7,843 research outputs found
Fluorescent visualization of a spreading surfactant
The spreading of surfactants on thin films is an industrially and medically
important phenomenon, but the dynamics are highly nonlinear and visualization
of the surfactant dynamics has been a long-standing experimental challenge. We
perform the first quantitative, spatiotemporally-resolved measurements of the
spreading of an insoluble surfactant on a thin fluid layer. During the
spreading process, we directly observe both the radial height profile of the
spreading droplet and the spatial distribution of the fluorescently-tagged
surfactant. We find that the leading edge of spreading circular layer of
surfactant forms a Marangoni ridge in the underlying fluid, with a trough
trailing the ridge as expected. However, several novel features are observed
using the fluorescence technique, including a peak in the surfactant
concentration which trails the leading edge, and a flat, monolayer-scale
spreading film which differs from concentration profiles predicted by current
models. Both the Marangoni ridge and surfactant leading edge can be described
to spread as . We find spreading exponents, and for the ridge peak and
surfactant leading edge, respectively, which are in good agreement with
theoretical predictions of . In addition, we observe that the
surfactant leading edge initially leads the peak of the Marangoni ridge, with
the peak later catching up to the leading edge
Islands of conformational stability for Filopodia
Filopodia are long, thin protrusions formed when bundles of fibers grow outwardly from a cell surface while remaining closed in a membrane tube. We study the subtle issue of the mechanical stability of such filopodia and how this depends on the deformation of the membrane that arises when the fiber bundle adopts a helical configuration. We calculate the ground state conformation of such filopodia, taking into account the steric interaction between the membrane and the enclosed semiflexible fiber bundle. For typical filopodia we find that a minimum number of fibers is required for filopodium stability. Our calculation elucidates how experimentally observed filopodia can obviate the classical Euler buckling condition and remain stable up to several tens of . We briefly discuss how experimental observation of the results obtained in this work for the helical-like deformations of enclosing membrane tubes in filopodia could possibly be observed in the acrosomal reactions of the sea cucumber Thyone, and the horseshoe crab Limulus. Any realistic future theories for filopodium stability are likely to rely on an accurate treatment of such steric effects, as analysed in this work
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