Rheological design of thickened alcohol-based hand rubs.

The handleability and sensory perception of hand sanitisers by consumers affect the hygiene outcome. Spillage may result in under-dosing and poor sensory properties can lead to under-utilisation. We first propose four principles (low runoff, spreadability, smoothness and non-stickiness) for designing the rheology of thickened alcohol-based hand rubs with acceptable handleability and hand feel. We then evaluate a commercial hand gel and a variety of simplified formulations thickened with microgels (Carbopol 974P, Carbopol Ultrez 20 and Sepimax Zen), or linear polymers (Jaguar HP 120 COS), and evaluate them against these design criteria. All four additives provide acceptable spreadability by shear thinning to η ≈ 10 - 1 Pa s at γ ˙ ∼ 10 3 s - 1 . Either the finite yield stress conferred by the microgels ( σ y ≳ 10 Pa ) or the increase in low-shear viscosity provided by the linear polymer ( η ≳ 1 Pa s at γ ˙ ≲ 0.1 s - 1 ) give rise to acceptably low runoff. However, the formulation using the linear polymer shows a filament breakage time of τ b ≈ 1 s in capillary rheology, which may result in stickiness and therefore a less than optimal hand feel.

What is the role of dehydroepiandrosterone in gynecologic practice?

In small clinical trials, dehydroepiandrosterone (DHEA) has been found to relieve symptoms associated with postmenopausal conditions and infertility in women. DHEA may provide a cost-effective alternative to typical hormone therapies. Because of a lack of long-term and large-scale studies, only intravaginal DHEA supplementation is approved and recommended for treatment. Further investigation of DHEA supplementation is needed and encouraged to determine its safety and effectiveness.

Self-organization of particles with planar surface anchoring in a cholesteric liquid crystal.

We present birefringence and fluorescence confocal microscopy studies of melamine particles in a liquid-crystalline host solvent. The liquid crystal has a cholesteric phase at room temperature with a helical pitch that can be modified by changing the composition. The pitch employed here is always less than the particle diameter (3 microm). We demonstrate via birefringence that the mesogens preferentially anchor flat at the melamine surface. Via studies in a sample cell with aligning surfaces we show that although the particles form chains in a nematic liquid crystal they organize in 2D plates in a cholesteric. Fluorescence confocal microscopy and particle location analysis are used to determine the radial distribution function and to evaluate the particle aggregation number as a function of pitch length. We discuss possible explanations for the self-organization.

Characterization of microparticles with driven optical tweezers.

We discuss how actively-driven optical tweezers may be used to characterize Brownian microparticles. Two experiments are described in detail. We follow the thermal fluctuations of a charged particle in an oscillatory electric field and demonstrate that charges as low as a few elementary charges can be measured accurately and reproducibly. Secondly, we measure the orientational dynamics of a trapped rotating droplet and use circular polarimetry within optical tweezers to determine in situ birefringence.

Direct measurement of the effective charge in nonpolar suspensions by optical tracking of single particles.

The authors develop an ultrasensitive method for the measurement of the charge carried by a colloidal particle in a nonpolar suspension. The technique uses the phenomenon of the resonance of a particle held in an optical tweezer trap and driven by a sinusoidal electric field. The trapped particle forms a strongly damped harmonic oscillator whose fluctuations are a function of gamma, the ratio of the root-mean-square average of the electric and thermal forces on the particle. At low applied fields (gamma<<1) the particle is confined to the optical axis, while at high fields (gamma>>1) the probability distribution of the particle is double peaked. The periodically modulated thermal fluctuations are measured with nanometer sensitivity using an interferometric position detector. Charges, as low as a few elementary charges, can be measured with an uncertainty of about 0.25 e. This is significantly better than previous techniques and opens up new possibilities for the study of nonpolar suspensions.

Laser manipulation in liquid crystals: an approach to microfluidics and micromachines.

Laser trapping of particles in three dimensions can occur as a result of the refraction of strongly focused light through micrometre-sized particles. The use of this effect to produce laser tweezers is extremely common in fields such as biology, but it is only relatively recently that the technique has been applied to liquid crystals (LCs). The possibilities are exciting: droplets of LCs can be trapped, moved and rotated in an isotropic fluid medium, or both particles and defects can be trapped and manipulated within a liquid crystalline medium. This paper considers both the possibilities. The mechanism of transfer of optical angular momentum from circularly polarized light to small droplets of nematic LCs is described. Further, it is shown that droplets of chiral LCs can be made to rotate when illuminated with linearly polarized light and possible mechanisms are discussed. The trapping and manipulation of micrometre-sized particles in an aligned LC medium is used to provide a measure of local shear viscosity coefficients and a unique test of theory at low Ericksen number in LCs.