Dušan Babič

Self-assembled artificial cilia

Due to their small dimensions, microfluidic devices operate in the low Reynolds number regime. In this case, the hydrodynamics is governed by the viscosity rather than inertia and special elements have to be introduced into the system for mixing and pumping of fluids. Here we report on the realization of an effective pumping device that mimics a ciliated surface and imitates its motion to generate fluid flow. The artificial biomimetic cilia are constructed as long chains of spherical superparamagnetic particles, which self-assemble in an external magnetic field. Magnetic field is also used to actuate the cilia in a simple nonreciprocal manner, resulting in a fluid flow. We prove the concept by measuring the velocity of a cilia-pumped fluid as a function of height above the ciliated surface and investigate the influence of the beating asymmetry on the pumping performance. A numerical simulation was carried out that successfully reproduced the experimentally obtained data.

Observation of condensed phases of quasiplanar core-softened colloids.

We experimentally study the condensed phases of repelling core-softened spheres in two dimensions. The dipolar pair repulsion between superparamagnetic spheres trapped in a thin cell is induced by a transverse magnetic field and softened by suitably adjusting the cell thickness. We scan a broad density range and we materialize a large part of the theoretically predicted phases in systems of core-softened particles, including expanded and close-packed hexagonal, square, chainlike, stripe or labyrinthine, and honeycomb phase. Further insight into their structure is provided by Monte Carlo simulations.

Fiber optic catalytic probe for weakly ionized oxygen plasma characterization

Construction and operation of a novel fiber optic catalytic probe is presented. The probe is intended for measurements of atomic oxygen density in plasma postglows. The operation of the probe is based on a catalytic recombination of oxygen atoms and remote temperature sensing via optical fiber. Compared to the classical catalytic probes, the new approach results in smaller dimensions, better sensitivity, and higher immunity to the electromagnetic interference. Comparative measurements of atomic oxygen density performed simultaneously with both types of probes demonstrated a superior performance of the novel probes.

Comparison of fiber optics and standard nickel catalytic probes for determination of neutral oxygen atoms concentration

The density of neutral oxygen atoms in a plasma postglow chamber was measured with a standard Ni catalytic probe and a fiber optical catalytic probe. A highly dissociated oxygen plasma was created in the discharge chamber with an inductively coupled rf generator with a frequency of 27.12 MHz and output power of 200 W. The measurements were performed in the postglow chamber where a movable recombinator for oxygen atoms was placed. The recombinator enabled the adjustment of O density independently of discharge parameters. The density of neutral oxygen atoms was determined at different pressures and different positions of the recombinator. The O density was between 5×1020 and 2×1021 m−3. The fiber probe expressed excellent repeatability as compared to the standard probe. As long as the temperature of the catalyst was above 100 °C both probes gave similar results. At a lower temperature of the standard probe, however, the optical fiber probe gave more reasonable results. The discrepancy was attributed to the ...

Comparison of NO titration and fiber optics catalytic probes for determination of neutral oxygen atom concentration in plasmas and postglows

A comparative study of two different absolute methods NO titration and fiber optics catalytic probe (FOCP) for determination of neutral oxygen atom density is presented. Both methods were simultaneously applied for measurements of O density in a postglow of an Ar/O2 plasma created by a surfatron microwave generator with the frequency of 2.45 GHz an adjustable output power between 30 and 160 W. It was found that the two methods gave similar results. The advantages of FOCP were found to be as follows: it is a nondestructive method, it enables real time measuring of the O density, it does not require any toxic gas, and it is much faster than NO titration. The advantage of NO titration was found to be the ability to measure O density in a large range of dissociation of oxygen molecules.

A diagnostic method for real-time measurements of the density of nitrogen atoms in the postglow of an Ar–N2 discharge using a catalytic probe

We determined the density of neutral nitrogen atoms in an Ar–N2 postglow using a fiber-optics catalytic probe. The probe, which had a catalyst made of nitrided iron, was calibrated with a NO titration. The recombination coefficient for the heterogeneous recombination of N atoms on the nitrided iron surface was determined by comparing the probe signal with the NO titration. Within the limits of experimental error the coefficient was found to be independent of the catalyst temperature between 400 and 650K and had a value of 0.21±0.04. Real-time measurements of the N-atom density were performed at a nitrogen flow of 600SCCM (standard cubic centimeter per minute) for several discharge powers between 80 and 300W, and for argon flow rates between 200 and 3000SCCM. With increasing discharge power the N-atom density increased monotonously; with increasing Ar flow the N-atom density at first increased, reaching a broad maximum at about 1.8×1021m−3 for an Ar flow of 2000SCCM, after which the N-atom density decrease...

Colloids as model systems for problems in statistical physics

Owing to their mesoscopic length scales, colloidal suspensions provide ideal model systems suitable for addressing many problems in the field of statistical physics. Exemplarily, we highlight the versatile nature of such systems by discussing experiments with stochastic resonance and a practical realization of a recently proposed ratchet cellular automaton.

Magnetically actuated microrotors with individual pumping speed and direction control

We demonstrate an experimental realization of a microscale pump self-assembled from superparamagnetic colloidal spheres and driven by an external magnetic field, where a system of microelectrodes controls the pump rotor by dielectrophoretic force. Whereas an external magnetic field is used to assemble and drive the rotor, which allows parallel fabrication and actuation of many such devices, the microelectrodes enable control of an individual rotor and thus regulate pumping speed and direction of any single pump in the microfluidic device. Dielectrophoretically controlled micropumps can be fabricated with existing microfabrication techniques and can be easily integrated into complex microfluidic devices.

Pattern Formation and Coarse-Graining in Two-Dimensional Colloids Driven by Multiaxial Magnetic Fields

We study the pattern formation in a two-dimensional system of superparamagnetic colloids interacting via spatially coherent induced interactions driven by an external precessing magnetic field. On the pair level, upon changing the opening angle of the external field, the interactions smoothly vary from purely repulsive (opening angle equal to zero) to purely attractive (time-averaged pair interactions at an opening angle of 90°). In the experiments, we observed ordered hexagonal crystals at the repulsive end and coarsening frothlike structures for purely attractive interactions. In both of these limiting cases, the dense colloidal systems can be sufficiently accurately described by assuming pairwise additivity of the interaction potentials. However, for a range of intermediate angles, pronounced many-body depolarization effects compete with the direct induced interactions, resulting in inherently anisotropic effective interactions. Under such conditions, we observed the decay of hexagonal order with the concomitant formation of short chains and percolated networks of chains coexisting with free colloids. In order to describe and investigate these systems theoretically, we developed a coarse-grained model of a binary mixture of patchy and nonpatchy particles with the ratio of patchy and nonpatchy colloids as the order parameter. Combining genetic algorithms with Monte Carlo simulations, we optimized the model parameters and quantitatively reproduced the experimentally observed sequence of colloidal structures. The results offer new insight into the anisotropy induced by the many-body effects. At the same time, they allow for a very efficient description of the system by means of a pairwise-additive Hamiltonian, whereupon the original, one-component system features a two-component mixture of isotropic and patchy colloids.

Evanescent light scattering with magnetic colloids

We measure magnetic interactions between paramagnetic colloidal beads and an external magnetic field by using total internal reflection microscopy (TIRM). Our results demonstrate that TIRM can be applied to absorptive paramagnetic probe particles and thus extends the range of interaction types which can be addressed with this method. With our setup, we can detect magnetic forces on single superparamagnetic particle ranging from about 10 to 600 fN.