Suspensions of cornstarch in water exhibit strong dynamic shear-thickening. We show that partly replacing water by ethanol strongly alters the suspension rheology. We perform steady and nonsteady rheology measurements combined with atomic force microscopy to investigate the role of fluid chemistry on the macroscopic rheology of the suspensions and its link with the interactions between cornstarch grains. Upon increasing the ethanol content, the suspension goes through a yield-stress fluid state and ultimately becomes a shear-thinning fluid. On the cornstarch grain scale, atomic force microscopy measurements reveal the presence of polymers on the cornstarch surface, which exhibit a cosolvency effect. At intermediate ethanol content, a maximum of polymer solubility induces high microscopic adhesion which we relate to the macroscopic yield stress.Suspensions are mixtures of undissolved particles in a liquid. They are literally found all around us: mud, paints, pastes and blood [1]. The viscosity of a dense suspension can vary by orders of magnitude in a small shear rate interval [2]. Subjected to an increasing shear rate, dense suspensions first tend to become less viscous (shear-thinning) and then more viscous (shearthickening). The viscosity of some suspensions, especially non-Brownian ones, may increase so much that they effectively become solid [3]. Although standard rheology measurements provide a great tool to study this phenomenon [e.g. 4, 5], they are mainly limited to steadystate conditions.Many studies point out that dense suspensions exhibit remarkable dynamic phenomena emerging under non-steady-shear conditions: stable holes in thin vibrated layers [6], non-monotonic settling [7], dynamic compaction front [8] or fracturing [3]. Oscillatory rheology helps to describe some of these dynamic behaviours [9], but remains limited to constant volume conditions. Dynamic shear-thickening has been widely investigated [10], but its physical origin remains an active debate. Although several parameters seem to contribute to it (e.g. particles size [11], shape [12] or roughness [13]), it has become increasingly clear that frictional and non-contact interactions between particles play a key role [14,15]. Such interactions are easily modified in numerical simulations, but present a real challenge in experiments. Consequently, only few experimental studies addressthe role of particle-particle interactions in dense suspensions rheology [e.g. 5, 16] however lacking systematic variation of these interactions. Moreover, direct measurements of these interactions in relation to the rheology are also lacking so far.Here, we directly probe the microscopic interactions between individual particles and explore their link with * adeline.pons@normalesup.org the macroscopic rheology for dense cornstarch (CS) suspensions. The archetypical suspension of CS grains in water exhibits a strong dynamic shear-thickening [3,[6][7][8]. Interestingly, Taylor [17] shows that replacing water by polypropylene glycol in CS suspensions completely su...

Arbuscular mycorrhizal fungi function as conduits for underground nutrient transport. While the fungal partner is dependent on the plant host for its carbon (C) needs, the amount of nutrients that the fungus allocates to hosts can vary with context. Because fungal allocation patterns to hosts can change over time, they have historically been difficult to quantify accurately. We developed a technique to tag rock phosphorus (P) apatite with fluorescent quantum-dot (QD) nanoparticles of three different colors, allowing us to study nutrient transfer in an in vitro fungal network formed between two host roots of different ages and different P demands over a 3-week period. Using confocal microscopy and raster image correlation spectroscopy, we could distinguish between P transfer from the hyphae to the roots and P retention in the hyphae. By tracking QD-apatite from its point of origin, we found that the P demands of the younger root influenced both: (1) how the fungus distributed nutrients among different root hosts and (2) the storage patterns in the fungus itself. Our work highlights that fungal trade strategies are highly dynamic over time to local conditions, and stresses the need for precise measurements of symbiotic nutrient transfer across both space and time.

No abstract

High speed imaging was used to capture the fast dynamics of two injection methods. The first one and perhaps the oldest known, is based on solid needles and used for dermal pigmentation, or tattooing. The second, is a novel needle-free micro-jet injector based on thermocavitation. We performed injections in agarose gel skin surrogates, and studied both methods using ink formulations with different fluidic properties to understand better the end-point injection. Both methods were used to inject water and a glycerin-water mixture. Commercial inks were used with the tattoo machine and compared with the other liquids injected. The agarose gel was kept stationary or in motion at a constant speed, along a plane perpendicular to the needle. The agarose deformation process due to the solid needle injection was also studied. The advantages and limitations of both methods are discussed, and we conclude that micro-jet injection has better performance than solid injection when comparing several quantities for three different liquids, such as the energy and volumetric delivery efficiencies per injection, depth and width of penetrations. A newly defined dimensionless quantity, the penetration strength, is used to indicate potential excessive damage to skin surrogates. Needle-free methods, such as the micro-jet injector here presented, could reduce the environmental impact of used needles, and benefit the health of millions of people that use needles on a daily basis for medical and cosmetic use. arXiv:1811.00510v1 [physics.app-ph] 1 Nov 2018

Theory suggests that relatives will cooperate more, and compete less, because of an increased benefit for shared genes. In symbiotic partnerships, hosts may benefit from interacting with highly related symbionts because there is less conflict among the symbionts. This has been difficult to test empirically. We used the arbuscular mycorrhizal symbiosis to study the effects of fungal relatedness on host and fungal benefits, creating fungal networks varying in relatedness between two hosts, both in soil and in‐vitro. To determine how fungal relatedness affected overall transfer of nutrients, we fluorescently tagged phosphorus and quantified resource distribution between two root systems. We found that colonization by less‐related fungi was associated with increased fungal growth, lower transport of nutrients across the network, and lower plant benefit ‐ likely an outcome of increased fungal competition. More generally, we demonstrate how symbiont relatedness can mediate benefits of symbioses.

The behavior of a vertically vibrated granular bed is reminiscent of a liquid in that it exhibits many phenomena such as convection and Faraday-like surface waves. However, when the lateral dimensions of the bed are confined such that a quasi-one-dimensional geometry is formed, the only phenomena that remain are bouncing bed and the granular Leidenfrost effect. This permits the observation of the granular Leidenfrost state for a wide range of energy injection parameters and more specifically allows for a thorough characterization of the low-frequency oscillation (LFO) that is present in this state. In both experiments and particle simulations we determine the LFO frequency from the power spectral density of the center-of-mass signal of the grains, varying the amplitude and frequency of the driving, the particle diameter, and the number of layers in the system. We thus find that the LFO frequency (i) is inversely proportional to the fast inertial timescale and (ii) decorrelates with a typical decay time proportional to the slow dissipative timescale in the system. The latter is consistent with the view that the LFO is driven by the inherent noise that is present in the granular Leidenfrost state with a low number of particles.

Abstract. In this work we experimentally study a granular rotor which is similar to the famous Smoluchowski-Feynman device and which consists of a rotor with four vanes immersed in a granular gas. Each side of the vanes can be composed of two different materials, creating a rotational asymmetry and turning the rotor into a ratchet. When the granular temperature is high, the rotor is in movement all the time, and its angular velocity distribution is well described by the Brownian Limit discussed in previous works. When the granular temperature is lowered considerably we enter the so-called Single Kick Limit, where collisions occur rarely and the unavoidable external friction causes the rotor to be at rest for most of the time. We find that the existing models are not capable of adequately describing the experimentally observed distribution in this limit. We trace back this discrepancy to the non-constancy of the deceleration due to external friction and show that incorporating this effect into the existing models leads to full agreement with our experiments. Subsequently, we extend this model to describe the angular velocity distribution of the rotor for any temperature of the gas, and obtain a very good agreement between the model and experimental data.

Relatives are expected to cooperate more and compete less. In symbiotic partnerships, hosts may benefit from interacting with highly-related symbionts because there is less conflict. This has been difficult to test empirically. We used the arbuscular mycorrhizal symbiosis to study the effects of fungal relatedness on host and fungal benefits, creating fungal networks varying in relatedness (self vs. non-self) between two host plants, in both soil and in-vitro systems. To determine how fungal relatedness affected overall transfer of nutrients to host roots, we fluorescently tagged phosphorus and quantified resource distribution. We imaged fungal networks, and related fractal dimension to fungal growth strategies. We found that decreased relatedness was associated with increased fungal growth and complexity, lower transport of nutrients across the fungal network, and lower plant benefit, likely an outcome of increased fungal competition. More generally, we demonstrate how relatedness among symbionts can mediate the overall benefits of symbiotic partnerships.