Viktor Soprunyuk

Structural transformations of even-numbered n-alkanes confined in mesopores

The n-alkanes C12H26, C14H30, and C16H34 have been imbibed and solidified in mesoporous Vycor glass with a mean pore diameter of 10 nm. The samples have been investigated by x-ray diffractometry and calorimetric measurements. The structures and phase sequences have been determined. Apart from a reduction and the hysteresis of the melting-freezing transition, pore-confined C12 reproduces the liquid-triclinic phase sequence of the bulk material, but for C16 an orthorhombic rotator mesophase appears that in the bulk state is absent for C16 but well known from odd-numbered alkanes of similar length. In pore-confined C14 this phase shows up on cooling but not on heating.

Optical Transmission Measurements on Phase Transitions of O2 and CO in Mesoporous Glass

The optical transmission of O2 and CO condensates embedded in porous Vycor glass has been studied as function of the filling fraction and of the thermal history of the samples. The freezing transition as well as the solid-solid transitions (β-α of CO and γ-β of O2) induce a coarsening of the separation into empty and filled regions which results from the hysteretic behavior of the transitions in the presence of a pore size distribution. In the birefringent β phase of O2 domains with a fixed crystallographic orientation extend over distances much larger than the pore diameter (10 nm).

Faraday instability in a surface-frozen liquid

Faraday surface instability measurements of the critical acceleration, a(c), and wave number, k(c), for standing surface waves on a tetracosanol (C24H50) melt exhibit abrupt changes at T(s)=54 degrees C, approximately 4 degrees C above the bulk freezing temperature. The measured variations of a(c) and k(c) vs temperature and driving frequency are accounted for quantitatively by a hydrodynamic model, revealing a change from a free-slip surface flow, generic for a free liquid surface (T>T(s)), to a surface-pinned, no-slip flow, characteristic of a flow near a wetted solid wall (T<T(s)). The change at T(s) is traced to the onset of surface freezing, where the steep velocity gradient in the surface-pinned flow significantly increases the viscous dissipation near the surface.

Strain intermittency due to avalanches in ferroelastic and porous materials

The avalanche statistics in porous materials and ferroelastic domain wall systems has been studied for slowly increasing compressive uniaxial stress with stress rates between 0.2 and 17 kPa s-1. Velocity peaks [Formula: see text] are calculated from the measured strain drops and used to determine the corresponding Energy distributions [Formula: see text]. Power law distributions [Formula: see text] have been obtained over 4-6 decades. For most of the porous materials and domain wall systems an exponent [Formula: see text] was obtained in good agreement with mean-field theory of the interface pinning transition. For charcoal, shale and calcareous schist we found significant deviations of the exponents from mean-field values in agreement with recent acoustic emission experiments.

Segmental front line dynamics of randomly pinned ferroelastic domain walls

Dynamic mechanical analysis (DMA) measurements as a function of temperature, frequency, and dynamic force amplitude are used to perform a detailed study of the domain wall motion in LaAlO3. In previous DMA measurements Harrison et al.[Phys. Rev. B69,144101(2004)] found evidence for dynamic phase transitions of ferroelastic domain walls in LaAlO3. In the present work we focus on the creep-to-relaxation region of domain wall motion using two complementary methods. We determine, in addition to dynamic susceptibility data, waiting time distributions of strain jerks during slowly increasing stress. The present dynamic susceptibility data can be well fitted with a power law, where a crossover from stochastic DW motion to the pinned regime is well described using the scaling function of Fedorenko et al.[Phys. Rev. B70, 224104(2004)].

Towards a Quantitative Analysis of Crackling Noise by Strain Drop Measurements

The method of measuring strain drops with a Dynamic Mechanical Analyzer (DMA) at slowly varying stress has a considerable potential to become an interesting complementary tool for the study of mechanical failure and earthquake dynamics in micron-sized materials. Evidence for this claim is provided by measurements of the \(\mathrm {SiO_2}\)-based porous materials Vycor and Gelsil under slow uniaxial compression at constant force rates of \(10^{-4}{-}10^{-3}\,\mathrm{N s}^{-1}\) using a Diamond DMA (Dynamical Mechanical Analyzer, Perkin Elmer). The jerky evolution of the sample’s height with time is analyzed in order to determine the corresponding power-law exponents for the maximum velocity distribution, the squared maximum velocity distribution as well as the aftershock activity in the region before macroscopic failure. A comparison with recent results from acoustic emission (AE) data on the same materials (J. Baro, A. Corral, X. Illa, A. Planes, E. K. H. Salje, W. Schranz, D. E. Soto-Parra, and E. Vives, Phys. Rev. Lett. 110, 088702 (2013)) shows similitude in the statistics, although the two methods operate on different spatial and temporal scales. Moreover, the obtained power-law exponents are in reasonable agreement with theoretical mean-field values (M. LeBlanc, L. Angheluta, K. Dahmen, N. Goldenfeld, Phys. Rev. B 87, 022126 (2013)). The results indicate that the failure dynamics of materials can be well studied by measuring strain drops under slow compression, which opens the possibility to study earthquake dynamics in the laboratory also at non-ambient conditions, i.e. at high temperatures or under confining liquid pore pressure.

On the behaviour of supercooled liquids and polymers in nano-confinement

ABSTRACT Size effects play an important role in structural phase transitions, melting transitions, in martensitic materials, glass transitions, etc. Very often the question arises, whether a measured size effect originates from the geometrical confinement itself, or if it appears due to the interaction with the limiting surface. Using dynamic mechanical analysis (DMA) technique we have studied various microphase segregated polymers, molecular glass forming liquids and supercooled water confined in nanoporous silica as well as in biological tissues. Here we show on some selected examples that DMA measurements can be used to study relaxation processes in detail and to disentangle in favourable cases pure pore size effects from effects that are induced by the confining surface.