A new scenario for gravity detection in plants: the position sensor hypothesis.

The detection of gravity plays a fundamental role during the growth and evolution of plants. Although progress has been made in our understanding of the molecular, cellular and physical mechanisms involved in the gravity detection, a coherent scenario consistent with all the observations is still lacking. In this special issue article, we discuss recent experiments showing that the response to inclination of shoots is independent of the gravity intensity, meaning that the gravity sensor detects an inclination and not a force. This result questions some of the commonly accepted hypotheses and leads to propose a new 'position sensor hypothesis'. The implications of this new scenario are discussed in light of the different observations available in the literature.

Evidence of mechanically activated processes in slow granular flows.

We study how a shear band in a granular medium dramatically changes the mechanical behavior of the material further in the non sheared region. To this end, we carry out a microrheology experiment, where a constant force F is applied to a small rod immersed outside the shear band. In the absence of a shear band, a critical force F(c) is necessary to move the intruder. When a shear band exists, the intruder moves even for a force F less than the critical force F(c). We systematically study how the creep velocity V(creep) of the rod varies with F(c) - F and with the distance to the shear band, and show that the behavior can be described by an Eyring-like activated process.

Velocity correlations in dense granular flows.

Velocity fluctuations of grains flowing down a rough inclined plane are experimentally studied. The grains at the free surface exhibit fluctuating motions, which are correlated over a few grain diameters. The characteristic correlation length is shown to depend on the inclination of the plane and not on the thickness of the flowing layer. This result strongly supports the idea that dense granular flows are controlled by a characteristic length larger than the particle diameter.

Fluctuating particle motion during shear induced granular compaction.

Using a refractive index matching method, we investigate the trajectories of particles in three dimensional granular packing submitted to cyclic shear deformation. The particle motion observed during compaction is not diffusive but exhibits a transient cage effect, similar to the one observed in colloidal glasses. We precisely study the statistics of the step size between two successive cycles and observe that it is proportional to the shear amplitude. The link between the microscopic observations and the macroscopic evolution of the volume fraction during compaction is discussed.

Longitudinal vortices in granular flows.

We present a new instability observed in rapid granular flows down rough inclined planes. For high inclinations and flow rates, the free surface of the flow experiences a regular deformation in the transverse direction. Measurements of the surface velocities imply that this instability is associated with the formation of longitudinal vortices in the granular flow. From the experimental observations, we propose a mechanism for the longitudinal vortex formation based on the concept of granular temperature.

Segregation induced instabilities of granular fronts.

Experimental investigation of granular flows containing particles of several sizes and moving down slopes shows that segregation of coarse-grained, irregularly shaped particles induces a fingering instability at the propagating front. The size-segregation mechanism involves percolation of small particles downward and a corresponding migration of large ones toward the flow surface. Large particles at the flow surface experience velocities that are greater than average so that they migrate forward and begin to collect at the flow front. In the case of dry cohesionless flows, the instability depends upon these large particles at the flow perimeter being more angular and thus more resistant to flow than the smaller rounder ones in the interior. A simple analytical model predicts the fingering instability when friction of the flow front is greater than that of the following flow. The presence of viscous liquid inhibits both size-segregation and the development of the instability. Fluidization of dry flows permits segregation of large particles to flow perimeters, thus increasing permeability and permitting a similar instability that owes its development to the dry frictional perimeter that surrounds a partly fluidized interior. (c) 1999 American Institute of Physics.