Despite its widespread use in nanocomposites, the effect of embedding graphene in highly viscoelastic polymer matrices is not well-understood. We add graphene to a lightly crosslinked polysilicone, often encountered as Silly Putty, changing its electro-mechanical properties significantly. The resulting nanocomposites display unusual electromechanical behavior such as post-deformation temporal relaxation of electrical resistance and nonmonotonic changes in resistivity with strain. These phenomena are associated with the mobility of the nanosheets in the low-viscosity polymer matrix. By considering both the connectivity and mobility of the nanosheets, we develop a quantitative model that completely describes the electromechanical properties. These nanocomposites are sensitive electromechanical sensors with gauge factors >500 which can measure pulse, blood pressure and even the impact associated with the footsteps of a small spider.

Size separation of granular particlesG ranular media differ from other materials in their response to stirring or jostling -unlike two-fluid systems, bi-disperse granular mixtures will separate according to particle size when shaken, with large particles rising, a phenomenon termed the 'Brazil-nut effect' 1-8 . Mounting evidence indicates that differences in particle density affect size separation in mixtures of granular particles 9-11 . We show here that this density dependence does not follow a steady trend but is non-monotonic and sensitive to background air pressure. Our results indicate that particle density and interstitial air must both be considered in size segregation.Explanations of the Brazil-nut effect, which has been known since the 1930s, have focused either on infiltration of small particles into voids created underneath larger ones during shaking 1-5 or on granular convection 6-8 , and have implied densityindependent rising times for the larger 'intruder' particles. However, an increase in the velocity of a large intruder with increasing density has been reported 9,10 , suggesting that increased inertia might play a role. Furthermore, in computer simulations 10 , a 'reverse' Brazil-nut effect was found, in which groups of larger particles, if heavy enough, segregate to the bottom.A monotonic density dependence implied by such mechanisms 9-11 is incompatible with our measurements of intruder rising times over a wide range of size and density ratios (Fig. 1). We tracked an intruder particle in the presence of granular convection produced by vertically shaking a three-dimensional cylinder filled with smaller background particles (density, ț m ). A spherical intruder (diameter, D; density, ț) was placed at a depth z 0 below the surface; a hollow acrylic ball filled with foam and lead shot was used to tune the intruder density. Material properties other than density, such as coefficients of restitution and friction, had no measurable impact.For a fixed intruder diameter, the measured rising time, T rise , to the free surface exhibits a pronounced peak as a function of ț/ț m (Fig. 1). This peak is not affected by variations in shaking parameters, background medium (glass beads, poppy seeds) and system size. Compared with convection measured in the absence of an intruder (dotted line), the intruder rises faster both at large and small ț/ț m , but more slowly when ț/ț m ഠ0.5. A monotonic dependence, T rise ഠ(ț/ț m ) ǁ1/2 , proposed for a two-dimensional system 10 , is incompatible with our data. The presence of a large intruder perturbs the convective flow of the background particles. Data above the horizontal dotted lines in Fig. 1 therefore do not necessarily imply sinking intruders 9 in the absence of convection. The peak in T rise becomes significant for diameter ratios D/d>10, increasing with increasing intruder size (Fig. 1, inset).Measurements of intruder velocity as a function of depth show that the increase in T rise with ț/ț m to the left of the peak is caused by behaviour that takes place as th...

The shear flow of two-dimensional foams is probed as a function of shear rate and disorder. Disordered, bidisperse foams exhibit strongly shear rate dependent velocity profiles. This behavior is captured quantitatively in a simple model based on the balance of the time-averaged drag forces in the system, which are found to exhibit power-law scaling with the foam velocity and strain rate. Disorder makes the scaling of the bulk drag forces different from that of the local interbubble drag forces, which we evidence by rheometrical measurements. In monodisperse, ordered foams, rate independent velocity profiles are found, which lends further credibility to this picture.

The evolution of granular shear flow is investigated as a function of height in a split-bottom Couette cell. Using particle tracking, magnetic-resonance imaging, and large-scale simulations we find a transition in the nature of the shear as a characteristic height H * is exceeded. Below H * there is a central stationary core; above H * we observe the onset of additional axial shear associated with torsional failure. Radial and axial shear profiles are qualitatively different: the radial extent is wide and increases with height while the axial width remains narrow and fixed.PACS numbers: 45.70. Mg, 83.50.Ax Shear bands in dense granular materials are localized regions of large velocity gradients; they are the antithesis of the broad uniform flows seen in slowly-sheared Newtonian fluids [1,2,3,4,5,6]. Until recently it was generally assumed that all granular shear bands were narrow. However, in 2003 Fenistein et al. [7] discovered that in modified Couette cells granular shear bands can be made arbitrarily broad. In this geometry, the bottom of a cylindrical container is split at radius r = R s and shear is produced by rotating both the outer ring and the cylindrical boundary of the container while keeping the central disk (r < R s ) stationary. For very shallow packs, the shear band measured at the top surface is narrow and located at r = R s so that the inner region directly above the central disk is stationary while the remaining part rotates as a solid. As the filling height of the material, H, increases, the shear band increases in radial width and moves toward the cylinder axis. For sufficiently large H, the shear band overlaps the axis at r = 0 and one might expect qualitatively new behavior. Indeed, Unger et al. [8] predicted that the shape of the boundary between moving and stationary material would undergo a first-order transition as H is increased past a threshold value H * : the shearing region which for H < H * is open at the top and intersects the free surface abruptly collapses to a closed cupola completely buried inside the bulk.Previous experiments focused primarily on the surface flows in shallow containers and left unexplored many questions about the shape and evolution of the shear profiles for large H. Here, we combine magnetic resonance imaging (MRI) and high-speed video observations with large-scale simulations to explore shear flow both for shallow and tall packs. In addition to monitoring the evolution of the flow profiles in the radial direction, we also examine shear in the vertical direction. Instead of a first order collapse of the shear zone as proposed by Unger et al.[8], we find that above H * ≃ 0.6R s , the inner core of immobile material disappears gradually as shear along the central axis of the cylinder sets in. Our setup is similar to that of Fenistein et al. [7] except that we rotate the inner disk instead of the outer ring and cylinder (Fig.1b inset). In the absence of inertial effects, this makes no difference to the results. For surface observations with high-speed video ...

Thin streams of liquid commonly break up into characteristic droplet patterns owing to the surface-tension-driven Plateau-Rayleigh instability. Very similar patterns are observed when initially uniform streams of dry granular material break up into clusters of grains, even though flows of macroscopic particles are considered to lack surface tension. Recent studies on freely falling granular streams tracked fluctuations in the stream profile, but the clustering mechanism remained unresolved because the full evolution of the instability could not be observed. Here we demonstrate that the cluster formation is driven by minute, nanoNewton cohesive forces that arise from a combination of van der Waals interactions and capillary bridges between nanometre-scale surface asperities. Our experiments involve high-speed video imaging of the granular stream in the co-moving frame, control over the properties of the grain surfaces and the use of atomic force microscopy to measure grain-grain interactions. The cohesive forces that we measure correspond to an equivalent surface tension five orders of magnitude below that of ordinary liquids. We find that the shapes of these weakly cohesive, non-thermal clusters of macroscopic particles closely resemble droplets resulting from thermally induced rupture of liquid nanojets.

The electrolytic production of gas bubbles involves three steps--nucleation, growth, and detachment. Here the growth of hydrogen bubbles and their detachment from a platinum microelectrode of diameter 125 μm are studied using high-speed photography and overpotential frequency spectrum (noise) analysis. The periodic release of large <800 μm bubbles--gas oscillator behavior--was often observed, with a corresponding periodic oscillation of the overpotential which is reflected as a main peak and a series of harmonics in the power spectral density. The release frequency is inversely correlated with the bubble size and hydrogen production rate. When the coalescence of bubbles at the electrode surface is inhibited, either chemically with a surfactant or ethylene glycol or hydrodynamically by magnetically induced convection, swarms of small ∼50 μm bubbles are released in an aperiodic stream. The abrupt transition from periodic to aperiodic release occurs when the surface tension falls below 70 mN m(-1). Hydrogen bubble growth is also studied on a transparent platinum thin-film electrode, where the bubble coalescence can be observed directly. It leaves sessile droplets of electrolyte within the footprint of the growing bubble, showing that the growth involves scavenging smaller bubbles from solution due to hydrogen generated directly at the electrode. A possible role of nanobubbles in the lift-off process is discussed.

Herein we present the use of lanthanide directed self-assembly formation (Ln(III) = Eu(III), Tb(III)) in the generation of luminescent supramolecular polymers, that when swelled with methanol give rise to self-healing supramolecular gels. These were analyzed by using luminescent and (1)H NMR titrations studies, allowing for the identification of the various species involved in the subsequent Ln(III)-gel formation. These highly luminescent gels could be mixed to give a variety of luminescent colors depending on their Eu(III):Tb(III) stoichiometric ratios. Imaging and rheological studies showed that these gels prepared using only Eu(III) or only Tb(III) have different morphological and rheological properties, that are also different from those determined upon forming gels by mixing of Eu(III) and Tb(III) gels. Hence, our results demonstrate for the first time the crucial role the lanthanide ions play in the supramolecular polymerization process, which is in principle a host-guest interaction, and consequently in the self-healing properties of the corresponding gels, which are dictated by the same host-guest interactions.

We experimentally investigate flow of quasi two-dimensional disordered foams in Couette geometries, both for foams squeezed below a top plate and for freely floating foams (bubble rafts). With the top plate, the flows are strongly localized and rate dependent. For the bubble rafts the flow profiles become essentially rate-independent, the local and global rheology do not match, and in particular the foam flows in regions where the stress is below the global yield stress. We attribute this to nonlocal effects and show that the "fluidity" model recently introduced by Goyon et al. (Nature, 454 (2008)) captures the essential features of flow both with and without a top plate.