Effects of Orthogonal Rotating Electric Fields on Electrospinning Process

Electrospinning is a nanotechnology process whereby an external electric field is used to accelerate and stretch a charged polymer jet, so as to produce fibers with nanoscale diameters. In quest of a further reduction in the cross section of electrified jets hence of a better control on the morphology of the resulting electrospun fibers, we explore the effects of an external rotating electric field orthogonal to the jet direction. Through extensive particle simulations, it is shown that by a proper tuning of the electric field amplitude and frequency, a reduction of up to a $30 \%$ in the aforementioned radius can be obtained, thereby opening new perspectives in the design of future ultra-thin electrospun fibres. Applications can be envisaged in the fields of nanophotonic components as well as for designing new and improved filtration materials.Comment: 22 pages, 8 figure

Slip-controlled thin film dynamics

In this study, we present a novel method to assess the slip length and the viscosity of thin films of highly viscous Newtonian liquids. We quantitatively analyse dewetting fronts of low molecular weight polystyrene melts on Octadecyl- (OTS) and Dodecyltrichlorosilane (DTS) polymer brushes. Using a thin film (lubrication) model derived in the limit of large slip lengths, we can extract slip length and viscosity. We study polymer films with thicknesses between 50 nm and 230 nm and various temperatures above the glass transition. We find slip lengths from 100 nm up to 1 micron on OTS and between 300 nm and 10 microns on DTS covered silicon wafers. The slip length decreases with temperature. The obtained values for the viscosity are consistent with independent measurements.Comment: 4 figure

Drop Splashing on a Dry Smooth Surface

The corona splash due to the impact of a liquid drop on a smooth dry substrate is investigated with high speed photography. A striking phenomenon is observed: splashing can be completely suppressed by decreasing the pressure of the surrounding gas. The threshold pressure where a splash first occurs is measured as a function of the impact velocity and found to scale with the molecular weight of the gas and the viscosity of the liquid. Both experimental scaling relations support a model in which compressible effects in the gas are responsible for splashing in liquid solid impacts.Comment: 11 pages, 4 figure

Drop impact upon micro- and nanostructured superhydrophobic surfaces

We experimentally investigate drop impact dynamics onto different superhydrophobic surfaces, consisting of regular polymeric micropatterns and rough carbon nanofibers, with similar static contact angles. The main control parameters are the Weber number \We and the roughness of the surface. At small \We, i.e. small impact velocity, the impact evolutions are similar for both types of substrates, exhibiting Fakir state, complete bouncing, partial rebouncing, trapping of an air bubble, jetting, and sticky vibrating water balls. At large \We, splashing impacts emerge forming several satellite droplets, which are more pronounced for the multiscale rough carbon nanofiber jungles. The results imply that the multiscale surface roughness at nanoscale plays a minor role in the impact events for small \We \apprle 120 but an important one for large \We \apprge 120. Finally, we find the effect of ambient air pressure to be negligible in the explored parameter regime \We \apprle 150Comment: 8 pages, 7 figure

Random field sampling for a simplified model of melt-blowing considering turbulent velocity fluctuations

In melt-blowing very thin liquid fiber jets are spun due to high-velocity air streams. In literature there is a clear, unsolved discrepancy between the measured and computed jet attenuation. In this paper we will verify numerically that the turbulent velocity fluctuations causing a random aerodynamic drag on the fiber jets -- that has been neglected so far -- are the crucial effect to close this gap. For this purpose, we model the velocity fluctuations as vector Gaussian random fields on top of a k-epsilon turbulence description and develop an efficient sampling procedure. Taking advantage of the special covariance structure the effort of the sampling is linear in the discretization and makes the realization possible

Investigation of flow through P[NIPAM/MMA] copolymer coated glass capillary tubes, and glass copolymer adhesion improvements with hydrofluoric acid etching

This study aims to display the retention of the thermo-responsive properties of the copolymer poly(N-isopropyl acrylamide-methyl methacrylate) [P(NIPAM/MMA)] when coated on the inner diameter of a glass capillary tube, and to prove the stability of the copolymer coating when subjected to pressure driven fluid flow. The study shows that the fluid flow through such a capillary tube follows Hagen-Poiseuille flow. Furthermore, this study examines methods of improving polymer adhesion to glass by hydrofluoric acid etching. Such a coated tube system is applicable in drug delivery, self cleaning tubes, and microelectromechanical systems (MEMS)

Electrospinning: A study in the formation of nanofibers

Electrospinning is a technique to produce nanofibers more efficiently. In electrospinning, electricity spins fibers by extracting the polymer from the solvent and stretching it, all in one continuous electric field. Basics of electrospinning are discussed in sequence from simple homogenous fibers through a single nozzle to heterogeneous core-shell fibers from double, concentric nozzles. Experimental set-up is described and the effect of different variables in the process of electrospinning on nanofiber quality is illustrated. Formation of carbon nanotube fibers with porous walls from polyethylene oxide/polyacrylnitrile (PEO/PAN) core-shell fibers is the definitive objective.Zipped LaTex fil

Shapes, contact angles, and line tensions of droplets on cylinders

Using an interface displacement model we calculate the shapes of nanometer-size liquid droplets on homogeneous cylindrical surfaces. We determine effective contact angles and line tensions, the latter defined as excess free energies per unit length associated with the two contact lines at the ends of the droplet. The dependences of these quantities on the cylinder radius and on the volume of the droplets are analyzed.Comment: 26 pages, RevTeX, 10 Figure

Hydrodynamics of back spatter by blunt bullet gunshot with a link to bloodstain pattern analysis

A theoretical model describing the blood spatter pattern resulting from a blunt bullet gunshot is proposed. The predictions are compared to experimental data acquired in the present work. This hydrodynamic problem belongs to the class of the impact hydrodynamics with the pressure impulse generating the blood flow. At the free surface, the latter is directed outwards and accelerated toward the surrounding air. As a result, the Rayleigh-Taylor instability of the flow of blood occurs, which is responsible for the formation of blood drops of different sizes and initial velocities. Thus, the initial diameter, velocity, and acceleration of the atomized blood drops can be determined. Then, the equations of motion are solved, describing drop trajectories in air accounting for gravity, and air drag. Also considered are the drop-drop interactions through air, which diminish air drag on the subsequent drops. Accordingly, deposition of two-phase (blood-drop and air) jets on a vertical cardstock sheet located between the shooter and the target (and perforated by the bullet) is predicted and compared with experimental data. The experimental data were acquired with a porous polyurethane foam sheet target impregnated with swine blood, and the blood drops were collected on a vertical cardstock sheet which was perforated by the blunt bullet. The highly porous target possesses a low hydraulic resistance and therefore resembles a pool of blood shot by a blunt bullet normally to its free surface. The back spatter pattern was predicted numerically and compared to the experimental data for the number of drops, their area, the total stain area, and the final impact angle as functions of radial location from the bullet hole in the cardstock sheet (the collection screen). Comparisons of the predicted results with the experimental data revealed satisfactory agreement. The predictions also allow one to find the impact Weber number on the collection screen, which is necessary to predict stain shapes and sizes

Intercalation of Poly-acrylonitrile (PAN) into carbon nanotubes

The primary goal of this work is to fill 200 nm average diameter CVD Carbon nanotubes (CNTs) with Poly-acrylonitrile (PAN) - a carbonizable polymer - with the diffusion process reported by Bazilevsky et al. and to control the thickness and structure of the PAN inside the CNTs. Transmission electron microscope (TEM) was used as a tool to monitor the morphology of polymer filled nanotubes. TEM images of CNTs that were filled using five different PAN concentrations of the PAN/DMF solution - 0.1, 0.5, 1, 2, and 5 wt% - demonstrated that the intercalation process is independent of the initial PAN concentration in solution. Furthermore, a DMF rinse process was used to remove polymer that was clinging to the outer walls of the CNTs, allowing clearer visual of the PAN structure in the interior of the CNTs. Finally, TEM images of filled CNTs taken through a carbonization process demonstrated that the CNT samples were able to survive the high temperature, with some damage to the CNT walls