## Latest from PNAS: mass spectrometry, omniphobic surfaces, giant Stark effect, and heavy tails in email communication

First things first; I am a sucker for articles whose abstract reads something like this:

Patterns of deliberate human activity and behavior are of utmost importance in areas as diverse as disease spread, resource allocation, and emergency response. Because of its widespread availability and use, e-mail correspondence provides an attractive proxy for studying human activity. Recently, it was reported that the probability density for the inter-event time τ between consecutively sent e-mails decays asymptotically as $\tau^{-\alpha}$ with $\alpha \approx 1$. The slower-than-exponential decay of the inter-event time distribution suggests that deliberate human activity is inherently non-Poissonian. Here, we demonstrate that the approximate power-law scaling of the inter-event time distribution is a consequence of circadian and weekly cycles of human activity. We propose a cascading nonhomogeneous Poisson process that explicitly integrates these periodic patterns in activity with an individual’s tendency to continue participating in an activity. Using standard statistical techniques, we show that our model is consistent with the empirical data. Our findings may also provide insight into the origins of heavy-tailed distributions in other complex systems.

Control of the fundamental absorption edge of a quantum dot with an applied electric field has been limited by the breakdown fields of the solid-state material surrounding the dot. However, much larger fields can be applied at the interface of two immiscible electrolytic solutions (ITIES) in an electrochemical cell. These electric fields also localize the quantum dots at the ITIES. Our analysis shows that semiconductor nanocrystals localized at the ITIES should have electric-field-tunable optical properties across much of the visible spectrum. The transparency of the liquids in such cells indicates that this configuration would be well suited for electrically tunable optical filters with wide-angle acceptance.

Anish Tuteja et al report on robust omniphobic surfaces (surfaces that do not like any substance — almost; and the photos and micrographs that accompany the piece are also a great pleasure to look at):

Superhydrophobic surfaces display water contact angles greater than 150° in conjunction with low contact angle hysteresis. Microscopic pockets of air trapped beneath the water droplets placed on these surfaces lead to a composite solid-liquid-air interface in thermodynamic equilibrium. Previous experimental and theoretical studies suggest that it may not be possible to form similar fully-equilibrated, composite interfaces with drops of liquids, such as alkanes or alcohols, that possess significantly lower surface tension than water ($\gamma_{lv}$ = 72.1 mN/m). In this work we develop surfaces possessing re-entrant texture that can support strongly metastable composite solid-liquid-air interfaces, even with very low surface tension liquids such as pentane ($\gamma_{lv}$ = 15.7 mN/m). Furthermore, we propose four design parameters that predict the measured contact angles for a liquid droplet on a textured surface, as well as the robustness of the composite interface, based on the properties of the solid surface and the contacting liquid. These design parameters allow us to produce two different families of re-entrant surfaces— randomly-deposited electrospun fiber mats and precisely fabricated microhoodoo surfaces—that can each support a robust composite interface with essentially any liquid. These omniphobic surfaces display contact angles greater than 150° and low contact angle hysteresis with both polar and nonpolar liquids possessing a wide range of surface tensions.

Finally, the latest issue is a special issue on mass spectrometry; here is from the introduction to the issue:

This sampling represents only a tiny fraction of the mass spectrometers in daily use worldwide for identification and analysis of a wide range of molecules, including flavors, natural products, pollutants, drugs, metabolites, and those in chemical process streams. MS specificity is ideal for efficiently probing the molecular complexity found in many fields including agriculture, atmospheric chemistry, biomedicine, food, forensics, geochemistry, and so forth. Hopefully, the illustrations here will suggest further novel applications in other scientific fields represented by the uniquely broad readership of PNAS.

“This” in the first sentence of the quote above refers to mass spectrometric studies on characterization of petroleum, details of transition from condensed phase to gas phase, gaseous ion reactions, imaging bio-medically important molecules on tissue samples, and protein characterization, among other things.

Have fun!