Role of the membrane for mechanosensing by tethered channels

Biologically important membrane channels are gated by force at attached tethers. Here, we generically characterize the non-trivial interplay of force, membrane tension, and channel deformations that can affect gating. A central finding is that minute conical channel deformation under force leads to significant energy release during opening. We also calculate channel-channel interactions and show that they can amplify force sensitivity of tethered channels

Self-crumpling elastomers: bending induced by the drying stimulus of a nanoparticle suspension

We report an experimental study of the drying-induced peeling of a bilayer, consisting of an elastomeric disk coated with a suspension of nanoparticles. We show that although capillary forces associated with the scale of the droplet can not compete with the adhesion of the elastomer on a surface, nevertheless large tensile stresses develop in the coating, which results in a moment bending the bilayer. We attribute this stress to the nano-menisci in the pores of the colloidal material and we propose a model that describes successfully the early stage curvature of the bilayer. Thus, we show that the peeling can be conveniently controlled by the particle size and the coating thickness.Comment: 6 pages, 5 figures, 1 table, accepted in EP

Liquid acrobatics

We experiment with injecting a continuous stream of gas into a shallow liquid, similar to how one might blow into a straw placed at the bottom of a near-empty drink. By varying the angle of the straw (here a metal needle), we observe a variety of dynamics, which we film using a high-speed camera. Most noteworthy is an intermediate regime in which cyclical jets erupt from the air-liquid interface and breakup into air-born droplets. These droplets trace out a parabolic trajectory and bounce on the air-liquid interface before eventually coalescing. The shape of each jet, as well as the time between jets, is remarkably similar and leads to droplets with nearly identical trajectories. The following article accompanies the linked fluid dynamics video submitted to the Gallery of Fluid Motion in 2008.Comment: Accompanies video submission to APS DFD 2008 Gallery of Fluid Motion, low http://ecommons.library.cornell.edu/bitstream/1813/11469/3/Bird_DFD2008_mpeg1.mpg , and high resolution http://ecommons.library.cornell.edu/bitstream/1813/11469/2/Bird_DFD2008_mpeg2.mp

Shear dispersion in dense granular flows

We formulate and solve a model problem of dispersion of dense granular materials in rapid shear flow down an incline. The effective dispersivity of the depth-averaged concentration of the dispersing powder is shown to vary as the P\'eclet number squared, as in classical Taylor--Aris dispersion of molecular solutes. An extensions to generic shear profiles is presented, and possible applications to industrial and geological granular flows are noted.Comment: 6 pages, 2 figures, Springer svjour3 format; to appear in Granular Matte

Coffee-stain growth dynamics on dry and wet surfaces

The drying of a drop containing particles often results in the accumulation of the particles at the contact line. In this work, we investigate the drying of an aqueous colloidal drop surrounded by a hydrogel that is also evaporating. We combine theoretical and experimental studies to understand how the surrounding vapor concentration affects the particle deposit during the constant radius evaporation mode. In addition to the common case of evaporation on an otherwise dry surface, we show that in a configuration where liquid is evaporating from a flat surface around the drop, the singularity of the evaporative flux at the contact line is suppressed and the drop evaporation is homogeneous. For both conditions, we derive the velocity field and we establish the temporal evolution of the number of particles accumulated at the contact line. We predict the growth dynamics of the stain and the drying timescales. Thus, dry and wet conditions are compared with experimental results and we highlight that only the dynamics is modified by the evaporation conditions, not the final accumulation at the contact line

Coalescence of Liquid Drops

When two drops of radius $R$ touch, surface tension drives an initially singular motion which joins them into a bigger drop with smaller surface area. This motion is always viscously dominated at early times. We focus on the early-time behavior of the radius \rmn of the small bridge between the two drops. The flow is driven by a highly curved meniscus of length 2\pi \rmn and width \Delta\ll\rmn around the bridge, from which we conclude that the leading-order problem is asymptotically equivalent to its two-dimensional counterpart. An exact two-dimensional solution for the case of inviscid surroundings [Hopper, J. Fluid Mech. ${\bf 213}$, 349 (1990)] shows that \Delta \propto \rmn^3 and \rmn \sim (t\gamma/\pi\eta)\ln [t\gamma/(\eta R)]; and thus the same is true in three dimensions. The case of coalescence with an external viscous fluid is also studied in detail both analytically and numerically. A significantly different structure is found in which the outer fluid forms a toroidal bubble of radius \Delta \propto \rmn^{3/2} at the meniscus and \rmn \sim (t\gamma/4\pi\eta) \ln [t\gamma/(\eta R)]. This basic difference is due to the presence of the outer fluid viscosity, however small. With lengths scaled by $R$ a full description of the asymptotic flow for \rmn(t)\ll1 involves matching of lengthscales of order \rmn^2, \rmn^{3/2}, \rmn$, 1 and probably$\rmn^{7/4}$.Comment: 36 pages, including 9 figure

Collective force generation by groups of migrating bacteria

From biofilm and colony formation in bacteria to wound healing and embryonic development in multicellular organisms, groups of living cells must often move collectively. While considerable study has probed the biophysical mechanisms of how eukaryotic cells generate forces during migration, little such study has been devoted to bacteria, in particular with regard to the question of how bacteria generate and coordinate forces during collective motion. This question is addressed here for the first time using traction force microscopy. We study two distinct motility mechanisms of Myxococcus xanthus, namely twitching and gliding. For twitching, powered by type-IV pilus retraction, we find that individual cells exert local traction in small hotspots with forces on the order of 50 pN. Twitching of bacterial groups also produces traction hotspots, however with amplified forces around 100 pN. Although twitching groups migrate slowly as a whole, traction fluctuates rapidly on timescales <1.5 min. Gliding, the second motility mechanism, is driven by lateral transport of substrate adhesions. When cells are isolated, gliding produces low average traction on the order of 1 Pa. However, traction is amplified in groups by a factor of ~5. Since advancing protrusions of gliding cells push on average in the direction of motion, we infer a long-range compressive load sharing among sub-leading cells. Together, these results show that the forces generated during twitching and gliding have complementary characters and both forces are collectively amplified in groups