First-Principles Approach to Electrorotation Assay

We have presented a theoretical study of electrorotation assay based on the spectral representation theory. We consider unshelled and shelled spheroidal particles as an extension to spherical ones. From the theoretical analysis, we find that the coating can change the characteristic frequency at which the maximum rotational angular velocity occurs. The shift in the characteristic frequency is attributed to a change in the dielectric properties of the bead-coating complex with respect to those of the uncoated particles. By adjusting the dielectric properties and the thickness of the coating, it is possible to obtain good agreement between our theoretical predictions and the assay data.Comment: 17 pages, 4 eps figures; minor revisions, accepted for publications by J. Phys.: Condens. Matte

Electrorotation of a pair of spherical particles

We present a theoretical study of electrorotation (ER) of two spherical particles under the action of a rotating electric field. When the two particles approach and finally touch, the mutual polarization interaction between the particles leads to a change in the dipole moment of the individual particle and hence the ER spectrum, as compared to that of the well-separated particles. The mutual polarization effects are captured by the method of multiple images. From the theoretical analysis, we find that the mutual polarization effects can change the characteristic frequency at which the maximum angular velocity of electrorotation occurs. The numerical results can be understood in the spectral representation theory.Comment: Minor revisions; accepted by Phys. Rev.

ac-field-induced fluid pumping in microsystems with asymmetric temperature gradients

We present two different designs of electrohydrodynamic micropumps for microfluidic systems. The micropumps have no movable parts, and their simple design allows for fabrication by microsystems technology. The pumps are operated by ac voltages from 1to60V and were tested with aqueous solutions in the conductivity range of 1–112mSm−1. The pump effect is induced by an ac electric field across a fluid medium with an inhomogeneous temperature distribution. It is constant over a wide range of the ac field frequency with a conductivity-dependent drop-off at high frequencies. The temperature-dependent conductivity and permittivity distributions in the fluid induce space charges that interact with the electric field and induce fluid motion. The temperature distribution can be generated either by Joule heating in the medium or by external heating. We present experimental results obtained with two prototypes featuring Joule heating and external heating by a heating filament. Experimental and numerical results are compared with an analytical model

Theory of ac electrokinetic behavior of spheroidal cell suspensions with an intrinsic dispersion

The dielectric dispersion, dielectrophoretic (DEP) and electrorotational (ER) spectra of spheroidal biological cell suspensions with an intrinsic dispersion in the constituent dielectric constants are investigated. By means of the spectral representation method, we express analytically the characteristic frequencies and dispersion strengths both for the effective dielectric constant and the Clausius-Mossotti factor (CMF). We identify four and six characteristic frequencies for the effective dielectric spectra and CMF respectively, all of them being dependent on the depolarization factor (or the cell shape). The analytical results allow us to examine the effects of the cell shape, the dispersion strength and the intrinsic frequency on the dielectric dispersion, DEP and ER spectra. Furthermore, we include the local-field effects due to the mutual interactions between cells in a dense suspension, and study the dependence of co-field or anti-field dispersion peaks on the volume fractions.Comment: accepted by Phys. Rev.

Dielectric behaviour of graded spherical cells with an intrinsic dispersion

The dielectric properties of single-shell spherical cells with an intrinsic dielectric dispersion has been investigated. By means of the dielectric dispersion spectral representation (DDSR) for the Clausius-Mossotti (CM) factor, we express the dispersion strengths as well as the characteristic frequencies of the CM factor analytically in terms of the parameters of the cell model. These analytic expressions enable us to assess the influence of various model parameters on the electrokinetics of cells. Various interesting behaviours have been reported. We extend our considerations to a more realistic cell model with a graded core, which can have spatial gradients in the conductivity and/or permittivity. To this end, we address the effects of a graded profile in a small-gradient expansion in the framework of DDSR.Comment: accepted by European Physical Journal

Dielectric Behavior of Nonspherical Cell Suspensions

Recent experiments revealed that the dielectric dispersion spectrum of fission yeast cells in a suspension was mainly composed of two sub-dispersions. The low-frequency sub-dispersion depended on the cell length, whereas the high-frequency one was independent of it. The cell shape effect was qualitatively simulated by an ellipsoidal cell model. However, the comparison between theory and experiment was far from being satisfactory. In an attempt to close up the gap between theory and experiment, we considered the more realistic cells of spherocylinders, i.e., circular cylinders with two hemispherical caps at both ends. We have formulated a Green function formalism for calculating the spectral representation of cells of finite length. The Green function can be reduced because of the azimuthal symmetry of the cell. This simplification enables us to calculate the dispersion spectrum and hence access the effect of cell structure on the dielectric behavior of cell suspensions.Comment: Preliminary results have been reported in the 2001 March Meeting of the American Physical Society. Accepted for publications in J. Phys.: Condens. Matte

Attachment of Rod-Like (BAR) Proteins and Membrane Shape

Previous studies have shown that cellular function depends on rod-like membrane proteins, among them Bin/Amphiphysin/Rvs (BAR) proteins may curve the membrane leading to physiologically important membrane invaginations and membrane protrusions. The membrane shaping induced by BAR proteins has a major role in various biological processes such as cell motility and cell growth. Different models of binding of BAR domains to the lipid bilayer are described. The binding includes hydrophobic insertion loops and electrostatic interactions between basic amino acids at the concave region of the BAR domain and negatively charged lipids. To shed light on the elusive binding dynamics, a novel experiment is proposed to expand the technique of single-molecule AFM for the traction of binding energy of a single BAR domain

The prolate-to-oblate shape transition of phospholipid vesicles in response to frequency variation of an AC electric field can be explained by the dielectric anisotropy of a phospholipid bilayer

The external electric field deforms flaccid phospholipid vesicles into spheroidal bodies, with the rotational axis aligned with its direction. Deformation is frequency dependent: in the low frequency range (~ 1 kHz), the deformation is typically prolate, while increasing the frequency to the 10 kHz range changes the deformation to oblate. We attempt to explain this behaviour with a theoretical model, based on the minimization of the total free energy of the vesicle. The energy terms taken into account include the membrane bending energy and the energy of the electric field. The latter is calculated from the electric field via the Maxwell stress tensor, where the membrane is modelled as anisotropic lossy dielectric. Vesicle deformation in response to varying frequency is calculated numerically. Using a series expansion, we also derive a simplified expression for the deformation, which retains the frequency dependence of the exact expression and may provide a better substitute for the series expansion used by Winterhalter and Helfrich, which was found to be valid only in the limit of low frequencies. The model with the anisotropic membrane permittivity imposes two constraints on the values of material constants: tangential component of dielectric permittivity tensor of the phospholipid membrane must exceed its radial component by approximately a factor of 3; and the membrane conductivity has to be relatively high, approximately one tenth of the conductivity of the external aqueous medium.Comment: 17 pages, 6 figures; accepted for publication in J. Phys.: Condens. Matte

Dynamic polarizability of rotating particles in electrorheological fluids

A rotating particle in electrorheological (ER) fluid leads to a displacement of its polarization charges on the surface which relax towards the external applied field ${\bf E}_0$, resulting in a steady-state polarization at an angle with respect to ${\bf E}_0$. This dynamic effect has shown to affect the ER fluids properties dramatically. In this paper, we develop a dynamic effective medium theory (EMT) for a system containing rotating particles of finite volume fraction. This is a generalization of established EMT to account for the interactions between many rotating particles. While the theory is valid for three dimensions, the results in a special two dimensional configuration show that the system exhibits an off-diagonal polarization response, in addition to a diagonal polarization response, which resembles the classic Hall effect. The diagonal response monotonically decreases with an increasing rotational speed, whereas the off-diagonal response exhibits a maximum at a reduced rotational angular velocity $\omega_0$ comparing to the case of isolated rotating particles. This implies a way of measurement on the interacting relaxation time. The dependencies of the diagonal and off-diagonal responses on various factors, such as $\omega_0$, the volume fraction, and the dielectric contrast, are discussed.Comment: 6 pages, 4 figures, accepted to J. Phys. Chem.

Transmembrane potential induced on the internal organelle by a time-varying magnetic field: a model study

<p>Abstract</p> <p>Background</p> <p>When a cell is exposed to a time-varying magnetic field, this leads to an induced voltage on the cytoplasmic membrane, as well as on the membranes of the internal organelles, such as mitochondria. These potential changes in the organelles could have a significant impact on their functionality. However, a quantitative analysis on the magnetically-induced membrane potential on the internal organelles has not been performed.</p> <p>Methods</p> <p>Using a two-shell model, we provided the first analytical solution for the transmembrane potential in the organelle membrane induced by a time-varying magnetic field. We then analyzed factors that impact on the polarization of the organelle, including the frequency of the magnetic field, the presence of the outer cytoplasmic membrane, and electrical and geometrical parameters of the cytoplasmic membrane and the organelle membrane.</p> <p>Results</p> <p>The amount of polarization in the organelle was less than its counterpart in the cytoplasmic membrane. This was largely due to the presence of the cell membrane, which "shielded" the internal organelle from excessive polarization by the field. Organelle polarization was largely dependent on the frequency of the magnetic field, and its polarization was not significant under the low frequency band used for transcranial magnetic stimulation (TMS). Both the properties of the cytoplasmic and the organelle membranes affect the polarization of the internal organelle in a frequency-dependent manner.</p> <p>Conclusions</p> <p>The work provided a theoretical framework and insights into factors affecting mitochondrial function under time-varying magnetic stimulation, and provided evidence that TMS does not affect normal mitochondrial functionality by altering its membrane potential.</p