Colloidal shear thickening presents a significant challenge because the macroscopic rheology becomes increasingly controlled by the microscopic details of short ranged particle interactions in the shear thickening regime. Our measurements here of the first normal stress difference over a wide range of particle volume fraction elucidate the relative contributions from hydrodynamic lubrication and frictional contact forces, which have been debated. At moderate volume fractions we find N1 < 0, consistent with hydrodynamic models, however at higher volume fractions and shear stresses these models break down and we instead observe dilation (N1 > 0), indicating frictional contact networks. Remarkably, there is no signature of this transition in the viscosity, instead this change in the sign of N1 occurs while the shear thickening remains continuous. These results suggest a scenario where shear thickening is driven primarily by the formation of frictional contacts, with hydrodynamic forces playing a supporting role at lower concentrations. Motivated by this picture, we introduce a simple model which combines these frictional and hydrodynamic contributions and accurately fits the measured viscosity over a wide range of particle volume fraction and shear stress. [3] suggesting that contact friction plays a dominant role in colloidal shear thickening, however this assertion is controversial because of contrary evidence. While friction-based models and simulations capture the viscosity increase observed in experiments, other experimental signatures, particularly the stress anisotropy, are at odds with expectations for frictional interactions [4].Shear thickening, where a suspension's viscosity η = σ/γ increases with increasing shear stress σ (or shear rateγ), is important in a wide array of industrial processes and applications, either something to be avoided or a desired, engineered property [5][6][7]. Shear thickening is observed in both granular suspensions, where the particle diameter d is generally d 10 µm, and colloidal suspensions, where d 10 µm. In granular suspensions, the evidence that friction drives shear thickening is well established [8][9][10][11][12][13][14][15][16] but in colloidal suspensions shear thickening is instead commonly attributed to diverging hydrodynamic lubrication forces, which lock particles together in correlated 'hydroclusters' [17][18][19][20][21].A key difference between friction and lubrication forces lies in the stress anisotropy generated by these two types of interactions. This difference is captured by the first normal stress difference N 1 ≡ σ xx − σ zz , where σ ij is the stress tensor for a shear flow in the x direction with a gradient along z. Simulations based on hydrodynamic interactions show that shear-induced distortions of the suspension microstructure and short ranged lubrication forces drive N 1 < 0 [7,18,19,22]. Including repulsive interactions or elastic particle deformations to these hydrodynamic models does not change the sign of N 1 [23][24][25], and N 1 is predicte...

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.

W hen a heavy sphere is dropped onto a bed of loose, fine sand, a remarkable phenomenon occurs: a large, focused jet of sand shoots upwards 1-4 . Although similar looking jets are observed on impact in fluid systems 5-7 , they are held together by surface tension. Surprisingly, the granular jet exists in the absence of both surface tension and cohesion, thus fluid jet models are of limited use. Previous work 1,2 proposed that the jet is created solely by the gravity-driven collapse of a void left by the sphere's descent through the pack. Here we present experimental evidence that granular jets are instead driven by a more complex process involving the interaction between the sand and interstitial air. Using high-speed X-ray radiography, and high-speed digital video, we observe the formation of the jet both inside and above the bed. We find that what previously was thought of as a single jet in fact consists of two components: a wispy, thin jet that varies little with pressure followed by a thick air-pressure-driven jet. This is further evidence that qualitatively new phenomena in granular systems can emerge as a function of air pressure 8-11 . Our results highlight the importance of the dynamic coupling between gas and granule motion.As discovered in ref.

The disordered static structure and chaotic dynamics of frictional granular matter has occupied scientists for centuries, yet there are few organizational principles or guiding rules for this highly hysteretic, dissipative material. We show that cyclic shear of a granular material leads to dynamic self-organization into several phases with different spatial and temporal order. Using numerical simulations, we present a phase diagram in strain-friction space that shows chaotic dispersion, crystal formation, vortex patterns, and most unusually a disordered phase in which each particle precisely retraces its unique path. However, the system is not reversible. Rather, the trajectory of each particle, and the entire frictional, many-degrees-of-freedom system, organizes itself into a limit cycle absorbing state. Of particular note is that fact that the cyclic states are spatially disordered, whereas the ordered states are chaotic.granular | self-organization | limit cycles | friction S elf-organization under periodic driving is a common feature in many disparate far-from-equilibrium systems (1-4). Recently, there has been substantial interest in self-organization in suspensions under slow, cyclic, low-Reynolds number shear (1, 5, 6). Non-Brownian suspensions were found to exhibit a phase transition from a dynamic fluctuating state to a reversible absorbing state depending on the shear amplitude and particle density. Here, there is a clear route to reversibility, because at low Reynolds numbers the equations of motion for the fluid are reversible, and irreversibility is only introduced through particle collisions or potential interactions. A similar cyclic behavior has been observed in periodically driven superconducting vortices (4), which interact via a nonlinear potential.If one instead considers a pack of frictional grains, this picture changes dramatically and the conditions for reversibility/irreversibility and periodic motion remain poorly understood (7-9). Here, the equations of motion are not time-reversible and particles remain in contact with their neighbors at all times, interacting via hysteretic frictional forces instead of simple collisions. Although a priori the system is hysteretic and irreversible, it is still possible to exhibit correlations and self-organization. Indeed, experimental studies of grains under cyclic shear have found spontaneous crystallization (10, 11), dynamic heterogeneities (12), and subdiffusive caged motion (9, 13). Recent experiments on 2D frictional disks found limit cycles in the shear stress and pressure in shear-jammed packings (14), although the individual grain motion remains diffusive. However, experiments are limited to a narrow range of friction and one cannot easily vary this crucial parameter that determines the possible packing configurations (15) and the response of the granular system to applied shear. Although we do not find reversible states, we do find that, depending on the strain amplitude and grain friction, the granular system can organize itself and evolve i...

Competing time scales generate novelty. Here, we show that a coupling between the time scales imposed by instrument inertia and the formation of inter-particle frictional contacts in shear-thickening suspensions leads to highly asymmetric shear-rate oscillations. Experiments tuning the presence of oscillations by varying the two time-scales support our model. The observed oscillations give access to a shear-jamming portion of the flow curve that is forbidden in conventional rheometry. Moreover, the oscillation frequency allows us to quantify an intrinsic relaxation time for particle contacts. The coupling of fast contact network dynamics to a slower system variable should be generic to many other areas of dense suspension flow, with instrument inertia providing a paradigmatic example.

No abstract

As demonstrated by means of DNA nanoconstructs[1], as well as DNA functionalization of nanoparticles[2-4] and micrometre-scale colloids[5-8], complex self-assembly processes require components to associate with particular partners in a programmable fashion. In many cases the reversibility of the interactions between complementary DNA sequences is an advantage[9]. However, permanently bonding some or all of the complementary pairs may allow for flexibility in design and construction[10]. Here, we show that the substitution of a pair of complementary bases by a cinnamate group provides an efficient, addressable, UV light-based method to covalently bond complementary DNA. To show the potential of this approach, we wrote micrometre-scale patterns on a surface via UV light and demonstrate the reversible attachment of conjugated DNA and DNA-coated colloids. Our strategy enables both functional DNA photolithography and multi-step, specific binding in self-assembly processes.

Abstract. -We examine the impact of a solid sphere into a fine-grained granular bed. Using high-speed X-ray radiography we track both the motion of the sphere and local changes in the bed packing fraction. Varying the initial packing density as well as the ambient gas pressure, we find a complete reversal in the effect of interstitial gas on the impact response of the bed: The dynamic coupling between gas and grains allows for easier penetration in initially loose beds but impedes penetration in more densely packed beds. High-speed imaging of the local packing density shows that these seemingly incongruous effects have a common origin in the resistance to bed packing changes caused by interstitial air.Introduction. -Granular materials often exhibit behavior intermediate between that of conventional solids and liquids. Probing the resulting combination of liquidand solid-like properties a number of recent studies investigated the impact of a large object into a bed of dry grains. These studies focused on issues such as the drag on the impacting object [1-6], crater formation [7-10] the corona-like splash formed immediately after the impactor hits the bed surface [10], and the subsequent jet of grains formed by the collapse of the cavity left by the impactor [11][12][13][14][15][16]. So far however, almost all work considered the limit of loosely packed, marginally stable beds that readily compact in response to perturbations. On the other hand, densely packed beds must dilate in order for grains to move out of the way of inserted objects. This implies different resistance not only for slow, quasi-static perturbations [17] but also suggests that there should be a significant change in the dynamics for faster impacts.An important feature of the impact dynamics in granular systems is the coupling between the interstitial gas, typically air, and the grain packing. For fine grained beds (grain diameters below ∼ 150 µm) this interaction can drastically change the impact dynamics. In particular, in