Piotr Habdas

Thermal vestige of the zero-temperature jamming transition

When the packing fraction is increased sufficiently, loose particulates jam to form a rigid solid in which the constituents are no longer free to move. In typical granular materials and foams, the thermal energy is too small to produce structural rearrangements. In this zero-temperature (T = 0) limit, multiple diverging and vanishing length scales characterize the approach to a sharp jamming transition. However, because thermal motion becomes relevant when the particles are small enough, it is imperative to understand how these length scales evolve as the temperature is increased. Here we used both colloidal experiments and computer simulations to progress beyond the zero-temperature limit to track one of the key parameters—the overlap distance between neighbouring particles—which vanishes at the T = 0 jamming transition. We find that this structural feature retains a vestige of its T = 0 behaviour and evolves in an unusual manner, which has masked its appearance until now. It is evident as a function of packing fraction at fixed temperature, but not as a function of temperature at fixed packing fraction or pressure. Our results conclusively demonstrate that length scales associated with the T = 0 jamming transition persist in thermal systems, not only in simulations but also in laboratory experiments.

Forced motion of a probe particle near the colloidal glass transition

We use confocal microscopy to study the motion of a magnetic bead in a dense colloidal suspension, near the colloidal glass transition volume fraction g. For dense liquid-like samples near g, below a threshold force the magnetic bead exhibits only localized caged motion. Above this force, the bead is pulled with a fluctuating velocity. The relationship between force and velocity becomes increasingly nonlinear as g is approached. The threshold force and nonlinear drag force vary strongly with the volume fraction, while the velocity fluctuations do not change near the transition.

Video microscopy of colloidal suspensions and colloidal crystals

Colloidal suspensions are simple model systems for the study of phase transitions. Video microscopy is capable of directly imaging the structure and dynamics of colloidal suspensions in different phases. Recent results related to crystallization, glasses, and 2D systems complement and extend previous theoretical and experimental studies. Moreover, new techniques allow the details of interactions between individual colloidal particles to be carefully measured. Understanding these details will be crucial for designing novel colloidal phases and new materials, and for manipulating colloidal suspensions for industrial uses.

Isothermal and high-pressure studies of dielectric relaxation in supercooled glycerol

Measurements of isothermal (), high-pressure behaviour (up to 270 MPa) of complex electric permittivity in supercooled glycerol are presented. The observed peaks of relaxation in absorption curves show increasing relaxation time with increasing pressure. The behaviour of can be reproduced by the pressure version of an Arrhenius relation or by a modified Vogel - Fulcher - Tammann equation. The data obtained can be superimposed by applying the scaling form used by Dixon et al 1990 Phys. Rev. Lett. 65 1108.

Hydraulic disruption and passive migration by a bacterial pathogen in oak tree xylem

Xylella fastidiosa (Xf) is a xylem-limited bacterial pathogen that causes leaf scorch symptoms in numerous plant species in urban, agricultural, and natural ecosystems worldwide. The exact mechanism of hydraulic disruption and systemic colonization of xylem by Xf remains elusive across all host plants. To understand both processes better, the functional and structural characteristics of xylem in different organs of both healthy and Xf-infected trees of several Quercus species were studied. Hydraulic conductivity (Ks) in Xf-infected petioles of Q. palustris and Q. rubra decreased significantly compared with healthy trees as the season progressed and plummeted to zero with the onset of scorch symptoms. Prior to the onset of symptoms, embolism was as much as 3.7 times higher in Xf-infected petioles compared with healthy controls and preceded significant reductions in Ks. Embolism likely resulted from pit membrane degradation during colonization of new petiole xylem and triggered the process that eventually led to vessel occlusion. Pit membrane porosity was studied using the following four methods to determine if a pathway exists in the xylem network of woody stems that allows for passive Xf migration: (i) calculations based on vulnerability to cavitation data, (ii) scanning electron micrographs, (iii) microsphere injections, and (iv) air seeding thresholds on individual vessels. All four methods consistently demonstrated that large pit membrane pores (i.e. greater than the diameter of individual Xf) occur frequently throughout the secondary stem xylem in several Quercus species. These large pores probably facilitate systemic colonization of the secondary xylem network and contribute to the high susceptibility to bacterial leaf scorch exhibited among these species.

The influence of pressure on dielectric relaxation for phthalate derivatives in the supercooled state

The dielectric relaxation of phthalate derivatives in the supercooled state has been studied at pressures up to 300 MPa, covering a frequency range from to . The behaviour of the relaxation time is very well reproduced by the relation . The analysis of the absorption curves shows that pressure increases the amplitude of the -relaxation more than the equivalent temperature change. The scaling form presented by Dixon and co-workers for the -process in supercooled liquids is re-examined in the light of pressure - dielectric relaxation data. It was found that Dixon scaling distinguishes isothermal measurements from isobaric measurements, by means of the greater strength of the second relaxation process for the pressure path.

Electric Permittivity in the One- and Two-Phase Region of 1- Nitropropane-Hexadecane Near Critical Solution

Abstract Results of dielectric constant measurements in a one- and two-phase region of near-critical 1-nitropropane-hexadecane solution are presented. For the critical anomaly in the homogeneous region the critical exponent 1 − α = 0.87 ± 0.03 was found. The influence of the correction-to-scaling has been discussed. Below T c , the analysis of the binodale gave 1 − α = 0.88 ± 0.12 for the diameter and s = 0.328 ± 0.06 for the order parameter, neglecting the influence of the correction-to-scaling in the tested temperature range T − T c ≈ 5 K. All above values are in very good agreement with theoretical predictions. For the pretransitional anomaly in the homogeneous region the analysis of the low-frequency Maxwell-Wagner dispersion effect gave the critical exponent x = 0.41 ± 0.02, a bit higher than the theoretically predicted value x ≈ 0.3.

PRESSURE AND TEMPERATURE STUDIES OF DIELECTRIC PERMITTIVITY IN THE HOMOGENEOUS PHASE OF NITROBENZENE-DODECANE BINARY MIXTURE

Abstract Results of isothermal high-pressure (up to 120 MPa) and isobaric ( P =0.1 and 51 MPa) temperature studies of dielectric permittivity in nitrobenzene–dodecane binary solution are presented. The dependencies of e ( P ) and e ( T ) obtained can be well portrayed by isomorphic relations with the same value of the critical exponent φ =1− α ≈0.88. The influence of correction-to-scaling terms for the temperature and pressure paths is shown. It has been found that the strength of the pretransitional anomaly of the isothermal pressure path is definitely larger than that of the isobaric temperature path. The influence of the Maxwell–Wagner effect was found to be insignificant in the isothermal pressure studies even for a frequency of 100 Hz. Data were completed by the results of low-frequency (250 kHz) measurements of the shift of dielectric permittivity due to application of a strong electric field (the non-linear dielectric effect).

Particle dynamics in colloidal suspensions above and below the glass-liquid re-entrance transition

We study colloidal particle dynamics of a model glass system using confocal and fluorescence microscopy as the sample evolves from a hard-sphere glass to a liquid with attractive interparticle interactions. The transition from hard-sphere glass to attractive liquid is induced by short-range depletion forces. The development of liquid-like structure is indicated by particle dynamics. We identify particles which exhibit substantial motional events and characterize the transition using the properties of these motional events. As samples enter the attractive liquid region, particle speed during these motional events increases by about one order of magnitude, and the particles move more cooperatively. Interestingly, colloidal particles in the attractive liquid phase do not exhibit significantly larger displacements than particles in the hard-sphere glass.

Temperature Studies of Dielectric Permittivity and Mass Density Pretransitional Anomalies in Binary Mixtures

Abstract Results of temperature studies of dielectric permittivity and mass density in binary mixtures are presented. The anomaly of dielectric permittivity as a function of temperature in nitrobenzene-n-alkanes binary mixtures has been studied. Molar dielectric permittivity which takes into account the anomaly of dielectric permittivity and the mass density anomaly has been introduced.