Showing papers by "Juan C. del Álamo published in 2007"

Spatio-temporal analysis of eukaryotic cell motility by improved force cytometry

Abstract: Cell motility plays an essential role in many biological systems, but precise quantitative knowledge of the biophysical processes involved in cell migration is limited. Better measurements are needed to ultimately build models with predictive capabilities. We present an improved force cytometry method and apply it to the analysis of the dynamics of the chemotactic migration of the amoeboid form of Dictyostelium discoideum. Our explicit calculation of the force field takes into account the finite thickness of the elastic substrate and improves the accuracy and resolution compared with previous methods. This approach enables us to quantitatively study the differences in the mechanics of the migration of wild-type (WT) and mutant cell lines. The time evolution of the strain energy exerted by the migrating cells on their substrate is quasi-periodic and can be used as a simple indicator of the stages of the cell motility cycle. We have found that the mean velocity of migration v and the period of the strain energy T cycle are related through a hyperbolic law v = L/T, where L is a constant step length that remains unchanged in mutants with adhesion or contraction defects. Furthermore, when cells adhere to the substrate, they exert opposing pole forces that are orders of magnitude higher than required to overcome the resistance from their environment.

Vorticity organization in the outer layer of turbulent channels with disturbed walls

Abstract: The vortex clusters in the turbulent outer region of rough- and smooth-walled channels, and their associated velocity structures, are compared using data from numerical experiments at friction Reynolds numbers Reτ 674. The results indicate that the roughness of the wall does not affect their properties, particularly the existence of wall-detached and wall-attached populations, and the self-similar size distribution of the latter. The average flow field conditioned to the attached clusters reveals similar conical structures of low streamwise velocity for the rough- and smoothwalled cases, which eventually grow into the global modes previously identified from spectral analysis. We conclude that the vortex clusters of the turbulent outer region either originate away from the wall, or quickly forget their origin, in agreement with Townsend’s similarity hypothesis. Turbulent wall flows are a challenging research subject with applications ranging from drag reduction to atmospheric dispersion. The analysis of how their outer layers may be influenced by the structures of the near-wall region is particularly useful, because the latter are altered by the hydraulically rough surfaces often found in engineering and geophysical flows, and because flow control strategies are usually implemented through actuators located at the wall. It has long been proposed that the inner–outer interactions are due to vortex loops connecting the two layers (see the review by Robinson 1991a). The best-known theoretical models are variations of the vortex hierarchies proposed by Perry & Chong (1982) and Perry, Henbest & Chong 1986, loosely based on Townsend’s (1976, pp. 150– 162) attached-eddy hypothesis. Although those models were initially constructed from