Shrinking of Rapidly Evaporating Water Microdroplets Reveals their Extreme Supercooling.

The fast evaporative cooling of micrometer-sized water droplets in a vacuum offers the appealing possibility to investigate supercooled water-below the melting point but still a liquid-at temperatures far beyond the state of the art. However, it is challenging to obtain a reliable value of the droplet temperature under such extreme experimental conditions. Here, the observation of morphology-dependent resonances in the Raman scattering from a train of perfectly uniform water droplets allows us to measure the variation in droplet size resulting from evaporative mass losses with an absolute precision of better than 0.2%. This finding proves crucial to an unambiguous determination of the droplet temperature. In particular, we find that a fraction of water droplets with an initial diameter of 6379±12 nm remain liquid down to 230.6±0.6 K. Our results question temperature estimates reported recently for larger supercooled water droplets and provide valuable information on the hydrogen-bond network in liquid water in the hard-to-access deeply supercooled regime.

Erratum: Shrinking of Rapidly Evaporating Water Microdroplets Reveals their Extreme Supercooling [Phys. Rev. Lett. 120, 015501 (2018)].

This corrects the article DOI: 10.1103/PhysRevLett.120.015501.

Clustered DNA damage induces pan-nuclear H2AX phosphorylation mediated by ATM and DNA-PK.

DNA double-strand breaks (DSB) are considered as the most deleterious DNA lesions, and their repair is further complicated by increasing damage complexity. However, the molecular effects of clustered lesions are yet not fully understood. As the locally restricted phosphorylation of H2AX to form γH2AX is a key step in facilitating efficient DSB repair, we investigated this process after localized induction of clustered damage by ionizing radiation. We show that in addition to foci at damaged sites, H2AX is also phosphorylated in undamaged chromatin over the whole-cell nucleus in human and rodent cells, but this is not related to apoptosis. This pan-nuclear γH2AX is mediated by the kinases ataxia telangiectasia mutated and DNA-dependent protein kinase (DNA-PK) that also phosphorylate H2AX at DSBs. The pan-nuclear response is dependent on the amount of DNA damage and is transient even under conditions of impaired DSB repair. Using fluorescence recovery after photobleaching (FRAP), we found that MDC1, but not 53BP1, binds to the nuclear-wide γH2AX. Consequently, the accumulation of MDC1 at DSBs is reduced. Altogether, we show that a transient dose-dependent activation of the kinases occurring on complex DNA lesions leads to their nuclear-wide distribution and H2AX phosphorylation, yet without eliciting a full pan-nuclear DNA damage response.

Species conserved DNA damage response at the inactive human X chromosome.

Chromatin modifications are long known as an essential part of the orchestrated response resulting in the repair of radiation-induced DNA double-strand breaks (DSBs). Only recently, however, the influence of the chromatin architecture itself on the DNA damage response has been recognised. Thus for heterochromatic DSBs the sensing and early recruitment of repair factors to the lesion occurs within the heterochromatic compartments, but the damage sites are subsequently relocated from the inside to the outside of the heterochromatin. While previous studies were accomplished at the constitutive heterochromatin of centromeric regions in mouse and flies, here we examine the DSB repair at the facultative heterochromatin of the inactive X chromosome (Xi) in humans. Using heavy ion irradiation we show that at later times after irradiation the DSB damage streaks bend around the Xi verifying that the relocation process is conserved between species and not specialised to repetitive sequences only. In addition, to measure chromatin relaxation at rare positions within the genome, we established live cell microscopy at the GSI microbeam thus allowing the aimed irradiation of small nuclear structures like the Xi. Chromatin decondensation at DSBs within the Xi is clearly visible within minutes as a continuous decrease of the DNA staining over time, comparable to the DNA relaxation revealed at DSBs in mouse chromocenters. Furthermore, despite being conserved between species, slight differences in the underlying regulation of these processes in heterochromatic DSBs are apparent.

DNA double-strand breaks in heterochromatin elicit fast repair protein recruitment, histone H2AX phosphorylation and relocation to euchromatin.

DNA double-strand breaks (DSBs) can induce chromosomal aberrations and carcinogenesis and their correct repair is crucial for genetic stability. The cellular response to DSBs depends on damage signaling including the phosphorylation of the histone H2AX (γH2AX). However, a lack of γH2AX formation in heterochromatin (HC) is generally observed after DNA damage induction. Here, we examine γH2AX and repair protein foci along linear ion tracks traversing heterochromatic regions in human or murine cells and find the DSBs and damage signal streaks bending around highly compacted DNA. Given the linear particle path, such bending indicates a relocation of damage from the initial induction site to the periphery of HC. Real-time imaging of the repair protein GFP-XRCC1 confirms fast recruitment to heterochromatic lesions inside murine chromocenters. Using single-ion microirradiation to induce localized DSBs directly within chromocenters, we demonstrate that H2AX is early phosphorylated within HC, but the damage site is subsequently expelled from the center to the periphery of chromocenters within ∼ 20 min. While this process can occur in the absence of ATM kinase, the repair of DSBs bordering HC requires the protein. Finally, we describe a local decondensation of HC at the sites of ion hits, potentially allowing for DSB movement via physical forces.

An infrastructure for accurate characterization of single-event transients in digital circuits.

We present the architecture and a detailed pre-fabrication analysis of a digital measurement ASIC facilitating long-term irradiation experiments of basic asynchronous circuits, which also demonstrates the suitability of the general approach for obtaining accurate radiation failure models developed in our FATAL project. Our ASIC design combines radiation targets like Muller C-elements and elastic pipelines as well as standard combinational gates and flip-flops with an elaborate on-chip measurement infrastructure. Major architectural challenges result from the fact that the latter must operate reliably under the same radiation conditions the target circuits are exposed to, without wasting precious die area for a rad-hard design. A measurement architecture based on multiple non-rad-hard counters is used, which we show to be resilient against double faults, as well as many triple and even higher-multiplicity faults. The design evaluation is done by means of comprehensive fault injection experiments, which are based on detailed Spice models of the target circuits in conjunction with a standard double-exponential current injection model for single-event transients (SET). To be as accurate as possible, the parameters of this current model have been aligned with results obtained from 3D device simulation models, which have in turn been validated and calibrated using micro-beam radiation experiments at the GSI in Darmstadt, Germany. For the latter, target circuits instrumented with high-speed sense amplifiers have been used for analog SET recording. Together with a probabilistic analysis of the sustainable particle flow rates, based on a detailed area analysis and experimental cross-section data, we can conclude that the proposed architecture will indeed sustain significant target hit rates, without exceeding the resilience bound of the measurement infrastructure.

Experimental evidence of a size-dependent sign change of the Seebeck coefficient of Bi nanowire arrays.

The electrical transport in bismuth nanowires is strongly influenced by both sample geometry and crystallinity. Compared to bulk bismuth, the electrical transport in nanowires is dominated by size effects and influenced by surface states, which gain increasing relevance with increasing surface-to-volume ratios, i.e. with decreasing wire diameter. Bismuth nanowires with tailored diameter and crystallinity constitute, therefore, excellent model systems, allowing to study the interplay of the different transport phenomena. Here, we present temperature-dependent Seebeck coefficient and relative electrical resistance measurements of parallel bismuth nanowire arrays with diameters between 40 and 400 nm synthesized by pulsed electroplating in polymer templates. Both electrical resistance and Seebeck coefficient exhibit a non-monotonic temperature dependence, with the sign of the Seebeck coefficient changing from negative to positive with decreasing temperature. The observed behavior is size-dependent and is attributed to limitations of the mean free path of the charge carriers within the nanowires. The observed size-dependent Seebeck coefficient and in particular the size-dependent sign change opens a promising avenue for single-material thermocouples with p- and n-legs made from nanowires with different diameters.

Polymeric lithography editor: Editing lithographic errors with nanoporous polymeric probes.

A new lithographic editing system with an ability to erase and rectify errors in microscale with real-time optical feedback is demonstrated. The erasing probe is a conically shaped hydrogel (tip size, ca. 500 nm) template-synthesized from track-etched conical glass wafers. The "nanosponge" hydrogel probe "erases" patterns by hydrating and absorbing molecules into a porous hydrogel matrix via diffusion analogous to a wet sponge. The presence of an interfacial liquid water layer between the hydrogel tip and the substrate during erasing enables frictionless, uninterrupted translation of the eraser on the substrate. The erasing capacity of the hydrogel is extremely high because of the large free volume of the hydrogel matrix. The fast frictionless translocation and interfacial hydration resulted in an extremely high erasing rate (~785 µm2/s), which is two to three orders of magnitude higher in comparison with the atomic force microscopy-based erasing (~0.1 µm2/s) experiments. The high precision and accuracy of the polymeric lithography editor (PLE) system stemmed from coupling piezoelectric actuators to an inverted optical microscope. Subsequently after erasing the patterns using agarose erasers, a polydimethylsiloxane probe fabricated from the same conical track-etched template was used to precisely redeposit molecules of interest at the erased spots. PLE also provides a continuous optical feedback throughout the entire molecular editing process-writing, erasing, and rewriting. To demonstrate its potential in device fabrication, we used PLE to electrochemically erase metallic copper thin film, forming an interdigitated array of microelectrodes for the fabrication of a functional microphotodetector device. High-throughput dot and line erasing, writing with the conical "wet nanosponge," and continuous optical feedback make PLE complementary to the existing catalog of nanolithographic/microlithographic and three-dimensional printing techniques. This new PLE technique will potentially open up many new and exciting avenues in lithography, which remain unexplored due to the inherent limitations in error rectification capabilities of the existing lithographic techniques.