Rubber-like elasticity in laser-driven free surface flow of a Newtonian fluid.

The energy needed to deform an elastic solid may be recovered, while in Newtonian fluids, like water and glycerol, deformation energy dissipates on timescales of the intermolecular relaxation time [Formula: see text] . For times considerably longer than [Formula: see text] the existence of shear elasticity requires long-range correlations, which challenge our understanding of the liquid state. We investigated laser-driven free surface bubbles in liquid glycerol by analyzing their expansion and bursting dynamics, in which we found a flow-dominating, rubber-like elasticity unrelated to surface tension forces. In extension to findings of a measurable liquid elasticity at even very low deformation frequencies [L. Noirez, P. Baroni, J. Mol. Struct. 972, 16-21 (2010), A. Zaccone, K. Trachenko, Proc. Natl. Acad. Sci. U.S.A. 117, 19653-19655 (2020)], that is difficult to access under increased strain, we find a robust, strain rate driven elasticity. The recovery of deformation energy allows the bursting bubble to reach Taylor-Culick velocities 20-fold higher than expected. The elasticity is persistent for microseconds, hence four orders of magnitude longer than [Formula: see text] . The dynamic shows that this persistence cannot originate from the far tail of a distribution of relaxation times around [Formula: see text] but must appear by frustrating the short molecular dissipation. The longer time should be interpreted as a relaxation of collective modes of metastable groups of molecules. With strain rates of 106 s-1, we observe a metastable glycerol shell exhibiting a rubber-like solid behavior with similar elasticity values and characteristic tolerance toward large strains, although the molecular interaction is fundamentally different.

Peptide Mass Spectra from Micrometer-Thick Ice Films Produced with Femtosecond Pulses.

We present a cryogenic mass spectrometry protocol with the capability to detect peptides in the attomole dilution range from ice films. Our approach employs femtosecond laser pulses and implements neither substrate modification nor proton donor agents in the aqueous solution, known to facilitate analyte detection in mass spectrometry. In a systematic study, we investigated the impact of temperature, substrate composition, and irradiation wavelength (513 and 1026 nm) on the bradykinin signal onset. Our findings show that substrate choice and irradiation wavelength have a minor impact on signal intensity once the preparation protocol is optimized. However, if the temperature is increased from -140 to 0 °C, which is accompanied by ice film thinning, a somehow complex picture of analyte desorption and ionization is recognizable, which has not been described in the literature yet. Under cryogenic conditions (-140 °C), obtaining a signal is only possible from isolated sweet spots across the film. If the thin ice film is between -100 and -70 °C of temperature, these sweet spots appear more frequently. Ice sublimation triggered by temperatures above -70 °C leads to an intense and robust signal onset that could be maintained for several hours. In addition to the above findings, we notice that a vibrant fragmentation pattern produced is strikingly similar with both wavelengths. Our findings suggest that while following an optimized protocol, femtosecond mass spectrometry has excellent potential to analyze small organic molecules and peptides with a mass range of up to 2.5 kDa in aqueous solution without any matrix, as employed in matrix-assisted laser desorption/ionization (MALDI) or any substrate surface modification, found in surface-assisted laser desorption/ionization (SALDI).