On angled bounce-off impact of a drop impinging on a flowing soap film

Small drops impinging angularly on thin flowing soap films frequently demonstrate the rare emergence of bulk elastic effects working in-tandem with the more common-place hydrodynamic interactions. Three collision regimes are observable: (a) drop piercing through the film, (b) it coalescing with the flow, and (c) it bouncing off the film surface. During impact, the drop deforms along with a bulk elastic deformation of the film. For impacts that are close-to-tangential, the bounce-off regime predominates. We outline a reduced order analytical framework assuming a deformable drop and a deformable three-dimensional film, and the idealization invokes a phase-based parametric study. Angular inclination of the film and the ratio of post and pre impact drop sizes entail the phase parameters. We also perform experiments with vertically descending droplets impacting against an inclined soap film, flowing under constant pressure head. Model predicted phase domain for bounce-off compares well to our experimental findings. Additionally, the experiments exhibit momentum transfer to the film in the form of shed vortex dipole, along with propagation of free surface waves. On consulting prior published work, we note that for locomotion of water-walking insects using an impulsive action, the momentum distribution to the shed vortices and waves are both significant, taking up respectively 2/3-rd and 1/3-rd of the imparted streamwise momentum. In view of the potentially similar impulse actions, this theory is applied to the bounce-off examples in our experiments, and the resultant shed vortex dipole momenta are compared to the momenta computed from particle imaging velocimetry data. The magnitudes reveal identical order ($10^{-7}$ N$\cdot$s), suggesting that the bounce-off regime can be tapped as a simple analogue for interfacial bio-locomotion relying on impulse reactions

Ionic effect on combing of single DNA molecules and observation of their force-induced melting by fluorescence microscopy

Molecular combing is a powerful and simple method for aligning DNA molecules onto a surface. Using this technique combined with fluorescence microscopy, we observed that the length of lambda-DNA molecules was extended to about 1.6 times their contour length (unextended length, 16.2 micrometers) by the combing method on hydrophobic polymethylmetacrylate (PMMA) coated surfaces. The effects of sodium and magnesium ions and pH of the DNA solution were investigated. Interestingly, we observed force-induced melting of single DNA molecules.Comment: 12 page

Photoinduced Doughnut-Shaped Nanostructures

We show that an incoherent unpolarized single-beam illumination is able to photoinduce nano-doughnuts on the surface of azopolymer thin films. We demonstrate that individual doughnut-shaped nano-objects as well as clusters of several adjacent nano-doughnuts can be formed and tailored with wide range of typical sizes, thus providing a rich field for applications in nanophotonics and photochemistry.Comment: 13 pages, 3 figures, first version to chem. phys. lett. 201

Experimental and theoretical evidence for molecular forces driving surface segregation in photonic colloidal assemblies

Surface segregation in binary colloidal mixtures offers a simple way to control both surface and bulk properties without affecting their bulk composition. Here, we combine experiments and coarse-grained molecular dynamics (CG-MD) simulations to delineate the effects of particle chemistry and size on surface segregation in photonic colloidal assemblies from binary mixtures of melanin and silica particles of size ratio (Dlarge/Dsmall) ranging from 1.0 to similar to 2.2. We find that melanin and/or smaller particles segregate at the surface of micrometer-sized colloidal assemblies (supraballs) prepared by an emulsion process. Conversely, no such surface segregation occurs in films prepared by evaporative assembly. CG-MD simulations explain the experimental observations by showing that particles with the larger contact angle (melanin) are enriched at the supraball surface regardless of the relative strength of particle-interface interactions, a result with implications for the broad understanding and design of colloidal particle assemblies

Stratospheric constituent measurements using UV solar occultation technique

The photochemistry of the stratospheric ozone layer was studied as the result of predictions that trace amounts of pollutants can significantly affect the layer. One of the key species in the determination of the effects of these pollutants is the OH radical. A balloon flight was made to determine whether data on atmospheric OH could be obtained from lower resolution solar spectra obtained from high altitude during sunset

Study of Particles at Fluid-Fluid Interfaces

Particles are known to adsorb at fluid-fluid interfaces in small molecule systems such as oil/water emulsions. These particle stabilized emulsions are called Pickering emulsions. This thesis aims to extend the phenomenon of particle adsorption as observed in Pickering emulsions to polymer blends. Polymer blends are high viscosity analogs of emulsions. They present an economical way of obtaining a material with desired properties by blending two immiscible polymers. The goal of this work is to examine the effects of interfacial adsorption of particles in polymer blends.We examine the effect of the simultaneous adsorption of silica particles at two polymer-polymer interfaces in polyisobutylene/polydimethylsiloxane (PIB/PDMS) and polyethyleneoxide/polyisobutylene (PEO/PIB) blends, leading to the bridging of drops. Microscopically and rheologically, the particle mediated drop bridging is shown to result in the formation of clusters and networks of drops. This is reported to impart weak gel-like characteristics to the blend.A variety of commercially available particles viz. polytetrafluoroethylene (PTFE), iron (Fe), iron oxyhydroxide (FeOOH) and titanium dioxide (TiO2) are shown to be interfacially active at chemically different polyisoprene/polydimethylsiloxane (PI/PDMS) and not so different polyisoprene/polyisobutylene (PI/PIB) interfaces. This has led to the possibility of exploiting the phenomenon of interfacial adsorption of particles, as particulate compatibilizers, to suppress the drop coalescence in PI/PDMS blends. Rheology is presented as a microstructural tool to qualitatively probe the effect of interfacial activity of particles on the drop size. Our rheology and microscopy results with 0.5vol% of particles show that none of the particle types suppress coalescence of drops in the blends. Instead, PTFE and Fe particles promote coalescence of the drops in PI/PDMS blends.We also examine the stabilization of polymer foams, specifically polystyrene (PS) and polyisobutylene (PIB) by PTFE particles. Our experimental results show that PTFE particles can significantly enhance the stabilization of PS and PIB foams, making them stable for extended periods of time. We believe that this approach of using PTFE particles to stabilize PS and PIB foams may prove useful in a variety of other polymers as well, and may extend the range of polymers and processing conditions under which foaming can be conducted

A mesoscopic model for microscale hydrodynamics and interfacial phenomena: Slip, films, and contact angle hysteresis

We present a model based on the lattice Boltzmann equation that is suitable for the simulation of dynamic wetting. The model is capable of exhibiting fundamental interfacial phenomena such as weak adsorption of fluid on the solid substrate and the presence of a thin surface film within which a disjoining pressure acts. Dynamics in this surface film, tightly coupled with hydrodynamics in the fluid bulk, determine macroscopic properties of primary interest: the hydrodynamic slip; the equilibrium contact angle; and the static and dynamic hysteresis of the contact angles. The pseudo- potentials employed for fluid-solid interactions are composed of a repulsive core and an attractive tail that can be independently adjusted. This enables effective modification of the functional form of the disjoining pressure so that one can vary the static and dynamic hysteresis on surfaces that exhibit the same equilibrium contact angle. The modeled solid-fluid interface is diffuse, represented by a wall probability function which ultimately controls the momentum exchange between solid and fluid phases. This approach allows us to effectively vary the slip length for a given wettability (i.e. the static contact angle) of the solid substrate