Rafael D. Schulman

Liquid Droplets on a Highly Deformable Membrane.

We examine the deformation produced by microdroplets atop thin elastomeric and glassy free-standing films. Because of the Laplace pressure, the droplets deform the elastic membrane thereby forming a bulge. Thus, two angles define the droplet or membrane geometry: the angles the deformed bulge and the liquid surface make with the film. These angles are measured as a function of the film tension, and are in excellent agreement with a force balance at the contact line. Finally, we find that if the membrane has an anisotropic tension, the droplets are no longer spherical but become elongated along the direction of high tension.

The Effect of Stellar Metallicity on the Sizes of Star Clusters

Observations indicate that blue globular clusters have half-light radii systematically larger than those of red globular clusters. In this paper, we test whether the different metallicitydependent stellar evolution time-scales and mass-loss rates within the clusters can impact their early dynamical evolution. By means of N-body simulations including stellar evolution recipes we simulate the early evolution of small centrally concentrated clusters with and without primordial mass segregation. Our simulations include accurate metallicity-dependent mass loss from massive stars. We find blue clusters to be larger than red clusters regardless of whether the clusters have been primordially mass segregated. In addition, the size difference is found to be larger and consistent with observations for initial models with a low central concentration. These results indicate that the systematic size difference found between red and blue clusters can, at least in part, be attributed to the dynamical effects of differing stellar evolution histories, driven by metallicity.

Undulatory microswimming near solid boundaries

The hydrodynamic forces involved in the undulatory microswimming of the model organism C. elegans are studied in proximity to solid boundaries. Using a micropipette deflection technique, we attain direct and time-resolved force measurements of the viscous forces acting on the worm near a single planar boundary as well as confined between two planar boundaries. We observe a monotonic increase in the lateral and propulsive forces with increasing proximity to the solid interface. We determine normal and tangential drag coefficients for the worm, and find these to increase with confinement. The measured drag coefficients are compared to existing theoretical models. The ratio of normal to tangential drag coefficients is found to assume a constant value of 1.5 ± 0.1(5) at all distances from a single boundary, but increases significantly as the worm is confined between two boundaries. In response to the increased drag due to confinement, we observe a gait modulation of the nematode, which is primarily characteri...

The effects of viscosity on the undulatory swimming dynamics of C. elegans

The undulatory swimming dynamics of the millimetric nematode Caenorhabditis elegans was investigated in fluids with different viscosities. The technique of micropipette deflection was used to directly measure the drag forces experienced by the swimming worm in both the lateral and propulsive directions. Gait modulation due to increasing viscosity in our tethered system was found to be qualitatively similar to that of freely swimming worms. Resistive force theory was used to determine the drag coefficients of the slender swimmer, and the experimental values were compared to the classical theories of Lighthill as well as Gray and Hancock. The gait modulation was shown to be independent of how the environmental resistance is changed, indicating the relevance of only the fluid resistance on the swimming kinematics and dynamics of the nematode.

Surface energy of strained amorphous solids

Surface stress and surface energy are fundamental quantities which characterize the interface between two materials. Although these quantities are identical for interfaces involving only fluids, the Shuttleworth effect demonstrates that this is not the case for most interfaces involving solids, since their surface energies change with strain. Crystalline materials are known to have strain-dependent surface energies, but in amorphous materials, such as polymeric glasses and elastomers, the strain dependence is debated due to a dearth of direct measurements. Here, we utilize contact angle measurements on strained glassy and elastomeric solids to address this matter. We show conclusively that interfaces involving polymeric glasses exhibit strain-dependent surface energies, and give strong evidence for the absence of such a dependence for incompressible elastomers. The results provide fundamental insight into our understanding of the interfaces of amorphous solids and their interaction with contacting liquids.Whether or not amorphous solids exhibit strain-dependent surface energies, like those of crystalline materials, is still a matter of debate. Here, Schulman et al. monitor the contact angle of droplets on strained polymeric glasses and elastomers, which directly probes energy variation at the interfaces.

Liquid Droplets Act as “Compass Needles” for the Stresses in a Deformable Membrane

We examine the shape of droplets atop deformable thin elastomeric films prepared with an anisotropic tension. As the droplets generate a deformation in the taut film through capillary forces, they assume a shape that is elongated along the high tension direction. By measuring the contact line profile, the tension in the membrane can be completely determined. Minimal theoretical arguments lead to predictions for the droplet shape and membrane deformation that are in excellent agreement with the data. On the whole, the results demonstrate that droplets can be used as probes to map out the stress field in a membrane.

Liquid droplets on a free-standing glassy membrane: Deformation through the glass transition

Abstract.In this study, micro-droplets are placed on thin, glassy, free-standing films where the Laplace pressure of the droplet deforms the free-standing film, creating a bulge. The film’s tension is modulated by changing temperature continuously from well below the glass transition into the melt state of the film. The contact angle of the liquid droplet with the planar film as well as the angle of the bulge with the film are measured and found to be consistent with the contact angles predicted by a force balance at the contact line.Graphical abstract