Diacamma ants adjust liquid foraging strategies in response to biophysical constraints.

Ant foragers provide food to the rest of the colony, often requiring transport over long distances. Foraging for liquid is challenging because it is difficult to transport and share. Many social insects store liquids inside the crop to transport them to the nest, and then regurgitate to distribute to nest-mates through a behaviour called trophallaxis. Some ants instead transport fluids with a riskier behaviour called pseudotrophallaxis-holding a drop of liquid between the mandibles through surface tension. Ants share this droplet with nest-mates without ingestion or regurgitation. We hypothesised that ants optimize their liquid-collection approach depending on viscosity. Using an ant that employs both trophallaxis and pseudotrophallaxis, we investigated the conditions where each liquid-collection behaviour is favoured by measuring biophysical properties, collection time and reaction to food quality for typical and viscosity-altered sucrose solutions. We found that ants collected more liquid per unit time by mandibular grabbing than by drinking. At high viscosities ants switched liquid collection method to mandibular grabbing in response to viscosity and not to sweetness. Our results demonstrate that ants change transport and sharing methods according to viscosity-a natural proxy for sugar concentration-thus increasing the mass of sugar returned to the nest per trip.

Dynamics of bubbles spontaneously entering into a tube.

When an open tube of small diameter touches a bubble of a larger diameter, the bubble spontaneously shrinks and pushes a soap film into the tube. We characterize the dynamics for different bubble sizes and number of soap films in the tube. We rationalize this observation from a mechanical force balance involving the Laplace pressure of the bubble and the viscous force from the advancing soap lamellae in the tube. We propose a numerical resolution of this model, and an analytical solution in an asymptotic regime. These predictions are then compared to the experiments. The emptying duration is primarily affected by the initial bubble to tube diameter ratio and by the number of soap films in the tube.

Roughness-Induced Friction on Liquid Foams.

Complex liquids flow is known to be drastically affected by the roughness condition at the interfaces. We combined stresses measurements and observations of the flow during the motion of different rough surfaces in dry liquid foams. We visually show that three distinct friction regimes exist: slippage, stick-slip motion, and anchored soap films. Our stress measurements are validated for slippage and anchored regimes based on existing models, and we propose a leverage rule to describe the stresses during the stick-slip regime. We find that the occurrence of the stick-slip or anchored regimes is controlled by the roughness factor, defined as the ratio between the size of the surface asperities and the radius of curvature of the Plateau borders.

Generation of Silicone Poly-HIPEs with Controlled Pore Sizes via Reactive Emulsion Stabilization.

Macrocellular silicone polymers are obtained after solidification of the continuous phase of a poly(dimethylsiloxane) emulsion, which contains poly(ethylene glycol) drops of sub-millimetric dimensions. Coalescence of the liquid template emulsion is prohibited by a reactive blending approach. The relationship is investigated in detail between the interfacial properties and the emulsion stability, and micro- and millifluidic techniques are used to generate macrocellular polymers with controlled structural properties over a wider range of cell sizes (0.2-2 mm) and volume fractions of the continuous phase (0.1%-40%). This approach could easily be transferred to a wide range of polymeric systems.

Memory of shear flow in soft jammed materials.

Cessation of flow in yield stress fluids results in a stress relaxation process that eventually leads to a finite residual stress. Both the rate of stress relaxation and the magnitude of the residual stresses systematically depend on the preceding flow conditions. To assess the microscopic origin of this memory effect, we combine experiments with large-scale computer simulations, exploring the behavior of jammed suspensions of soft repulsive particles. A spatiotemporal analysis of particle motion reveals that memory formation during flow is primarily governed by the emergence of domains of spatially correlated nonaffine displacements. These domains imprint the configuration of stress imbalances that drive dynamics upon flow cessation, as evidenced by a striking equivalence of the spatial correlation patterns in particle displacements observed during flow and upon flow cessation. Additional contributions to stress relaxation result from the particle packing that reorganizes to minimize the resistance to flow by decreasing the number of locally stiffer configurations. Regaining rigidity upon flow cessation drives further relaxation and effectively sets the magnitude of the residual stress. Our findings highlight that flow in yield stress fluids can be seen as a training process during which the material stores information of the flowing state through the development of domains of correlated particle displacements and the reorganization of particle packings optimized to sustain the flow. This encoded memory can then be retrieved in flow cessation experiments.