N. Arnold

Röntgen’s electrode-free elastomer actuators without electromechanical pull-in instability

Electrical actuators made from films of dielectric elastomers coated on both sides with stretchable electrodes may potentially be applied in microrobotics, tactile and haptic interfaces, as well as in adaptive optical elements. Such actuators with compliant electrodes are sensitive to the pull-in electromechanical instability, limiting operational voltages and attainable deformations. Electrode-free actuators driven by sprayed-on electrical charges were first studied by Röntgen in 1880. They withstand much higher voltages and deformations and allow for electrically clamped (charge-controlled) thermodynamic states preventing electromechanical instabilities. The absence of electrodes allows for direct optical monitoring of the actuated elastomer, as well as for designing new 3D actuator configurations and adaptive optical elements.

Capacitive extensometry for transient strain analysis of dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) are promising structural units for artificial muscles and robotic elements. Understanding the safe and failure mode regimes of such DEAs is essential for controlling the actuator. We develop an electrical characterization technique for obtaining information on the transient strain in the actuator and analyze the behavior of the actuator in safe and failure operation regimes, in particular in the pull-in instability mode. Additionally, the technique allows the strain-dependent measurement of the electrode resistance. The current measurement based technique can be also applied for actuator control with feedback loops.

Spectral and directional reshaping of fluorescence in large area self-assembled plasmonic-photonic crystals.

Spectral and directional reshaping of fluorescence from dye molecules embedded in self-assembled hybrid plasmonic-photonic crystals has been examined. The hybrid crystals comprise two-dimensional hexagonal arrays of dye-doped dielectric nanospheres, capped with silver semishells. Comparing the reshaped fluorescence spectra with measured transmission/reflection spectra and numerical calculations reveals that the spectral and directional reshaping of fluorescence is the result of its coupling to photonic crystal Bloch modes and to void plasmons localized inside the silver caps.

Single-step fabrication of silicon-cone arrays

A regular lattice of SiO2 microspheres on a quartz support is used as a microlens system for laser-induced single-step fabrication of arrays of silicon cones on a (100) Si surface. The experiments were performed with single-pulse 248 nm KrF laser radiation.

Theoretical description of dry laser cleaning

Dry laser cleaning (DLC) is considered as an escape from an adhesion potential under the forces induced by thermal expansion. Important temporal and spatial scales are: period of small oscillations τ0, and equilibrium deformation h0. Possible cleaning regimes are discussed. With laser pulse duration τ τ0 (small particles) cleaning proceeds in the inefficient inertial force regime. If the fronts of the laser pulse are steep enough, tf⪡τ0, cleaning occurs when kinetic energy of the particle exceeds that of adhesion. Utilization of resonance effects by modulation of laser pulse or employing the train of pulses and influence of damping on these regimes are discussed. Effects of the near field focusing by dielectric spheres and decrease in cleaning threshold due to three-dimensional (3D) effects are analyzed and compared with the experimental results for SiO2 particles on Si surface.

In situ Analysis of Metal Melts in Metallurgic Vacuum Devices by Laser-Induced Breakdown Spectroscopy

We report on rapid in situ analysis of liquid metal melts under reduced ambient pressure by laser-induced breakdown spectroscopy (LIBS) using a transportable system. LIBS denotes a method in which characteristic optical emission line intensities of excited species in laser-generated plasma plumes are used for a quantitative chemical analysis of target materials. It is a fast, noncontact method that can be carried out under various atmospheric conditions, allowing large working distances between the sample under investigation and the detection system. For these reasons, LIBS is applicable in particular for process control in metallurgy under reduced ambient pressure. This was demonstrated for two types of vacuum devices under production conditions at a steel mill. The results of these experiments, including calibration curves for Cr, Ni, and Mg in liquid steel, are presented. The influence of variations in the ambient pressure on the results of the LIBS analysis is discussed within the frame of a generalized shock-wave model for the expansion of the laser-induced plasma plume.

Single-pulse ultraviolet laser-induced surface modification and ablation of polyimide

Single-pulse laser ablation of polyimide was investigated by using focused UV-Ar+-laser radiation (λ≈302 nm, 140 ns⩽τl⩽50 ms) and atomic force microscopy. The results clearly demonstrate that the ablation rates do not depend on the total dose only, but depend as well on the duration of the laser pulses, τl. The experimental results can be interpreted almost quantitatively on the basis of a purely thermal model.

A fast quantitative modelling of ns laser ablation based on non-stationary averaging technique

A semi-analytical approach to a quantitative analysis of thermal ns laser ablation is presented. The nonlinear heat equation is reduced to three ordinary differential equations for the surface temperature, spatial width of the enthalpy distribution, and the ablated depth. Due to its speed and flexibility, the method provides a convenient tool for the fast . analysis of experimental data. The influence of different factors on ablation curves ablated depth h vs. fluence f is . studied. Analytical formulas for threshold fluence f and h f dependences are discussed. The ablation curves reveal three th regions of fluence: Arrhenius, linear, and the screening region. Small vaporization enthalpy results in a sub-linear but faster . than logarithmic h f dependence. Weakly-absorbing materials may exhibit two different ablation regimes - without or with ablation of the heated subsurface layer. The method is applied to the analysis of the single pulse ablation of polyimide. The single thermal evaporation energy f 1.51 eV describes satisfactory the ablation at 248, 308 and 351 nm wavelengths. q 1998 Elsevier Science B.V.

The role of excited species in ultraviolet-laser materials ablation III. Non-stationary ablation of organic polymers

The role of non-stationary effects in nano-second ultraviolet (UV) excimer-laser ablation of organic polymers is discussed. The model includes reversible changes in absorption related to darkening and bleaching effects. Comparison of calculations and experimental data for polyimide demonstrates that the photophysical model describes the ablation kinetics quite well.

Bulk model of laser ablation of polymers

Abstract The model of laser ablation driven by a bulk photothermal reaction is considered. Stationary wave solutions are obtained and analyzed. In the case of a simple photothermal bond-breaking reaction, it is possible to reduce this bulk ablation model to the surface evaporation model with some apparent activation energy. A more complicated model of laser ablation, taking into consideration a photothermal modification of broken bonds within the bulk of material, is also studied. The existence of the second threshold related to multiple-shot and long pulse irradiation experiments has been demonstrated. It is shown that the effect of modification on the ablation kinetics qualitatively depends on the interrelation between activation energies of two considered bulk reactions.