Physikingenieurwesen (PHY)
Silicon microprotrusions with tailored chirality enabled by direct femtosecond laser ablation
(2020)
Here, we report on formation of nanoprotrusions on the surface of a bulk crystalline silicon wafer under femtosecond-laser ablation with a donut-shaped laser beam. By breaking circular symmetry of the irradiating donut-shaped fs-pulse beam, a switch in geometry of the formed surface nanoprotrusions from regular to chiral was demonstrated. The chirality of the obtained Si nanostructures was promoted with an asymmetry degree of the laser beam. An uneven helical flow of laser-melted Si caused by asymmetry of the initial intensity and temperature pattern on the laser-irradiated Si surface explains this phenomenon. Chirality of the formed protrusions was confirmed by visualizing cross-sectional cuts produced by focused ion beam milling as well as Raman activity of these structures probed by circularly polarized light with opposite handedness. Our results open a pathway towards easy-to-implement inexpensive …
The potential use of an EMG-20 mass-spectrometric gas analyzer for process control and safe operation and environmental monitoring in metallurgy (oxygen converter process) is considered. The EMG-20 parameters are adequate for rapid, automatic on-line analysis of metallurgical waste gases. A multichannel system of sampling. sample preparation, and transportation of sampled gases for analysis is proposed.
Generation of microfluidic flow using an optically assembled and magnetically driven microrotor
(2014)
In this manuscript, a new approach in surface plasmon resonance microscopy is presented. The method provides optical real-time detection of single nanoparticles on surfaces. The potential of the method is demonstrated recording spherical dielectric particles as small as 40 nm in diameter and single HIV virus-like particles having diameters of similar to 100 nm both immobilized on functionalized surfaces. The surface plasmon resonance signal in the binding spots was found to be almost linearly proportional to the size of the particles and, therefore, surpasses the intensity of Mie scattering on spherical particle (dependence similar to r(-6)) by orders of magnitude for small objects. The physical reason leading to this strong effect is discussed.
Current self-sustained oscillations in a Townsend discharge are studied analytically. The proposed method for solving simple equations is applicable even when the Townsend coefficient of electron reproduction g (the main factor governing the oscillations) cannot be determined theoretically. The coefficient g is related to the discharge current-voltage characteristic, which can be obtained experimentally or from numerical simulations. Self-oscillating solutions (limit cycles) are found under various conditions. The mechanisms governing the excitation and stabilization of the solutions obtained are interpreted. It is shown that the waveform of the undamped oscillations may change significantly (the current peaks are smoothed, and the oscillation period decreases) when a weak constant cathode current, which is presumably related to the emission caused by slowly diffusing metastable molecules, is included in the equations.
Self-sustained current oscillations in a Townsend discharge that is excited in a short plane discharge gap and is uniform along the electrode surface are investigated experimentally. A thin semiconductor plane serves as a discharge cathode, a ballast resistor, and an additional capacitor in the dc power supply circuit. The domain of existence and the frequency and waveform, of oscillations are determined over a wide pressure range.
The dewetting of liquid filaments in linear grooves of a triangular cross section is studied experimentally and theoretically. Homogeneous filaments of glassy polystyrene (PS) are prepared in triangular grooves in a nonequilibrium state. At elevated temperatures, the molten PS restores its material contact angle with the substrate. Liquid filaments with a convex liquid-vapor interface decay into isolated droplets with a characteristic spacing depending on the wedge geometry, wettability, and filament width. This instability is driven by the interplay of local filament width and Laplace pressure and constitutes a wide class of ID instabilities that also include the Rayleigh -Plateau instability as a special case. Our results show an accurately exponential buildup of the instability, suggesting that fluctuations have a minor influence in our system.
In this paper a novel concept of ablation cell for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is presented. Suppression of the turbulence in the flush gas flow in the ablation region reduces the wash-out time of the ablation cell considerably. An ablation chamber which enables ICP-MS pulse duration down to several ten milliseconds has been designed. Dependence of the ICP-MS peak amplitude, width, and shape on the gas flow parameters is studied experimentally for aerosol ablated under laminar and turbulent conditions. Experiments demonstrate that the ICP-MS peak becomes sharper and the amplitude of the signal grows as the turbulence in the ablation cell is suppressed. Furthermore, the possibility of the LA-ICP-MS analysis with a sampling rate of more than 10 Hz has been demonstrated. Express in-depth profiling in the new ablation cell is demonstrated on examples of an Al -Zn multilayer structure and an industrial Mg -Zn coating.