Accelerated ripening in chemically fueled emulsions

Chemically fueled emulsions are solutions with droplets made of phase-separated molecules that are activated and deactivated by a chemical reaction cycle. These emulsions play a crucial role in biology as a class of membrane-less organelles. Moreover, theoretical studies show that droplets in these emulsions can evolve to the same size or spontaneously self-divide when fuel is abundant. All of these exciting properties, i. e., emergence, decay, collective behavior, and self-division, are pivotal to the functioning of life. However, these theoretical predictions lack experimental systems to test them quantitively. Here, we describe the synthesis of synthetic emulsions formed by a fuel-driven chemical cycle, and we find a surprising new behavior, i. e., the dynamics of droplet growth is regulated by the kinetics of the fuel-driven reaction cycle. Consequently, the average volume of these droplets grows orders of magnitude faster compared to Ostwald ripening. Combining experiments and theory, we elucidate the underlying mechanism

Genomic epidemiology of SARS-CoV-2 in a UK university identifies dynamics of transmission

AbstractUnderstanding SARS-CoV-2 transmission in higher education settings is important to limit spread between students, and into at-risk populations. In this study, we sequenced 482 SARS-CoV-2 isolates from the University of Cambridge from 5 October to 6 December 2020. We perform a detailed phylogenetic comparison with 972 isolates from the surrounding community, complemented with epidemiological and contact tracing data, to determine transmission dynamics. We observe limited viral introductions into the university; the majority of student cases were linked to a single genetic cluster, likely following social gatherings at a venue outside the university. We identify considerable onward transmission associated with student accommodation and courses; this was effectively contained using local infection control measures and following a national lockdown. Transmission clusters were largely segregated within the university or the community. Our study highlights key determinants of SARS-CoV-2 transmission and effective interventions in a higher education setting that will inform public health policy during pandemics.</jats:p

The Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

We explore the influence of salt addition on the structure of water interacting closely with a charged silica surface. Isolating these surface effects is challenging, even with surface-specific techniques like sum frequency generation (SFG), because of the presence of aligned water nanometers to microns away from the charged silica. Here we combine zeta potential and SFG intensity measurements with the maximum entropy method and reported heterodyne second harmonic and sum frequency generation results to deconvolute from the total signal intensity the SFG spectral contributions of the waters adjacent to the surface. Deconvolution reveals that at very low ionic strength the surface water structure is similar to that of a neutral silica surface near the point of zero charge with waters oriented in opposite directions. This result suggests the known metastability of silica near the PZC and the stability of silica in low ionic strength solutions may originate from the same source, these oppositely oriented surface-bound waters. Orientation changes are induced upon adding salt, which lead to a decrease in the total amount of aligned water at the surface

Influence of the Excitation Wavelength on First Order Hyperpolarizabilities and Optimal Gap Tuning of Range Separated Hybrid Functionals

In this study, we compute the hyperpolarizability of the nitroaniline isomers, para-nitroaniline (pNA), ortho-nitroaniline (oNA), and meta-nitroaniline (mNA), by density functional theory (DFT), including with optimally tuned range separated hybrid (RSH) functionals. By utilizing the nitroanilines hyperpolarizability trend based on charge transfer (pNA>oNA>mNA), we can uncover how the excitation wavelength affects the prediction of the hyperpolarizabilities in both on and off resonant regimes, and optimal gap tuning of RSH functionals. In non-resonant regions, with reference to CCSD/aug-cc-pVDZ and experimental studies, we find that some computational approaches do not always reproduce the nitroanilines trend at specific excitation wavelengths. For example, RSH functionals require optimal gap tuning to reproduce the trend. In resonant regions, we find that the damped response theory predicts that the trend is maintained at the two-photon absorption, however, it breaks near the one photon pole. This suggests that the underlying charge transfer characteristics are undermined in the one-photon pole which in comparison to the two-state model suggests that this is due to the presence of other electronic states in some of the isomers. Furthermore, we find that cases where optimal gap tuning is ineffective (pathological behavior) are dependent on the excitation wavelength

Relating the Phase in Vibrational Sum Frequency Spectroscopy and Second Harmonic Generation with the Maximum Entropy Method

Nonlinear optical methods such as vibrational sum frequency generation (vSFG) and second harmonic generation (SHG) are powerful techniques to study the elusive structures at charged buried interfaces such as the silica/water interface. However, for an accurate evaluation of the structure formed at these buried interfaces, the complex vSFG spectra and hence the absolute phase needs to be retrieved. The maximum entropy method is a useful tool for the retrieval of complex spectra from the intensity spectra; however, one caveat is that an understanding of the error phase is required. Here we provide a physically motivated understanding of the error phase, where we show that for broadband vSFG spectra such as the silica/water, the good spectral overlap between water in the diffuse and Stern (or bonded interfacial) layers results in the absolute phase correlating with the error phase. This correlation makes the error phase sensitive to changes in Debye length from varying the ionic strength amongst other variations at the interface. Furthermore, the change in the magnitude error phase can be related to the absolute SHG phase permitting the use of an error phase model that can utilize the SHG phase to predict the error phase and hence the complex vSFG spectra. We highlight limitations of the model for narrow vSFG spectra with poor overlap between the diffuse and Stern layer spectra, such as the silica/HOD in D2O system

Bimodal or Trimodal? The Influence of Starting pH on Site Identity and Distribution at the Low Salt Aqueous/Silica Interface

Second harmonic generation (SHG) is commonly employed to monitor processes at mineral oxide/liquid interfaces. Using SHG, we determine how the starting pH affects the acid–base chemistry of the silica/aqueous interface. We observe three different sites with p<i>K</i>_a values of approximately 3.8, 5.2, and ∼9 (p<i>K</i>_a-I, p<i>K</i>_a-II, p<i>K</i>_a-III, respectively), but the presence and relative abundance of these sites is very sensitive to the starting pH. For titrations initiated at pH 12, all three sites are observed, whereas only two sites are observed for titrations initiated at pH 2 or pH 7. Moreover, exposure to pH 2 facilitates the formation of p<i>K</i>_a-II and p<i>K</i>_a-III sites, while exposure to pH 7 results in p<i>K</i>_a-I and p<i>K</i>_a-III sites. Based on previous computational work, we assign these sites to three different hydrogen bonding environments at the interface including a hydrophobic site for the most acidic silanol corresponding to p<i>K</i>_a-I

Monitoring DNA Hybridization and Thermal Dissociation at the Silica/Water Interface Using Resonantly Enhanced Second Harmonic Generation Spectroscopy

The immobilization of oligonucleotide sequences onto glass supports is central to the field of biodiagnostics and molecular biology with the widespread use of DNA microarrays. However, the influence of confinement on the behavior of DNA immobilized on silica is not well understood owing to the difficulties associated with monitoring this buried interface. Second harmonic generation (SHG) is an inherently surface specific technique making it well suited to observe processes at insulator interfaces like silica. Using a universal 3-nitropyrolle nucleotide as an SHG-active label, we monitored the hybridization rate and thermal dissociation of a 15-mer of DNA immobilized at the silica/aqueous interface. The immobilized DNA exhibits hybridization rates on the minute time scale, which is much slower than hybridization kinetics in solution but on par with hybridization behavior observed at electrochemical interfaces. In contrast, the thermal dissociation temperature of the DNA immobilized on silica is on average 12 °C lower than the analogous duplex in solution, which is more significant than that observed on other surfaces like gold. We attribute the destabilizing affect of silica to its negatively charged surface at neutral pH that repels the hybridizing complementary DNA

Influence of the Hydrogen Bonding Environment on Vibrational Coupling in the Electrical Double Layer at the Silica/Aqueous Interface

Vibrational spectroscopy is a powerful tool for determining the local hydrogen-bonding environment. However, vibrational coupling present in H2O can make it difficult to relate vibrational spectra to a molecular description of the system. While numerous bulk studies have shed light on this phenomenon, the influence of both intra- and intermolecular vibrational coupling on the resulting electrical double layer spectra at buried interfaces remains largely unexplored. By utilizing the combination of vibrational sum frequency generation (vSFG), electrokinetic measurements, and the maximum entropy method on isotopically diluted water (HOD) at the silica/aqueous interface, we reveal the influence of vibrational coupling on the Stern and diffuse layer spectra as the surface charge density is varied. By comparing our HOD spectra with the corresponding H2O spectra from pH 10 to 2, we find that the diffuse layer spectra of H2O are dominated by both intra- and intermolecular coupling leading to significant differences in the H2O and HOD spectra. In contrast, the spectral response of HOD and H2O in the Stern layer as a function of pH is similar, providing strong evidence that the O-H oscillators in the Stern layer are evolving in a similar manner for both the H2O and HOD systems. However, we observe differences in the frequency centers at low pH that are less significant at higher pH suggesting that intermolecular coupling in the Stern layer is evolving as the surface charge density is varied