be seen after considering a simple model for liquid flow, in which the flow results from the instantaneous storage of elastic energy and its subsequent dissipation. The modulus then corresponds to the short-time elastic modulus and the timescale is that associated with particle rearrangements. Interestingly, applying conservation of energy, as introduced in continuum mechanics, naturally provides the basis for this way to think about liquid flow. Additionally, the result naturally connects to viscoelastic behavior, emphasizing its applicability to liquids in general, and opening up interesting possibilities to expand the contents often taught in continuum mechanics courses.

Many active systems comprised of self-propelled particles, including living systems, are known to undergo collective motion and exhibit giant number fluctuations (GNFs). Fire ants at sufficiently high density exhibit activity cycles consistent in time-instances where nearly all ants in the system move and time-instances where a large fraction of the ants assemble in non-moving clusters. We here quantify number fluctuations and demonstrate that fire ants in their actively moving state exhibit GNFs, suggesting that this state could correspond to the collective-motion state associated to the activity waves found in vertical columns. The corresponding scaling exponent is, in fact, consistent with that found for other polar active systems. Interestingly, similar behavior is found in the cluster phase, but in this case due to the presence of density heterogeneities. Our experimental results underscore that the underlying reason for GNFs depends on both structural as well as dynamical properties.

Granular matter exhibits unusual mechanical properties. For instance, when filling a cylindrical container with grains, the weight measured at the bottom of the granular column does not scale linearly with the added mass, but asymptotically saturates towards a constant apparent weight. This observation is the well-known Janssen effect, ultimately reflecting that the "lost" weight is supported by frictional forces between the grains and the wall of the container confining the grains. When filling narrower containers with spherical grains, however, an intermediate regime consisting of an overshoot in measured weight appears; in this case, before the usual Janssen saturation is observed, frictional forces contribute to increase the apparent weight. Here, we experimentally study the role of particle shape in this reverse Janssen effect and find that it disappears faster with increasing cylinder diameter for non-spherical grains compared to spherical grains. We then discuss the experimental results, analyzing some of the possible aspects that could potentially explain the influence of grain shape.

Structure most frequently constrains material behavior, resulting in the coupling between structural and dynamical properties. Using numerical simulations, we show that single-particle elasticity can cause the breakdown of this coupling, enabling liquidlike dynamics within crystalline configurations. Our findings reveal that the elastic heterogeneity of the many-particle system is key to this behavior, allowing the transition from a disordered to an ordered structure, where particles remain mobile; the resultant ordered material is thus characterized by the presence of rearrangements, a characteristic feature of the liquid state. Our findings clearly identify elasticity as a definitive aspect for achieving the decoupling of structure and dynamics.

The accepted paradigm in materials science is that the internal structure of a material determines its macroscopic properties. This connection is reflected in the particle dynamics, which are known to become slower at length scales comparable to the mean interparticle distance. This implies that the q -dependent relaxation time, with q the magnitude of the scattering wave vector, correlates with the structure factor of the material. This is known as de Gennes narrowing in simple atomic liquids, and is a behavior also seen in colloidal suspensions, where the quantities at play are more easily accessible experimentally. We here find that this familiar correlation can breakdown for soft-colloid suspensions. In both experiments and simulations, we find that the q -dependent relaxation time of the suspension does not follow the structure factor, but that instead, it remains unchanged relative to the length scale at which it is measured. We justify this unusual behavior by alluding to single-particle elasticity and how this aspect allows additional relaxation pathways for the characteristic time of the suspension to remain unaffected by the suspension structure. Our findings challenge the prevailing wisdom that the structure of a material unequivocally determines its properties.

In recent experiments, ants were constrained to a two-dimensional rectangular domain by enclosing them between two flat transparent surfaces and their dynamic behavior was studied. The large sides of the domain were in the vertical direction and thus, gravity affected the dynamics of the ants. The experiments showed that the collective dynamics of the ants displayed wave-like behavior. In this article, we develop and analyze a mathematical model of the mentioned experiments. Our work contributes to the understanding of the dynamics of active matter and its mathematical modeling.

Hyperuniformity and giant number fluctuations represent opposite ends in a spectrum of statistical correlations found in physical systems outside of thermal equilibrium. Dynamic phase transitions exhibit critical points that are hyperuniform, while anomalously large fluctuations are often quoted as a hallmark of active matter. Here, we show that the apparently disordered state of active nematic defects exhibits suppression of fluctuations on scales as large as the system size, reminiscent of what is seen for ordered lattices. Modeling the nematic defects in terms of continuum densities, we show that their distribution becomes asymptotically hyperuniform in the limit where their proliferation is solely driven by activity, rather than by thermal fluctuations. When both effects are at play, the resultant hyperuniform structure is limited to a finite range of length scales, ℓ < ℓ × , with ℓ × → ∞ as thermal unbinding is suppressed. The system of active nematic defects can therefore be said to possess hidden order across scales provided ℓ is comparable to the system size. This organization is most pronounced when considering the subpopulation of either the positive or the negative defects, illustrating lack of multihyperuniformity. Our experimental findings are supported by hydrodynamic theory and agent-based simulations, which further allow elucidating the role of both creation/annihilation of defect pairs and of their mutual repulsion and attraction in achieving hyperuniformity.