Computer Science and Engineering School Applied Physics  

Dielectric Spectroscopy Laboratory

The research interests of my laboratory are centred on the area of soft condensed matter physics for investigation of the structure, dynamics, and macroscopic behaviour of complex systems. (CS). CS is a very broad and general class of materials, which include associated liquids, polymers, biomolecules, colloids, porous materials and liquid crystals.

The dynamical processes occurring in Complex Systems involve different length and time scales. Fast as well as ultra-slow molecular rearrangements take place in the presence of the microscopic, mesoscopic and macroscopic organization of the systems. Commonly, the complete characterization of these relaxation behaviours requires the use of variety techniques in order to span the relevant ranges in frequency. In this view, the use of Dielectric Spectroscopy (DS) is very advantageous


Broad Band Dielectric Spectroscopy in Time and Frequency Domain

The dielectric spectroscopy (DS) method occupies a special place among the numerous modern methods used for physical and chemical analysis of material, because it enables investigation of dielectric relaxation processes in an extremely wide range of characteristic times (106 - 10-12 s), see Figure 1. Intermolecular interactions and cooperative processes may be monitored by help of DS, which provides a link between the properties of the individual constituents of a complex material to the characterization of its bulk properties. The successful development of the time domain dielectric spectroscopy method (generally called time domain spectroscopy - TDS) and Broadband Dielectric Spectroscopy (BDS) have radically changed the attitude towards DS, making it an effective tool for investigation of solids and liquids, on the macroscopic, mesoscopic and microscopic levels.

Time domain spectroscopy

Figure 1. The frequency band of dielectric spectroscopy.

Dielectric Relaxation in Complex Liquids at Mesoscale

The presented contribution is centred on the area of condensed matter physics for investigation of the structure, dynamics, and macroscopic behaviour of materials which can be characterized as complex liquids (CL). CL is a very broad and general class of materials that include associated liquids, polymers, biomolecules, colloids, etc.) CL involves the appearance of a new ("mesoscopic") length scale, intermediate between molecular and macroscopic. The complete characterization of these relaxation behaviours requires the use of variety techniques in order to span the relevant ranges in frequency. To obtain this information, non-invasive methods such as Dielectric Spectroscopy (DS) are very advantageous. The unique technique with wide frequency (10-5 - 1012 Hz) and temperature (-170 °C +300 °C) ranges of that method is more then any others appropriate for such different scales of molecular motions.

The experimental results obtained by dielectric spectroscopy for complex liquids (microemulsions, lyposomes, cell suspensions etc.) with different level of dc conductivity are studied. Dynamic behavior at mesoscale in the vicinity to the percolation threshold was described in terms of two fractal models
a. Recursive fractal model
b. Statistical fractal model
The scaling in frequency and time domain allowed the construction of time - space models and the elaboration of the relationships for structural and dynamical properties at the mesoscale

Figure 2. Three-dimensional plots of the frequency and temperature dependence of the dielectric permittivity ε’ (a) and the three-dimensional plot of experimental dipole correlation function versus time and temperature (b). The percolation threshold temperature Tp= 26.5°C.

Dielectric Relaxation and Transport in Porous Materials

The research is aimed in development of a method which enables us to infer the geometrical features of the porous medium from its dielectric response and electric conductivity and focused on a systematic study of how the pore-space geometry of an insulating porous material influences the low- and high-frequency dielectric relaxation and electric conductivity of the system when the pore space is filled with a conductor or another dielectric. These parameters are determined by the geometry of pore size distributions and fractal dimensions, and provide information about the cooperative relaxation processes and the mesostructural features of the matrix. The dielectric spectroscopy data describe an overall picture of the relaxation dynamics associated with the polarization of the system and the charge transport mechanisms.

Figure 3 The typical three-dimensional plot of the complex dielectric permittivity real ε‘(a) and imaginary part  ε” (b) versus frequency and temperature for porous glass. The dielectric permittivity and losses associated with the relaxation of water molecules of the adsorptive layer for the studied porous glasses versus frequency and temperature  can be can be described in terms of the four distributed relaxation processes (shown by figures).

Relaxation Dynamics and Mesoscopic Feature of Ferroelectric Crystals

The Dielectric Spectroscopy Laboratory in interested in complex relaxation behaviour of ion inside ferroelectric lattices. The main goal of this particular research is dopant, for instance Cu ions inside a KTN lattice.  Here the mismatch in ionic radii results in a multiwall potential and the creation of a virtual dipole as the Cu ion moves between local troughs inside the multi well.  This oscillation can be cooperative as long range interactions between ions begin to dominate with the approaching ferroelectric transition. These interactions lead to thermal behaviour reminiscent of glass forming liquids.  In the ferroelectric phase the same ionic oscillation can become restricted due to the large internal fields present after the spontaneous polarization of the ferroelectric transition. Such confinement can find parallels in soft condensed matter physics.  Transport phenomena such an electron hopping is also visible in the dielectric landscape.  How the local environment modifies this behaviour is a central theme of dielectric research, illuminating topics like Anomalous diffusion.  Phenomena such as percolation can also be observed whereby the accumulation of the local polarization vectors of individual clusters coalesces to an infinite percolation cluster.  A final theme in this multifaceted field is the design of smart marterials based on a thorough understanding of their ferroelectric behaviour.

Figure 4. The dielectric losses, ε”, for copper-doped KTN crystal. The three phase transitions are evident at T= 295.6 K, 291.1 K, and 230 K, respectively

Relaxation Dynamics and Mesoscopic Feature of Glass Forming Glycerol-Water mixtures

The main goal of this particular research is an understanding of dynamics and structure of hydrogen bonding liquids and the study of fluctuations and transport in the hydrogen-bonded network at the mesoscopic scale. Broadband Dielectric Spectroscopy will be used in conjunction with calorimetric and high precision density measurements. That allows to study dynamics of the systems in the entire frequency (time) range from 10 GHz to 1 mHz (total broader than 13 orders). At first, glycerol-water mixtures will be studied as an excellent model system. Systematic study covering with wide frequency, temperature, and concentration ranges will clarify the mechanisms of slow- and fast- dynamics through dc-conductivity, main relaxation process and high frequency excess wing that can be relating to the cage dynamics. For example, Figure 4 shows so-called master plot of glycerol-water mixture (75 mol% of glycerol) in which 32 spectra at different temperatures were normalized and presented. The fact that all normalised spectra traced the same single curve indicates that dielectric response at both high and low frequencies follows the same temperature dependence of the main relaxation process. Therefore, it leads to the hypotheses that the relaxation mechanism of the excess wing, the main process and dc-conductivity are based on the same origin in glycerol-rich mixtures.

Figure 5. Typical master plots of glycerol-water mixtures (100-, 80-, 60-, 40- and 25- mol% glycerol). Fitting result by relation for 75 mol% glycerol is also shown by solid curve in the small-inserted figure. Here, a master plot of a 35 mol% glycerol mixture is additionally presented as a typical example in water-rich .region.

Dielectric Spectroscopy as a monitoring tool for the blood cells response on the bioactive compounds influence

The main direction of that research is the monitoring of dielectric properties of Red Blood Cells for understanding of the structure and physical processes occurring in biological systems at different levels of complexity. The development of high-specialized and speedy non-invasive methods based on investigation of alteration of static and dynamic dielectric properties of blood cells as a response on exposure by bioactive compounds and external factors for research and clinical treatment is a central target of this interdisciplinary project. Such approach can be very useful not only for clinical applications, but also for understanding of molecular mechanisms of cellular response.

Figure 6. One-cell dielectric spectra of spherical erythrocytes, measured at 25°C in the presence of increasing concentrations of D-glucose.


Human Skin as Arrays of Helical Antennas in the Millimeter and Submillimeter Wave Range

Recent studies of the minute morphology of the skin by optical coherence tomography showed that the sweat ducts in human skin are helically shaped tubes, filled with a conductive aqueous solution. A computer simulation study of these structures in millimeter and submillimeter wave bands show that the human skin functions as an array of low-Q helical antennas. Experimental evidence is presented that the spectral response in the sub-Terahertz region is governed by the level of activity of the perspiration system. It is also correlated to physiological stress as manifested by the pulse rate and the systolic blood pressure.

Figure 7. 3D optical coherence tomography image (reproduced with permission from ISIS GmbH) of a single human eccrine sweat gland embedded in the human skin and a schematic presentation of the duct as a helical antenna embedded in the skin, where the dermis-epidermis interface acts as a dielectric reflector. The respective permittivities of the skin layers are marked. They were estimated for the specific frequency range based on the water content of the layers.