C.W. Wu

Measurement and Prediction of Insertion Force for the Mosquito Fascicle Penetrating into Human Skin

Mosquitoes are exceptional in their ability to pierce into human skin with a natural ultimate painless microneedle, named fascicle. Here the structure of the Aedes albopictus mosquito fascicle is obtained using a Scanning Electron Microscope (SEM), and the whole process of the fascicle inserting into human skin is observed using a high-speed video imaging technique. Direct measurements of the insertion force for mosquito fascicle to penetrate into human skin are reported. Results show that the mosquito uses a very low force (average 18 µN) to penetrate into the skin. This force is at least three orders of magnitude smaller than the reported lowest insertion force for an artificial microneedle with an ultra sharp tip to insert into the human skin. In order to understand the piercing mechanism of mosquito fascicle tip into human multilayer skin tissue, a numerical simulation is conducted to analyze the insertion process using a nonlinear finite element method. A good agreement occurs between the numerical results and the experimental measurements.

Influence of mixed particle size on electrorheological response

Two sizes (6 and 100 μm average diameter) glass spheres suspended in silicone oil (volume fraction φ=0.3) were used to investigate the influence of particle size on electrorheological response at a shear rate 2 s−1. The larger particles gave a higher shear yield stress but a lower current density at zero shear than the smaller ones. When the two particle sizes were mixed together, the shear yield stress decreased, reaching a minimum when the volume fraction of the small particles equals that of the large particles. The minimum in the current density occurred when the ratio of the volume of small to large particles was ∼1/3.

Low Friction and High Load Support Capacity of Slider Bearing With a Mixed Slip Surface

The classical Reynolds theory reveals that a converging gap is the first necessary condition to generate a hydrodynamic pressure in a viscous fluid film confined between two solid surfaces with a relative sliding/rolling motion. For hundreds of years, the classical lubrication mechanics has been based on the frame of the Reynolds theory with no slip assumption. Recent studies show that a large boundary slip occurs on an ultrahydrophobic surface, which results in a very small friction drag. Unfortunately, such a slip surface also produces a small hydrodynamic pressure in a fluid film between two solid surfaces. This paper studies the lubrication behavior of infinite width slider bearings involving a mixed slip surface (MSS). The results of the study indicate that any geometrical wedges (gaps), i.e., a convergent wedge, a parallel gap, and even a divergent wedge, can generate hydrodynamic pressure in an infinite slider bearing with a mixed slip surface. It is found that with an MSS, the maximum fluid load support capacity occurs at a slightly divergent wedge (roughly parallel sliding gap) for an infinite width slider bearing, but not at a converging gap as what the classical Reynolds theory predicts. Surface optimisation of a parallel sliding gap with a slip surface can double the hydrodynamic load support and reduce the friction drag by half of what the Reynolds theory predicts for an optimal wedge of a traditional slider bearing.

On the conductivity model for the electrorheological response of dielectric particles with a conducting film

An analytical approach is given for the electric current and attractive force between spherical dielectric particles surrounded by a conducting film and situated in a nonpolar liquid. Good agreement occurs between the calculated values and those measured on humidified glass beads in silicone oil. Also, the model predicts a decrease from a quadratic electric field dependence of the force as the field strength is increased, in accord with the results for many electrorheological fluids. The effects of certain parameters which govern the conductivity of the host liquid and of the particles are considered.

Influence of a surface film on conducting particles on the electrorheological response with alternating current fields

A method was developed for the frequency dependence of the electrorheological (ER) response of suspensions of highly conducting particles with a low conducting surface film. At dc or low frequency ac field, the shear yield stress of the ER fluids considered is determined by the conductivity ratio σI/σf of the film to the host oil; at high frequency ac field it is determined by the permittivity ratio eI/ef. The critical frequency separating the conduction domain from the dielectric domain is proportional to σf/ef, the ratio of the conductivity to the permittivity of the host oil. To obtain a high shear yield stress at high frequency ac field, a high ratio of the permittivity of the surface film to the oil is desired and a reasonably thin surface film. Too thin a film increases the possibility of electrical breakdown in the film especially at a small ratio of eI/ef. An effective method for overcoming electrical breakdown in the surface film is to increase the permittivity of the film. Good agreement exists ...

Performance of hydrodynamic lubrication journal bearing with a slippage surface

Purpose – The aim of this paper is to propose a design idea for an infinite journal bearing with the optimized slip zone on the bearing sleeve surface.Design/methodology/approach – The approach is to use finite element analysis and the quadratic programming algorithm to study the performance of the journal bearing with a slip zone on the sleeve surface. The fluid film pressure and slip velocity can be obtained in one solution step.Findings – A journal bearing with a slip zone on the sleeve surface produces many different advantages over the traditional journal bearing. Even in a parallel sliding gap there is still a considerable large load support, but a very low friction drag. The effect of the enhancement of such a slip wedge on the journal bearing performance is much greater at a small eccentricity ratio than at a large eccentricity ratio. Numerical analyses indicate that the location and size of the slip zone greatly affect the journal performance. When the eccentricity ratio ϵ=0.8, the maximum load s...

Influence of a surface film on the particles on the electrorheological response

A conduction model is developed for the dc electrorheological (ER) response of highly conducting particles (e.g., metal particles) suspended in a weakly conducting oil. The numerical analyses show that a surface film with some conductivity is desired, but not a completely insulating film as previously proposed. Increasing the film conductivity leads to an increase in the ER yield stress. However, too high a conductivity will give an unacceptable level of current density. The film should also have an intermediate thickness. A small thickness increases the possibility of electrical breakdown in the film; too large a thickness decreases the ER effect. Good agreement exists between the yield stress and the current density predicted by our model and those measured.

Three-Point Bending Young's Modulus of Nanowires

The three-point bending Youngs modulus (E) of nanowires was studied by a finite element method. It was found that the constraint conditions at the nanowire ends, the ratio of the suspended length of the wire to the wire diameter and the deflection of the wire all work together to contribute to the measurement errors of the Youngs modulus. Yielding of the deposited material used to fix the nanowire ends leads to a decrease in the slope of the load–deflection curve. The three-point bending criteria for nanowires have been established. A modified equation for the measurement of the three-point bending Youngs modulus of nanowires is proposed.

Shear strength of electrorheological particle clusters

The electrorheological shear strength of clusters of small, humidified glass spheres with an average diameter ∼220 μm in silicone oil forming a rectangular or cubic lattice was determined with d.c. electrical field. The arrangement, shearing, and the yield and fracture processes of the clusters were observed. The shear strength of the cluster increased with the number of chains in a cluster and with the applied electrical field. The shear modulus was essentially independent of the number of chains in a cluster, but increased with applied field.

CONDUCTIVITY AND FORCE BETWEEN PARTICLES IN A MODEL ELECTRORHEOLOGICAL FLUID : I : CONDUCTIVITY

The conductivity of silicone oil with various water contents and that of a single-row chain of glass beads in silicone oil were determined as a function of electric field and spacing between the particles produced by tensile and shear strain. The conductivity of the silicone oil depended sensitively on its water content; that of the chain was three orders of magnitude greater than the silicone oil alone, but decreased rapidly with separation of the particles. The conductivity of the water film on the glass bead surface and the electric field enhancement in the oil gap between the beads were derived from the data. The measured current density along a chain of particles was in accord with that predicted by a conductivity model for electrorheological response. At large strains the separation of the beads in a chain became localized between one set of adjacent beads and they oscillated back-and-forth in the gap, leading ultimately to rupture of the chain with increased strain.