Search

Proceedings of the National Academy of Sciences of the United States of America 117(39), 1-7 (2020). doi:10.1073/pnas.2003337117, Dynamics and kinetics in soft matter physics, biology, and nanoscience frequently occur on fast (sub)microsecond but not ultrafast timescales which are difficult to probe experimentally. The European X-ray Free-Electron Laser (European XFEL), a megahertz hard X-ray Free-Electron Laser source, enables such experiments via taking series of diffraction patterns at repetition rates of up to 4.5 MHz. Here, we demonstrate X-ray photon correlation spectroscopy (XPCS) with submicrosecond time resolution of soft matter samples at the European XFEL. We show that the XFEL driven by a superconducting accelerator provides unprecedented beam stability within a pulse train. We performed microsecond sequential XPCS experiments probing equilibrium and nonequilibrium diffusion dynamics in water. We find nonlinear heating on microsecond timescales with dynamics beyond hot Brownian motion and superheated water states persisting up to 100 μs at high fluences. At short times up to 20 μs we observe that the dynamics do not obey the Stokes–Einstein predictions., Published by National Acad. of Sciences, Washington, DC

ChemPhysChem 21, 1318-1325 (2020). doi:10.1002/cphc.202000114, We study the structure and dynamics of poly(N‐isopropylacrylamide) (PNIPAm) core‐shell nanogels dispersed in aqueous trimethylamine N‐oxide (TMAO) solutions by means of small‐angle X‐ray scattering and X‐ray photon correlation spectroscopy (XPCS). Upon increasing the temperature above the lower critical solution temperature of PNIPAm at 33 °C, a colloidal gel is formed as identified by an increase of I (q ) at small q as well as a slowing down of sample dynamics by various orders of magnitude. With increasing TMAO concentration the gelation transition shifts linearly to lower temperatures. Above a TMAO concentration of approximately 0.40 mol/L corresponding to a 1 : 1 ratio of TMAO and NIPAm groups, collapsed PNIPAm states are found for all temperatures without any gelation transition. This suggests that reduction of PNIPAm‐water hydrogen bonds due to the presence of TMAO results in a stabilisation of the collapsed PNIPAm state and suppresses gelation of the nanogel., Published by Wiley-VCH Verl., Weinheim

Journal of applied crystallography 52(4), 777 - 782 (2019). doi:10.1107/S1600576719007568, An X-ray cross-correlation study of the local orientational order in self-assembled films made from PEGylated gold nanoparticles is presented. The local structure of this model system is dominated by four- and sixfold order. Coadsorption of shorter ligands in the particle's ligand layer and variation of salt concentration in the suspension prior to self-assembly result in a change of local orientational order. The degree of sixfold order is reduced after salt addition. This decrease of order is less pronounced for the fourfold symmetry. The results presented here suggest complex symmetry-selective order formation upon ligand exchange and salt addition and demonstrate the versatility of X-ray cross-correlation methods for nanoparticle superlattices., Published by IUCr, Chester

Physical review / E 104, L012602 (1-7) (2021). doi:10.1103/PhysRevE.104.L012602, We study the structure and dynamics of colloidal particles with a spherical hard core and a thermo-responsive soft shell over the whole phase diagram by means of small-angle x-ray scattering and x-ray photon correlation spectroscopy. By changing the effective volume fraction by temperature and particle concentration, liquid, repulsive glass. and attractive gel phases are observed. The dynamics slow down with increasing volume fraction in the liquid phase and reflect a Vogel-Fulcher-Tamann behavior known for fragile glass formers. We find a liquid-glass transition above 50 vol.% that is independent of the particles' concentration and temperature. In an overpacked state at effective volume fractions above 1, the dispersion does not show a liquid phase but undergoes a gel-glass transition at an effective volume fraction of 34 vol.%. At the same concentration, extrema of subdiffusive dynamics are found in the liquid phase at lower weight fractions. We interpret this as dynamic precursors of the glass-gel transition., Published by Inst., Woodbury, NY

Soft matter 16(11), 2864-2872 (2020). doi:10.1039/C9SM02230A, We investigate the out-of-equilibrium dynamics of a colloidal gel obtained by quenching a suspension of soft polymer-coated gold nanoparticles close to and below its gelation point using X-ray Photon Correlation Spectroscopy (XPCS). A faster relaxation process emergent from the localized motions of the nanoparticles reveals a dynamically-arrested network at the nanoscale as a key signature of the gelation process. We find that the slower network dynamics is hyperdiffusive with a compressed exponential form, consistent with stress-driven relaxation processes. Specifically, we use direction-dependent correlation functions to characterize the anisotropy in dynamics. We show that the anisotropy is greater for the gel close to its gelation point than at lower temperatures, and the anisotropy decreases as the gel ages. We quantify the anisotropic dynamical heterogeneities emergent in such a stress-driven dynamical system using higher order intensity correlations, and demonstrate that the aging phenomenon contributes significantly to the properties evaluated by the fluctuations in the intensity correlations. Our results provide important insights into the structural origin of the emergent anisotropic and cooperative heterogeneous dynamics, and we discuss analogies with previous work on other soft disordered systems., Published by Royal Soc. of Chemistry, London

Soft matter 16(2), 466-475 (2020). doi:10.1039/C9SM01884K, We study the structure and dynamics of aqueous dispersions of densely packed core–shell nanoparticles composed of a silica core and a poly(N-isoproylacrylamide) (PNIPAm) shell as a function of temperature and concentration. With small angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS) we shed light on the structural and dynamical changes of the thermo-responsive colloidal nanogel during its volume phase transition at a lower critical solution temperature (LCST) of 32 °C. A transition of the dynamics and its distinct dependency on the particle number concentration could be determined by analysing the intensity autocorrelation function while the structural transition remains concentration independent. We found the dynamics of a jammed system beyond a critical concentration. In addition, by variation of the PNIPAm shell size we tuned both the effective volume fraction and the underlying nature of the dynamics in the system. With our results we can present a full phase diagram of a PNIPAm core–shell system that spans from diluted suspensions, where the system behaves like a liquid, to an effective volume fraction of more than ninety percent where after exceeding a critical concentration the system undergoes a temperature-induced transition from a liquid towards a colloidal gel., Published by Royal Soc. of Chemistry, London

The journal of physical chemistry letters 10, 5231 - 5236 (2019). doi:10.1021/acs.jpclett.9b01690, We present the structure and dynamics of highly concentrated core–shell nanoparticles composed of a silica core and a poly(N-isoproylacrylamide) (PNIPAm) shell suspended in water. With X-ray photon correlation spectroscopy, we are able to follow dynamical changes over the volume phase transition of PNIPAm at LCST = 32 °C. On raising the temperature beyond LCST, the structural relaxation times continue to decrease. The effect is accompanied by a transition from stretched to compressed exponential shape of the intensity autocorrelation function. Upon further heating, we find a sudden slowing down for the particles in their collapsed state. The q dependence of the relaxation time shows an anomalous change from $τ_c ∝ q^{–3}$ to $τ_c ∝ q^{–1}$. Small angle X-ray scattering data evidence a temperature-induced transition from repulsive to attractive forces. Our results indicate a temperature-induced phase transition from a colloidal liquid with polymer-driven dynamics toward a colloidal gel., Published by ACS, Washington, DC

The journal of physical chemistry / C C, Nanomaterials and interfaces 123, 29994-30000 (2019). doi:10.1021/acs.jpcc.9b08173, We demonstrate the pressure-induced formation of supercrystalsmade from PEGylated gold nanorods (NRs) in aqueous suspension.Utilizing the combined effect of hydrostatic pressure and salt on the solubilityof the organic poly(ethylene glycol) (PEG) shell that passivates the NRs, thereversible formation of two-dimensional hexagonal supercrystals has beenobserved by means of small-angle X-ray scattering. The pressure dependenceof the crystal lattice’s structural parameters is determined. By time-resolvedmeasurements performed after a pressure jump, the growth process of thecrystals is found to be completed already after a few seconds. The presented results demonstrate that by PEGylatingnanoparticles, pressure-induced homogeneous supercrystals can be formed for different particle shapes, in particular, anisotropicNRs, which determine the resulting lattice type., Published by Soc., Washington, DC

Physical chemistry, chemical physics 21(38), 21349 - 21354 (2019). doi:10.1039/C9CP04658E, Colloidal nanocrystals (NC) are known to self-organize into superlattices that promise many applications ranging from medicine to optoelectronics. Recently, the formation of high-quality PEGylated gold NC was reported at high hydrostatic pressure and high salt concentrations. Here, we study the formation kinetics of these superlattices after pressure jumps beyond their crystallisation pressure by means of small-angle X-ray scattering with few ms experimental resolution. The timescale of NC formation was found to be reduced the larger the width of the pressure jump. This is connected to an increase of crystal quality, i.e., the faster the NC superlattice forms, the better the crystal quality. In contrast to the formation kinetics, the melting of the NC superlattice is approximately one order of magnitude slower and shows linear kinetics., Published by RSC Publ., Cambridge