Melting at the Limit of Superheating

Theories on superheating-melting mostly involve vibrational and mechanical instabilities, catastrophes of entropy, volume and rigidity, and nucleation-based kinetic models. The maximum achievable superheating is dictated by nucleation process of melt in crystals, which in turn depends on material properties and heating rates. We have established the systematics for maximum superheating by incorporating a dimensionless nucleation barrier parameter and heating rate, with which systematic molecular dynamics simulations and dynamic experiments are consistent. Detailed microscopic investigation with large-scale molecular dynamics simulations of the superheating-melting process, and structure-resolved ultrafast dynamic experiments are necessary to establish the connection between the kinetic limit of superheating and vibrational and mechanical instabilities, and catastrophe theories

Time-Resolved X-Ray Diffraction Investigation of Superheating-Melting of Crystals under Ultrafast Heating

The maximum superheating of a solid prior to melting depends on the effective dimensionless nucleation energy barrier, heterogeneities such as free surfaces and defects, and heating rates. Superheating is rarely achieved with conventional slow heating due to the dominant effect of heterogeneous nucleation. In present work, we investigate the superheating-melting behavior of crystals utilizing ultrafast heating techniques such as exploding wire and laser irradiation, and diagnostics such as time-resolved X-ray diffraction combined with simultaneous measurements on voltage and current (for exploding wire) and particle velocity (for laser irradiation). Experimental designs and preliminary results are presented

Expansion of the non-native Mississippi Silverside, Menidia audens (Pisces, Atherinopsidae), into fresh and marine waters of coastal southern California

Abstract--Mississippi Silversides, Menidia audens, were first recorded in southern California reservoirs and nearby outflows in the late 1980s and early 1990s. In 1997-2000 they were taken in King Harbor, Redondo Beach,and in 2000 in the Santa Ana River. By 2005-2006 they were found in several other coastal drainages from the San Gabriel River in Orange and Los Angeles counties northward to Arroyo Burro, Santa Barbara County. Initial invasion was via the California Aqueduct in the late 1980s and early 1990s and more recently dispersal has taken place along the southern California coast. The records from King Harbor occurred for a relatively short period, mid-1997-mid-2000 (mostly 1997 and 1998) before they were established in coastal drainages. Their impact on native species is not known but on some occasions Mississippi Silversides have outnumbered native Topsmelt, Atherinops affinis, in small coastal lagoons estuaries. Mississippi Silversides are known to prey on eggs and larvae of other fishes and could be increasing predation on small native animals as well as serving as prey for larger piscivores like steelhead and terns.

Poly(acrylic acid) interpolymer complexes

YesInterpolymer complex formation of poly(acrylic acid) with other macromolecules can occur via several mechanisms that vary depending on the pH. At low pH the protonated acid functional group can form bonds with both donor and acceptor moieties, resulting in desolvated structures consisting of two polymers. Complexes were formed in dilute solutions of PAA, functionalised with acenaphthylene, with a range of other polymers including: poly(NIPAM); poly(ethylene oxide) (PEO); poly(dimethylacrylamide) (PDMA); poly(diethyl acrylamide) (PDEAM) poly(vinyl alcohol) (PVA) and poly(vinyl pyrolidinone) (PVP). Fluorescence anisotropy was used to demonstrate complex formation in each case by monitoring the reductions in segmental motion of the chain as the complexes formed. Considerations of the molecular structures of the complexing moieties suggest that solvation energies and pKas play an important role in complex formation

Binding of bacteria to poly (N-isopropylacrylamide) modified with vancomycin: Comparison of behavior of linear and highly branched polymers

YesThe behavior of a linear copolymer of N-isopropyl acrylamide with pendant vancomycin functionality was compared to an analogous highly branched copolymer with vancomycin functionality at the chain ends. Highly branched poly(N-isopropylacrylamide) modified with vancomycin (HB-PNIPAM-van) was synthesized by functionalization of the HB-PNIPAM, prepared using reversible addition-fragmentation chain transfer polymerization. Linear PNIPAM with pendant vancomycin functionality (L-PNIPAM-van) was synthesized by functionalization of poly(N-isopropyl acrylamide-co-vinyl benzoic acid). HB-PNIPAM-van aggregated S. aureus effectively whereas the L-PNIPAM-van polymer did not. It was found that when the HB-PNIPAM-van was incubated with S. aureus the resultant phase transition provided an increase in the intensity of fluorescence of a solvatochromic dye, nile red, added to the system. In contrast, a significantly lower increase in fluorescence intensity was obtained when L-PNIPAM-van was incubated with S. aureus. These data showed that the degree of desolvation of HB-PNIPAM-van was much greater than the desolvation of the linear version. Using microCalorimetry it was shown that there were no significant differences in the affinities of the polymer ligands for D-Ala-D-Ala and therefore differences in the interactions with bacteria were associated with changes in the probability of access of the polymer bound ligands to the D-Ala-D-Ala dipeptide. The data support the hypothesis that generation of polymer systems that respond to cellular targets, for applications such as cell targeting, detection of pathogens etc., requires the use of branched polymers with ligands situated at the chain ends.MR

Small variations in reaction conditions tune carbon dot fluorescence

The development of robust and reproducible synthetic strategies for the production of carbon dots with improved fluorescence quantum yields and distinct emission profiles is of great relevance given the vast range of applications of CDs. The fundamental understanding at a molecular level of their formation mechanism, chemical structure and how these parameters are correlated to their photoluminescence (PL) properties is thus essential. In this study, we describe the synthesis and structural characterization of a range of CDs with distinct physico-chemical properties. The materials were prepared under three minutes of microwave irradiation using the same common starting materials (GlcNH2·HCl 1 and EDA 2) but modifying the stoichiometry of the reagents. We show that small changes in reaction conditions leads to the tailoring of the fluorescent behaviour of the CDs from apparent blue to green emission. Structural analysis of the different CD samples suggested different reaction pathways during the CD formation and surface passivation, with the latter step being key to the observed differences. Moreover, we demonstrate that the different materials also respond reversibly to changes in pH, which we can attribute to different behaviour towards protonation/deprotonation events of distinct emission domains present within each nanomaterial. Our results highlight the importance of understanding the reaction pathways that lead to the formation of this carbon-based nanomaterials and how this can be exploited to develop tailored materials towards specific applications