Anna Y. Gyurova

Experimental verification of an equivalent circuit for the characterization of electrothermal micropumps: high pumping velocities induced by the external inductance at driving voltages below 5 V.

Electrothermal micropumps (ETμPs) use local heating to create conductivity and permittivity gradients in the pump medium. In the presence of such gradients, an external AC electric field influences smeared spatial charges in the bulk of the medium. When there is also a symmetry break, the field‐charge interaction results in an effective volumetric force resulting in medium pumping. The advantages of the ETμP principle are the absence of moving parts, the opportunity to passivate all the pump structures, homogeneous pump‐channel cross‐sections, as well as force plateaus in broad frequency ranges. The ETμPs consisted of a DC‐heating element and AC field electrodes arranged in a 1000 μm × 250 μm × 60 μm (length × width × height) channel. They were processed as platinum structures on glass carriers. An equivalent‐circuit diagram allowed us to model the frequency‐dependent pumping velocities of passivated and nonpassivated ETμPs, which were measured at medium conductivities up to 1.0 S/m in the 300 kHz to 52 MHz frequency range. The temperature distributions within the pumps were controlled by thermochromic beads. Under resonance conditions, an additional inductance induced a tenfold pump‐velocity increase to more than 50 μm/s at driving voltages of 5 Vrms. A further miniaturization of the pumps is viewed as quite feasible.

High frequency electric polarizability of bacteria E. coli: Dependence on the medium ionic strength

There are two main kinds of electric polarizability of bacteria: surface charge dependent (SChD) and Maxwell-Wagner (MW) polarizability. The aim of this article is to distinguish SChD and MW components on the external bacteria surface. An electro-optic method (electric turbidity) was used to study the polarizability of E. coli fixed by formaldehyde at the frequency range 20 kHz to 20 MHz. According to the literature the SChD polarization disappears at such high frequencies and MW one gives the main contribution. However we found unexpected dependence on the outer medium electrolyte concentration, which cannot be explained by MW polarization. The results show that the polarizability decreases by ionic strength increasing in the same way as the double electric layer thickness does. Such behaviour is a characteristic for SChD polarizability, which allows us to conclude that this component has the main contribution on the external bacteria surface at the experimental frequencies mentioned.

Influence of ethanol on the high frequency electric polarizability of E. coli

The interface electric polarizability of bacteria (charge dependent (ChD) and Maxwell-Wagner (MW) polarizabilities) gives information about their electric charge, determined by the structure and functional state. It is well known that the polarizability could be changed significantly by adding some substances to the suspension, and can be measured using an electro-optical (EO) method. There are some literature data, according to which the adding of ethanol decreases the electric polarizability of the cells. However the reason for the change in this parameter is not clear, as well as which component (ChD and/or MW) of polarizability has the main contribution. Generally the present work shows that the effect of ethanol is connected to the change of the internal (cytoplasm) MW polarizability and is mainly caused by increasing the cell membrane permeability. This results in an ionic flow through the membrane, which velocity and direction depends on the relative values of the inner (cytoplasm) and the outer medium ionic strength.

Influence of cytoplasm electrolyte concentration on Maxwell-Wagner polarizability of bacteria E. coli.

The electric polarizability of bacteria is considered in the literature to have a surface charge dependent (ChD) and a Maxwell-Wagner (MW) mechanism. We distinguish experimentally both the types of interface polarizability by the frequency of the electric field and the medium electrolyte concentration. It was shown in a previous work ( Zhivkov , A. M. ; Gyurova , A. Y. Colloids Surf., B 2008 , 66 , 201. ) that the ChD component is shown up on the outer bacteria surface even at megahertz frequencies. The MW polarizability is studied in the present work in the range from 20 kHz to 20 MHz by change in the inner (cytoplasm) electrolyte concentration. The ion transport through the cytoplasmic membrane of alive and fixed by formaldehyde E. coli K12 is accelerated by adding of ethanol in low concentration. The frequency dependence and the kinetics of the electric polarizability and the size of the bacteria are investigated by conservative electric dichroism, based on the alteration of the optical density at orientation of the cells in electric field. The conclusion is that the internal MW component has the main contribution to the change in the total bacteria polarizability, as well as the external MW and the internal ChD components are not shown up.

Do bacteria have an electric permanent dipole moment

In the scientific literature in the last 40 years, some data for the permanent dipole moment and the electric polarizability of Escherichia coli can be found [S.P. Stoylov, Colloid Electro-Optics - Theory, Techniques and Application, Academic Press, London, 1991]. In this paper the data based mainly on electro-optic investigation is considered as much as some dipolophoretic (most often called dielectrophoretic) studies. Serious grounds are found to doubt the conclusions made for the electric dipole moments of bacteria by one of the authors of this paper (SPS) and by some other researchers. This concerns both the permanent dipole moment and the electric charge dependent polarizabilities of E. coli. Here, along with the discussion of the old experimental data, new experimental data are shown for a strain of E. coli HB101. The conclusions from the analysis of the old and the new experimental data is that they do not provide correct evidence for the presence of a permanent dipole moment. It seems that all statements for the existence of electric permanent dipole moment in bacteria [S.P. Stoylov, Colloid Electro-Optics - Theory, Techniques and Application, Academic Press, London, 1991; S.P. Stoylov, S. Sokerov, I. Petkanchin, N. Ibroshev, Dokl. AN URSS 180 (1968) 1165; N.A. Tolstoy, A.A. Spartakov, A.A. Trusov, S.A. Schelkunova, Biofizika 11 (1966) 453; V. Morris, B. Jennings, J. Chem. Soc. Faraday Trans. II 71 (1975) 1948; V. Morris, B. Jennings, J. Colloid Interface Sci. 55 (1978) 313; S.P. Stoylov, V.N. Shilov, S.S. Dukhin, S. Sokerov, I. Petkanchin, in: S.S. Dukhin (Ed.), Electro-optics of Colloids, Naukova Dumka, Kiev, 1977 (in Russian).] based on electro-optic studies are result of incorrect interpretation. Therefore, they should be further ignored.

Low frequency electric polarizability and zeta-potential of Escherichia coli HB101 (K-12) cells during inactivation with ethanol

AbstractThe electric properties of bacteria determine their non-specific interactions with the environment, in particular their pathogenic activity. The electric polarizability of Escherichia coli HB101 (K-12 strain) was studied while inactivation with ethanol (20–40 vol.%). The current investigation might be regarded as a continuation of previous research on the polarizability of E. coli at lower ethanol concentration (≤ 20 vol.%) and higher frequencies (≥ 20 kHz).The bacteria polarizability at low frequencies (<104 Hz) shows anomalies (unexpected increase in the polarizability at certain ethanol concentrations), while the parameter decreases with an increase in the ethanol concentration at higher frequencies. We investigated for the possible reasons causing the anomalies — in our case reduced to the medium dielectric permittivity, the average cell length and the surface electric charge density distribution, related to bacterial lipopolysaccharides. We suggest a hypothesis for the molecular mechanism of changing the surface charge of E. coli, carried by lipopolysaccharides, induced by the non-ionic ethanol.

Electric polarizability changes during E. coli culture growth.

The electric polarizability of bacteria has two main components: surface-charge dependent (SChD) and Maxwell-Wagner (MW). It has been reported that the low frequency SChD component of Escherichia coli K12 still arise in the frequency range 20kHz - 2MHz, together with the high-frequency MW one. All the previous experiments were carried out with bacterial cultures of E. coli K12 in the stationary phase. In the present work we study electric polarizability during culture growth with the aim of finding out how it is influenced by the physiological state of the cells. The electro-optical method of electric turbidimetry is used, which is based on the change in the optical density as a result of orientation of bacterial cells under the action of an applied electric field. Our results show that until the cell concentration increases exponentially, the polarizability and the cell size change synchronously, so that the polarizability is approximately a quadratic function of the average bacterial length. We explain this with dominance of the SChD component. However, that after the polarizability deceases twofold at insignificant length oscillations and the power of the function decreases to 1.5. The last result is interpreted as an increase in the MW component.

Influence of the medium electrolyte concentration on the electric polarizability of bacteria Escherichia coli in presence of ethanol

The electric polarizability is an important parameter of bacteria, giving information about the electric properties of the cells. In our previous works [A.M. Zhivkov, A.Y. Gyurova, Colloids Surf. B: Biointerfaces 66 (2008) 201; A.Y. Gyurova, A.M. Zhivkov, Biophys. Chem., 139 (2009) 8; A.M. Zhivkov, A.Y. Gyurova, J. Phys. Chem. B, 113 (2009) 8375] we have applied an experimental approach to distinguish the contribution of the components of the two types of interface electric polarizability-surface charge dependent (ChD) and Maxwell-Wagner (MW) polarizability. It is based on electro-optical study of the separate influence of the outer and inner medium electrolyte concentration, which changes the external ChD and internal MW components of polarizability; the last effect is reached by the membrane permeability increase in low ethanol concentration. In the present work we investigate the behavior of electric polarizability of Escherichia coli K12 at increasing the outer KCl concentration in presence of 10 vol.% ethanol in order to check if the polarizability components change independently from one another. The conclusion is that the outer electrolyte concentration influence indirectly the internal MW component by the trans-membrane concentration gradient, but the polarizability components themselves change independently.

Aqueous solutions of random poly(methyl methacrylate-co-acrylic acid): effect of the acrylic acid content

Aqueous solutions from two types of PMMA–AA polymers (PMMA–25AA and PMMA–50AA) are investigated. The aim is to outline the effect of AA content on the bulk solution and air/solution interface properties. The experiments include dynamic light scattering, surface tension and interfacial rheology measurements. The drainage kinetics and stability of microscopic foam films are also investigated. It is established that at similar conditions (pH = 10, temperature 20 °C) the polymer molecules have almost the same mean molecular weight and size distribution of the bulk globules. Dynamic and equilibrium surface tension measurements reveal systematically higher values for PMMA–50AA as compared to PMMA–25AA. These outcomes are related to higher bulk electrophoretic mobility in the PMMA–50AA case and the overall more stretched configuration of the polymer at the air/solution interface. Surface dilational rheology characteristics are particularly sensitive to the polymer structural peculiarities: while no significant changes are registered in the case of PMMA–25AA, the solutions of PMMA–50AA exhibit a pronounced maximum in surface dilational elasticity for the concentration ∼1 × 10−4 mol L−1. This observation is a clear sign of specific bulk and/or interfacial (structure) transition which has to be investigated in further studies. Microscopic foam films provide additional evidence for the effect of fine-tuning the AA content of the copolymer. All microscopic films are stable which is due predominantly to the overwhelming electrostatic repulsion effects. The obtained results add new knowledge to the structure–property relationships of the PMMA–AA based aqueous formulations. They give valuable hints for further fine-tuning opportunities of these systems, that have high innovative potential for various applications.