Jaroslav Ricka

Dynamic light scattering with single-mode and multimode receivers

Single-mode optical fibers provide the ideal receiver optics for dynamic light-scattering measurements. Theoretical analysis shows that with a single-mode fiber one can achieve a theoretical limit of 1 for the coherence factor while maintaining a high light-collection efficiency. In fact, the sensitivity of the single-mode receiver surpasses that of a classical two-pinhole setup with a coherence factor of 0.8 by a factor of 4 and the advantage increases rapidly when a still higher coherence factor is desired. In addition, a single-mode fiber receiver offers the possibility of working with an arbitrary large scattering volume and with an arbitrary working distance. All these features are also demonstrated experimentally by a remarkably simple apparatus that consists, essentially, of a commercial laser beam delivery assembly.

Mode-selective dynamic light scattering: theory versus experimental realization

We present a quantitative experimental comparison of fiber-based, single- and few-mode dynamic light scattering with the classical pinhole-detection optics. The recently presented theory of mode-selective dynamic light scattering [Appl. Opt. 32, 2860 (1993)] predicts a collection efficiency and a signal-tobaseline ratio superior to that of a classical pinhole setup. These predictions are confirmed by our experiments. Using single-mode optical fibers with different cutoff wavelengths and commercially available mechanical components, we have constructed a mode-selective detection optics in a simple and compact dynamic light-scattering spectrometer that permits an optimal compromise between signal intensity and dynamical resolution.

Diffusional deposition of colloidal particles: electrostatic interaction and size polydispersity effects

Abstract In the deposition process of charged nanosized particles onto oppositely charged planar substrates two effects are being discussed: (1) The maximum surface coverage sensitively depends on the repulsive electrostatic particle–particle interactions. The ionic strength of the particle suspension defines the magnitude of the electrostatic repulsion between the particles, which in turn modifies the maximum surface coverage. The maximum surface coverage decreases with decreasing ionic strength, a trend that can be well described by an effective hard-sphere model based on random sequential adsorption (RSA), where the effective radius is estimated from the repulsive screened Coulomb potential. Measured radial pair-distribution functions also reveal ideal hard-sphere behavior as compared to RSA simulations for monodisperse disks. The magnitude of the interaction, however, is overestimated with the simple electrostatic model. (2) Particle size polydispersity does also influence strongly the deposition process. Small particles may fill voids left by larger particles such that the maximum surface coverage increases significantly. The size distribution of the deposited particles on the surface changes with time, whereby the small particles are adsorbed preferentially. These trends are observed experimentally and confirmed by computer simulation.

Absolute measurement of molecular two-photon absorption cross-sections using a fluorescence saturation technique

We have developed a fluorescence saturation technique for accurate measurements of the absolute molecular two-photon absorption (TPA) cross-section of fluorescent dyes. We determine the TPA crosssection both from measurements at excitation intensities well below saturation onset (in the square power-law regime) and from data obtained near the onset of saturation. The two estimates have different sensitivities to potential sources of errors. Using the square power-law regime requires calibration of the overall collection efficiency of the detection channel, including the quantum yield of the dye. In the saturation regime, the two key requirements are a good knowledge of the excitation profile and an adequate model of the two-photon excitation transition. To fulfill the former requirement, we developed diagnostic tools to characterize the tightly focussed excitation beam. To satisfy the latter requirement, we included the correct polarization dependent averaging over molecular orientations in our model. We measured the TPA cross-section of Rhodamine B (RhB) and Rhodamine 6g (Rh6g) in methanol at 798 nm for linear and circular polarization. For RhB we observed excellent agreement between the TPA cross-section estimate < sigma2 > obtained from the square power-law regime and that obtained from the saturation regime, < sigma2 >(sat). For the case of linear polarization we found: < sigma2 > = 12 +/- 2 GM and < sigma2 >(sat) = 10.5 +/- 2 GM. For the case of circular polarization we obtained: < sigma2 > = 8.4+/-2 GM and < sigma2 >(sat) = 7.5+/-2 GM. The results obtained with linear polarization are in good agreement with previously published non-linear transmission data (delta= 2sigma = 20.4 GM at 800nm). For Rh6g the difference between < sigma2 > and < sigma2 >(sat) is larger, but still considerably smaller than the variance of sigma2 values found in the literature.

Femtosecond lasers in fluid-inclusion analysis: overcoming metastable phase states

We have developed a new method to overcome metastable phase states that prevent microthermometric measurements in fluid inclusions. We use tightly focused femtosecond laser pulses to induce bubble nucleation in one-phase (all liquid) inclusions, nucleation of salt crystals in supersaturated brines, and nucleation of ice and salt hydrates. The threshold laser intensity necessary for phase nucleation was determined to be of the order of 10 TW/cm 2 . To avoid potential damage to the fluid inclusion, the threshold for producing ablation of quartz visible under the microscope was also determined and found to be about 25 % higher than the threshold for halite nucleation and 100 % higher than that for bubble nucleation. The experimental setup allows us to induce phase nucleation in selected fluid inclusions at different temperatures under microscopic observation. Subsequent microthermometric measurements can be performed in the same setup, making it suitable for routine applications.

Polydispersity in dilute microemulsions: a consequence of the monomer-droplet equilibrium

Abstract Formation of droplets in a dilute water-in-oil microemulsion can be viewed as an aggregation of water and surfactant monomers. To describe this process we develop an explicit model which combines features of thermodynamics of mixed micelle formation and microemulsion stability. We find that at very low solute concentrations there are only monomers of water and surfactant present whereas at higher concentrations nearly monodisperse droplets form whose radii are dictated by the surface-to-volume ratio (Schulman radius). At intermediate concentrations the microemulsion coexists with excess water. In this range the polydispersity is very high and decreases monotonically with increasing solute concentration.

Functional imaging of mucociliary phenomena

We present a technique for the investigation of mucociliary phenomena on trachea explants under conditions resembling those in the respiratory tract. Using an enhanced reflection contrast, we detect simultaneously the wave-like modulation of the mucus surface by the underlying ciliary activity and the transport of particles embedded in the mucus layer. Digital recordings taken at a speed of 500 frames per second are analyzed by a set of refined data processing algorithms. The simultaneously extracted data include not only ciliary beat frequency and its surface distribution, but also space–time structure of the mucociliary wave field, wave velocity and mucus transport velocity. Furthermore, we propose the analysis of the space and time evolution of the phase of the mucociliary oscillations to be the most direct way to visualize the coordination of the cilia. In particular, this analysis indicates that the synchronization is restricted to patches with varying directions of wave propagation, but the transport direction is strongly correlated with the mean direction of waves. The capabilities of the technique and of the data-processing algorithms are documented by characteristic data obtained from mammalian and avine tracheae.

A Compact and Portable Deposition Chamber to Study Nanoparticles in Air-Exposed Tissue

BACKGROUND Epidemiological studies show that elevated levels of particulate matter in ambient air are highly correlated with respiratory and cardiovascular diseases. Atmospheric particles originate from a large number of sources and have a highly complex and variable composition. An assessment of their potential health risks and the identification of the most toxic particle sources would require a large number of investigations. Due to ethical and economic reasons, it is desirable to reduce the number of in vivo studies and to develop suitable in vitro systems for the investigation of cell-particle interactions. METHODS We present the design of a new particle deposition chamber in which aerosol particles are deposited onto cell cultures out of a continuous air flow. The chamber allows for a simultaneous exposure of 12 cell cultures. RESULTS Physiological conditions within the deposition chamber can be sustained constantly at 36-37°C and 90-95% relative humidity. Particle deposition within the chamber and especially on the cell cultures was determined in detail, showing that during a deposition time of 2 hr 8.4% (24% relative standard deviation) of particles with a mean diameter of 50 nm [mass median diameter of 100 nm (geometric standard deviation 1.7)] are deposited on the cell cultures, which is equal to 24-34% of all charged particles. The average well-to-well variability of particles deposited simultaneously in the 12 cell cultures during an experiment is 15.6% (24.7% relative standard deviation). CONCLUSIONS This particle deposition chamber is a new in vitro system to investigate realistic cell-particle interactions at physiological conditions, minimizing stress on the cell cultures other than from deposited particles. A detailed knowledge of particle deposition characteristics on the cell cultures allows evaluating reliable dose-response relationships. The compact and portable design of the deposition chamber allows for measurements at any particle sources of interest.

Compressibility Anomalies in Stretched Water and Their Interplay with Density Anomalies

Water keeps puzzling scientists because of its numerous properties which behave oppositely to those of usual liquids: for instance, water expands upon cooling, and liquid water is denser than ice. To explain this anomalous behavior, several theories have been proposed, with different predictions for the properties of supercooled water (liquid at conditions where ice is stable). However, discriminating between those theories with experiments has remained elusive because of spontaneous ice nucleation. Here we measure the sound velocity in liquid water stretched to negative pressure and derive an experimental equation of state, which reveals compressibility anomalies. We show by rigorous thermodynamic relations how these anomalies are intricately linked with the density anomaly. Some features we observe are necessary conditions for the validity of two theories of water.

Exploration of the phase diagram of liquid water in the low-temperature metastable region using synthetic fluid inclusions

We present new experimental data of the low-temperature metastable region of liquid water derived from high-density synthetic fluid inclusions (996-916 kg m-3) in quartz. Microthermometric measurements include: (i) prograde (upon heating) and retrograde (upon cooling) liquid-vapour homogenisation. We used single ultrashort laser pulses to stimulate vapour bubble nucleation in initially monophase liquid inclusions. Water densities were calculated based on prograde homogenisation temperatures using the IAPWS-95 formulation. We found retrograde liquid-vapour homogenisation temperatures in excellent agreement with IAPWS-95. (ii) Retrograde ice nucleation. Raman spectroscopy was used to determine the nucleation of ice in the absence of the vapour bubble. Our ice nucleation data in the doubly metastable region are inconsistent with the low-temperature trend of the spinodal predicted by IAPWS-95, as liquid water with a density of 921 kg m-3 remains in a homogeneous state during cooling down to a temperature of -30.5 °C, where it is transformed into ice whose density corresponds to zero pressure. (iii) Ice melting. Ice melting temperatures of up to 6.8 °C were measured in the absence of the vapour bubble, i.e. in the negative pressure region. (iv) Spontaneous retrograde and, for the first time, prograde vapour bubble nucleation. Prograde bubble nucleation occurred upon heating at temperatures above ice melting. The occurrence of prograde and retrograde vapour bubble nucleation in the same inclusions indicates a maximum of the bubble nucleation curve in the ϱ-T plane at around 40 °C. The new experimental data represent valuable benchmarks to evaluate and further improve theoretical models describing the p-V-T properties of metastable water in the low-temperature region.