Characterization of oscillation modes in levitated droplets using image and non-image based techniques.

The dynamics of levitated liquid droplets can be used to measure their thermophysical properties by correlating the frequencies at which normal modes of oscillation most strongly resonate when subject to an external oscillatory force. In two preliminary works, it was shown via electrostatic levitation and processing of various metals and alloys that (1) the resonance of the first principal mode of oscillation (mode n = 2) can be used to accurately measure surface tension and (2) that so-called "higher-order resonance" of n = 3 is observable at a predictable frequency. It was also shown, in the context of future space-based experimentation on the Electrostatic Levitation Furnace (ELF), a setup on the International Space Station (ISS) operated by Japan Aerospace Exploration Agency (JAXA), that while the shadow array method in which droplet behavior is visualized would be challenging to identify the n = 3 resonance, the normal mode n = 4 was predicted to be more easily identifiable. In this short communication, experimental evidence of the first three principal modes of oscillation is provided using molten samples of Tin and Indium and it is subsequently shown that, as predicted, an "image-less" approach can be used to identify both n = 2 and n = 4 resonances in levitated liquid droplets. This suggests that the shadow array method may be satisfactorily used to obtain a self-consistent benchmark of thermophysical properties by comparing results from two successive even-mode natural frequencies.

Benchmarking surface tension measurement method using two oscillation modes in levitated liquid metals.

The Faraday forcing method in levitated liquid droplets has recently been introduced as a method for measuring surface tension using resonance. By subjecting an electrostatically levitated liquid metal droplet to a continuous, oscillatory, electric field, at a frequency nearing that of the droplet's first principal mode of oscillation (known as mode 2), the method was previously shown to determine surface tension of materials that would be particularly difficult to process by other means, e.g., liquid metals and alloys. It also offers distinct advantages in future work involving high viscosity samples because of the continuous forcing approach. This work presents (1) a benchmarking experimental method to measure surface tension by excitation of the second principal mode of oscillation (known as mode 3) in a levitated liquid droplet and (2) a more rigorous quantification of droplet excitation using a projection method. Surface tension measurements compare favorably to literature values for Zirconium, Inconel 625, and Rhodium, using both modes 2 and 3. Thus, this new method serves as a credible, self-consistent benchmarking technique for the measurement of surface tension.

Faraday forcing of high-temperature levitated liquid metal drops for the measurement of surface tension.

In this work, a method for the measurement of surface tension using continuous periodic forcing is presented. To reduce gravitational effects, samples are electrostatically levitated prior to forcing. The method, called Faraday forcing, is particularly well suited for fluids that require high temperature measurements such as liquid metals where conventional surface tension measurement methods are not possible. It offers distinct advantages over the conventional pulse-decay analysis method when the sample viscosity is high or the levitation feedback control system is noisy. In the current method, levitated drops are continuously translated about a mean position at a small, constant forcing amplitude over a range of frequencies. At a particular frequency in this range, the drop suddenly enters a state of resonance, which is confirmed by large executions of prolate/oblate deformations about the mean spherical shape. The arrival at this resonant condition is a signature that the parametric forcing frequency is equal to the drop's natural frequency, the latter being a known function of surface tension. A description of the experimental procedure is presented. A proof of concept is given using pure Zr and a Ti39.5Zr39.5Ni21 alloy as examples. The results compare favorably with accepted literature values obtained using the pulse-decay method.

Novel phenylalanine derived diamides as Factor XIa inhibitors.

The synthesis, structural activity relationships (SAR), and selectivity profile of a potent series of phenylalanine diamide FXIa inhibitors will be discussed. Exploration of P1 prime and P2 prime groups led to the discovery of compounds with high FXIa affinity, good potency in our clotting assay (aPPT), and high selectivity against a panel of relevant serine proteases as exemplified by compound 21. Compound 21 demonstrated good in vivo efficacy (EC50=2.8µM) in the rabbit electrically induced carotid arterial thrombosis model (ECAT).

Can an adverse density difference across a surface be stabilized by heating from above?

We investigate the possibility of stabilizing a Rayleigh-Taylor experiment by imposing a small upward temperature gradient. We find that if the two fluids have equal thermal conductivities nothing can be accomplished. If either thermal conductivity is much greater than the other, the small gradient is always stabilizing. If the thermal conductivities are of the same order of magnitude the small gradient can be stabilizing or destabilizing depending on the thermal expansion coefficients. We have used a Darcy model so that we can derive formulas and present a physical explanation of what we find.

Mixing generated by Faraday instability between miscible liquids.

The mixing between two miscible liquids subject to vertical vibrations is studied by way of experiments and a two-dimensional numerical model. The experimental setup consisted of a rectangular cell in which the lighter fluid was placed above the denser one. The diffuse interface was then visualized by a high-speed camera. After an initial period of diffusion growth, the interface becomes unstable with a defined wavelength, which depends on the amplitude and frequency of the acceleration. The waviness of the interfacial region disappears once the mixing of the two fluids takes place. The mixing is characterized by a mixing layer thickness (MLT) which measures the thickness of the mixed region between the two pure fluid domains. We find that the MLT shows an exponential growth with time due to an initial fingering that appears at the interface and then a growth with a defined slope after the mixing takes place. The MLT also increases with amplitude of driving motion. Experimentally determined MLTs are always greater than those determined by computations since the latter assume a jump discontinuity between the fluids prior to shaking, whereas in an experiment an initial diffusive region establishes itself prior to shaking and this is destabilizing. In addition, it is found from computations that mixing is best for low gravity levels at earlier times and high gravity levels at longer times. Explanations are advanced for each of these observations.

Onset of Rayleigh-Marangoni convection in a cylindrical annulus heated from below.

The onset of Rayleigh-Marangoni convection in a vertical annulus heated from below is investigated using linear stability analysis. The results of the present study also show the pattern transitions as a function of scaled gap width and aspect ratio. It is concluded that Marangoni convection can change the fluid pattern in an otherwise pure Rayleigh problem. It is also concluded that the gap width cannot significantly change the Marangoni effect as it is essentially the depth of liquid and Biot number that play a dominant role.