Nanocomposite gels formed by mixing nanoparticles and polymers offer a limitless creative space for the design of functional advanced materials with a broad range of applications in materials and biological sciences. Here, we focus on aqueous dispersions of hydrophobic colloidal soot particles, namely, carbon black (CB) dispersed with a sodium salt of carboxymethylcellulose (CMC), a food additive known as cellulose gum that bears hydrophobic groups, which are liable to bind physically to CB particles. Varying the relative content of CB particles and cellulose gum allows us to explore a rich phase diagram that includes a gel phase observed for a large enough CB content. We investigate this hydrogel using rheometry and electrochemical impedance spectroscopy. CB−CMC hydrogels display two radically different types of mechanical behaviors that are separated by a critical CMC-to-CB mass ratio r c . For r < r c , i.e., for low CMC concentration, the gel is electrically conductive and shows a glassy-like viscoelastic spectrum, pointing to a microstructure composed of a percolated network of CB particles decorated by CMC. In contrast, gels with a CMC concentration larger than r c are nonconductive, indicating that the CB particles are dispersed in the cellulose gum matrix as isolated clusters, and act as physical cross-linkers of the CMC network, hence providing mechanical rigidity but limited conductivity enhancement to the composite. Moreover, in the r > r c concentration range, CB−CMC gels display a power-law viscoelastic spectrum that depends strongly on the CMC concentration. These relaxation spectra can be rescaled onto a master curve, which exhibits a power-law scaling in the high-frequency limit, with an exponent that follows Zimm theory, showing that CMC plays a key role in the gel viscoelastic properties for r > r c . Our results offer an extensive experimental characterization of CB−CMC dispersions that will be useful for designing soft nanocomposite gels based on hydrophobic interactions.

Magnus gliders are spinning toys displaying spectacular looped trajectories when launched at large velocity. These trajectories originate from the large amplitude of the Magnus force due to translational velocities of a few meters per second combined with a backspin of a few hundred radians per seconds. In this article, we analyse the trajectories of Magnus gliders built from paper cups, easily reproducible in the laboratory. We highlight an analogy between the trajectory of the glider and the trajectory of charged particles in crossed electric and magnetic fields. The influence of the initial velocity and the initial backspin on the trajectories is analyzed using high speed imaging. The features of these trajectories are captured by a simple model of the evolution of the Magnus and drag forces as a function of the spin of the gliders. The experimental data and the modeling show that the type of trajectory—for instance, the occurrence of loops—depends mostly on the value and orientation of the initial translational velocity regardless of the value of the backspin, while the maximum height of the apex depends on both the initial translational velocity and initial backspin.