Dense suspensions of hard particles in a liquid can exhibit strikingly counter-intuitive behavior, such as discontinuous shear thickening (DST) [1,2,3,4,5,6,7,8] and reversible shear jamming (SJ) into a state with finite yield stress [9,10,11,12,13]. Recent studies identified a stress-activated crossover from hydrodynamic interactions to frictional particle contacts to be key for these behaviors [2,3,4,6,7,8,10,14]. However, many suspensions exhibit only DST and not SJ. Here we show that particle surface chemistry can play a central role in creating conditions that allow for SJ. We find the system's ability to form interparticle hydrogen bonds when sheared into contact elicits SJ. We demonstrate this with charge-stabilized polymer microspheres and non-spherical cornstarch particles, controlling hydrogen bond formation with solvents. The propensity for SJ is quantified by tensile tests [13] and linked to an enhanced friction by atomic force microscopy. Our results extend the fundamental understanding of the SJ mechanism and open new avenues for designing strongly non-Newtonian fluids.

Electrostatic charging of insulating fine particles can be responsible for numerous phenomena ranging from lightning in volcanic plumes to dust explosions. However, even basic aspects of how fine particles become charged are still unclear. Studying particle charging is challenging because it usually involves the complexities associated with many particle collisions. To address these issues we introduce a method based on acoustic levitation, which makes it possible to initiate sequences of repeated collisions of a single sub-millimeter particle with a flat plate, and to precisely measure the particle charge in-situ after each collision. We show that collisional charge transfer between insulators is dependent on the hydrophobicity of the contacting surfaces. We use glass, which we modify by attaching nonpolar molecules to the particle, the plate, or both. We find that hydrophilic surfaces develop significant positive charges after contacting hydrophobic surfaces. Moreover, we demonstrate that charging between a hydrophilic and a hydrophobic surface is suppressed in an acidic environment and enhanced in a basic one. Application of an electric field during each collision is found to modify the charge transfer, again depending on surface hydrophobicity. We discuss these results within the context of contact charging due to ion transfer and show that they lend strong support to OH − ions as the charge carriers.

This study investigates six university professors' reflections on the shift to remote instruction during the Spring 2020 semester in response to the COVID-19 global pandemic. The rapid shift in instructional platform presents an opportunity to learn from unresolved challenges that persisted through the semester.Here we present a qualitative study of how experienced (i.e., associate or full) chemistry professors report their teaching practices in light of the COVID-19 disruptions. We observed four major themes: personal factors, contextual factors of the structure and culture, teacher thinking, and teachers' practice. These themes revealed that the professors in this study adapted quickly using institutionally offered platforms, modified their courses as minimally as possible, struggled with assessment, and held diverging beliefs about teaching and students. The outcomes of this study have implications for ongoing efforts to reform instructional practices at the institutional and departmental level. Specifically, we recommend similar studies to ascertain current faculty beliefs and instructional practices in other departments in order to identify shared visions for change and effective supports for enacting that change.

Nanomaterials dispersed in different media, such as liquids or polymers, generate a variety of functional composites with synergistic properties. In this work we discuss liquid metals as the

Dense suspension of particles in a liquid exhibit rich, non-Newtonian behaviors such as shear thickening and shear jamming. Shear thickening is known to be enhanced by increasing the particles' frictional interactions and also by making their shape more anisotropic. For shear jamming, however, only the role of interparticle friction has been investigated, while the effect of changing particle shape has so far not been studied systematically. To address this we here synthesize smooth silica particles and design the particle surface chemistry to generate strong frictional interactions such that dense, aqueous suspensions of spheres exhibit pronounced shear jamming. We then vary particle aspect ratio from Γ=1 (spheres) to Γ=11 (slender rods), and perform rheological measurements to determine the effect of particle anisotropy on the onset of shear jamming and its precursor, discontinuous shear thickening. Keeping the frictional interactions fixed, we find that increasing aspect ratio significantly reduces φm, the minimum particle packing fraction at which shear jamming can be observed, to values as low φm = 33% for Γ=11. The ability to independently control particle interactions due to friction and shape anisotropy yields fundamental insights about the thickening and jamming capabilities of suspensions and provides a framework to rationally design shear jamming characteristics. arXiv:1901.04051v1 [cond-mat.soft]