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Our new type of on-chip micro-pump exploits the ac electro-kinetic forces acting in the volume of a fluid in the presence of a temperature gradient. No mechanically movable parts are used. The velocity of the pump flow observed depends on the frequency and strength of the driving ac field and on the temperature gradient across the pump channel. An integrated heating element allows the temperature gradient to be adjusted. Both ac field electrodes and heating element are platinum structures processed on a glass chip. The pump-channel walls and cover are made from polymer and thin-glass, respectively. In this paper, we present measurements of the fluid velocity as functions of the medium conductivity (0.1–1.3 S m−1) and field frequency (300 kHz–52 MHz), voltage across the field-electrode voltage (0–35 Vrms) and the heating element (1.1–3.6 V). Velocities of up to 120 µm s−1 were observed in the pump channel. The advantage of our new design is an evenly shaped cross-section of the pump channel, which reduces the risk of the channel becoming clogged by debris. Ac-electro-osmosis is not a predominant effect in our structures. Pumping could only be observed when the heating current and ac-pump field were applied simultaneously. The effects observed were simulated with the COMSOL Multiphysics program.

The induction of transmembrane potentials (Δ) by an external field is the basis of numerous applications in biotechnology, cell-technology and medicine. We have developed new, simplified analytical equations that avoid the complicated description by the depolarizing factors. The equations apply to the Δ induced in cells resembling ellipsoids of rotation, i.e. spheroids by homogeneous dc or ac fields. They will be especially useful for experimental scientists. The equations describe the dependence of the Δ on the electric media properties, the field frequency and the axis ratio for oblate and prolate spheroids for which the symmetry semiaxis (c) is shorter and longer than the other two semiaxes (a and b, with a = b), respectively. According to the Schwan equation, an electric field E may induce a maximum Δ of 1.5aE at the poles of a spherical cell. For the poles of spheroidal cells, the maxima can be easily described by Δ = (a + 2c)E/2 and Δ = a(a + 2c)E/(a + c) for fields oriented along and perpendicular to the symmetry axis, respectively. For practically important shapes the error in the magnitude of Δ is smaller than 5% except along the c-axis for axis ratios larger than 2. Nevertheless, the errors vanish for the three limiting shapes of infinitely thin disc, sphere and cylinder.

Potentially, lipid membranes possess a high tangential conductivity and permittivity due to their surface charges and the in-plane orientation of the headgroup dipoles. Electrically, membranes exhibit a sandwich structure with a largely isotropic centre formed by the fatty acid chains and confined by two anisotropic headgroup layers. Accordingly, we described spherical vesicles by an aqueous core covered by three shells. For a theoretical comparison, models with an anisotropic single shell and anisotropic homogeneous spheres were also considered. Two effects can be clearly demonstrated. (1) High tangential conductivities or permittivities may lead to cyclic variations in the phase of the electric potential in the radial direction, resulting in a hemi-shell structure of the electric potential inside the objects with oppositely charged facets. The thickness of the anisotropic shell restricts the number of phase oscillations. (2) Despite the strong local field inhomogeneities, an isotropic homogeneous Maxwellian equivalent body with an identical external field distribution exists for any of the anisotropic models. Its properties can be found from a comparison of the numerically calculated surface potential and the classical expression of the Clausius–Mossotti factor at any given frequency. The permittivity conductivity pairs obtained exhibit a sigmoidal-like frequency dependence.

Circular spatial filtering velocimetry (CSFV) was tested during the microscopic registration of the individual rotations of baker's yeast cells. Their frequency-dependent rotation (electrorotation; ER) was induced in rotating electric fields, which were generated in a glass chip chamber with four electrodes (600 μm tip-to-tip distance). The electrodes were driven with sinusoidal quadrature signals of 5 or 8 V PP with frequencies up to 3 MHz. The observed cell rotation was of the order of 1–100 s per revolution. At each measuring frequency, the independent rotations of up to 20 cells were simultaneously recorded with a high-speed camera. CSFV was software-implemented using circular spatial filters with harmonic gratings. ER was proportional to the phase shift between the values of the spatial filtering signal of consecutive frames. ER spectra obtained by CSFV from the rotation velocities at different ER-field frequencies agreed well with manual measurements and theoretical spectra. Oscillations in the rotation velocity of a single cell in the elliptically polarized field near an electrode, which were resolved by CSFV, could not be visually discerned. ER step responses after field-on were recorded at 2500 frames per second. Analysis proved the high temporal resolution of CSFV and revealed a largely linear torque-friction relation during the acceleration phase of ER. Future applications of CSFV will allow for the simple and cheap automated high-resolution analysis of rotational movements where mechanical detection has too low a resolution or is not possible, e.g. in polluted environments or for gas and fluid vortices, microscopic objects, etc.