Predicting frictional aging from bulk relaxation measurements.

The coefficient of static friction between solids normally increases with the time they have remained in static contact before the measurement. This phenomenon, known as frictional aging, is at the origin of the difference between static and dynamic friction coefficients but has remained difficult to understand. It is usually attributed to a slow expansion of the area of atomic contact as the interface changes under pressure. This is however challenging to quantify as surfaces have roughness at all length scales. In addition, friction is not always proportional to the contact area. Here we show that the normalized stress relaxation of the surface asperities during frictional contact with a hard substrate is the same as that of the bulk material, regardless of the asperities' size or degree of compression. This result enables us to predict the frictional aging of rough interfaces based on the bulk material properties of two typical polymers: polypropylene and polytetrafluoroethylene.

Surface-Mediated Molecular Transport of a Lipophilic Fluorescent Probe in Polydisperse Oil-in-Water Emulsions.

Emulsions often act as carriers for water-insoluble solutes that are delivered to a specific target. The molecular transport of solutes in emulsions can be facilitated by surfactants and is often limited by diffusion through the continuous phase. We here investigate this transport on a molecular scale by using a lipophilic molecular rotor as a proxy for solutes. Using fluorescence lifetime microscopy we track the transport of these molecules from the continuous phase toward the dispersed phase in polydisperse oil-in-water emulsions. We show that this transport comprises two time scales, which vary significantly with droplet size and surfactant concentration, and, depending on the type of surfactant used, can be limited either by transport across the oil-water interface or by diffusion through the continuous phase. By studying the time-resolved fluorescence of the fluorophore, accompanied by molecular dynamics simulations, we demonstrate how the rate of transport observed on a macroscopic scale can be explained in terms of the local environment that the probe molecules are exposed to.

Needle-free jet injection-induced small-droplet aerosol formation during intralesional bleomycin therapy.

OBJECTIVES: Needle-free jet injectors are frequently used in dermatological practice. Injection-generated small-droplet aerosols could be harmful upon inhalation when chemotherapeutics, like bleomycin, are used. Here, we aim to explore jet injector-induced small-droplet aerosol formation of bleomycin in relation to air ventilation and to provide safety measures for clinical practice. MATERIALS AND METHODS: With a professional particle sensor, we measured airborne aerosol particles (0.2-10.0 µm) after electronic pneumatic injection (EPI), spring-loaded jet injection (SLI), and needle injection (NI) of bleomycin and saline (100 µl) on ex vivo human skin. Three levels of air ventilation were explored: no ventilation, room ventilation, and room ventilation with an additional smoke evacuator. RESULTS: EPI and SLI induced significant small-droplet aerosol formation compared with none after NI (0.2-1.0 µm; no ventilation). The largest bleomycin aerosol generation was observed for the smallest particles (0.2-1.0 µm) with 673.170 (528.802-789.453) aerosol particles/liter air (EPI; no ventilation). Room ventilation and smoke evacuation led to a reduction of ≥99% and 100% of measured aerosols, respectively. CONCLUSION: Jet injectors generate a high number of small-droplet aerosols, potentially introducing harmful effects to patients and healthcare personnel. Room ventilation and smoke evacuation are effective safety measures when chemotherapeutics are used in clinical practice.

Oppositely Charged Ions at Water-Air and Water-Oil Interfaces: Contrasting the Molecular Picture with Thermodynamics.

The surface-active ions tetraphenylarsonium (Ph4As(+)) and tetraphenylboron (Ph4B(-)) have a similar structure but opposite charge. At the solution-air interface, the two ions affect the surface tension in an identical manner, yet sum-frequency generation (SFG) spectra reveal an enhanced surface propensity for Ph4As(+) compared with Ph4B(-), in addition to opposite alignment of interfacial water molecules. At the water-oil interface, the interfacial tension is 7 mN/m lower for Ph4As(+) than for Ph4B(-) salts, but this can be fully accounted for by the different bulk solubility of these ions in the hydrophobic phase, rather than inherently different surface activities. The different solubility can be accounted for by differences in electronic structure, as evidenced by quantum chemical calculations and NMR studies. Our results show that the surface propensity concluded from SFG spectroscopy does not necessarily correlate with interfacial adsorption concluded from thermodynamic measurements.

Conflicting effects of exercise on the establishment of a short-photoperiod phenotype in Syrian hamster.

In the Syrian hamster, winter seasonal inhibition of reproduction occurs in response to decreasing day length. This inhibitory response is modulated by nonphotic cues. In particular, access to a running wheel has been shown to produce incomplete gonadal regression. The present study sought to determine whether this occurs as a consequence of wheel effect on adaptation of the circadian system to short days or whether downstream physiological responses are involved. Short-day adaptation of the circadian clock, which is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, was tested by lengthening the photosensitive phase of the SCN (assayed by light-induced c-Fos expression in the SCN) as a parameter. We found that wheel-running activity does not inhibit the integration of the photoperiodic change by the SCN even if complete testicular regression is prevented. Moreover, this exercise was even capable of accelerating the lengthening of the photosensitive phase after the transfer to short day length. Thus, although wheel-running activity inhibits the short photoperiod-induced gonadal regression, it acts on the SCN to accelerate the integration of the photoperiodic change by the biological clock.