System and method for measuring surface tension of a levitated sample
Inventors
Brosius, Nevin B. • NARAYANAN, RANGANATHAN • SanSoucie, Michael P.
Assignees
National Aeronautics and Space Administration NASA • University of Florida Research Foundation Inc
Publication Number
US-12135269-B2
Publication Date
2024-11-05
Expiration Date
2041-10-19
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Abstract
Various examples are provided related to measuring surface tension. In one example, a method includes levitating a sample using electrostatic levitation; applying a signal to at least one electrode to excite the sample into a n=3 mode of oscillation; capturing images of the sample with a respective image being associated with a particular frequency that is applied to the sample when the respective image is captured; quantifying sample resonance using a projection method of Legendre polynomials based on the plurality of images; and determining a measured resonance frequency of the sample by an analysis of the sample resonance. The sample can be levitated using a feedback-controlled voltage and the applied signal can be swept over a range of frequencies. A system including electrodes, a position sensor, a camera device, and at least one computing device can be used to carry out the method.
Core Innovation
The present disclosure relates to systems and methods for measuring surface tension of levitated liquid samples using electrostatic levitation and continuous forcing. The method involves levitating a sample using electrostatic levitation achieved by applying a feedback-controlled voltage to electrodes, which balances forces such as gravity. The sample, which can be melted into a liquid state while levitated, is excited into specific oscillation modes (notably n=3 mode) by applying a sinusoidal signal to one or more electrodes. This signal is swept over a frequency range that includes a predicted resonance frequency determined by theories such as Rayleigh's.
During this frequency sweep, high-speed images of the sample are captured, with each image associated with the frequency applied at the time of capture. The images are analyzed using a projection method of Legendre polynomials to quantify the resonance amplitude of the sample at different frequencies. The frequency that results in maximum deviation from the sample's spherical shape is identified as the measured resonance frequency, which correlates with the sample's surface tension. The system enables excitation and measurement of various oscillation modes (n=2, n=3, n=4), allowing determination of the surface tension based on the resonance frequencies of these modes, individually or averaged.
The problem addressed is the difficulty in accurately measuring thermophysical properties such as surface tension for high-temperature, reactive liquid materials using conventional methods. Traditional levitation techniques face challenges such as contamination, viscous effects, shape deviations, and complexities in analyzing pulse-decay oscillations especially for high-viscosity samples. Electrostatic levitation offers advantages including greater sphericity of the levitated droplet and non-contact measurement capabilities. The disclosure improves upon previous surface tension measurement methods by introducing refined frequency sweeps at multiple oscillation modes and more robust resonance quantification using Legendre polynomial projections, overcoming limitations of pulse-decay and indirect methods.
Claims Coverage
The patent includes independent claims directed to a method and a system for measuring surface tension of a levitated sample, featuring key inventive aspects related to excitation, imaging, resonance quantification, and surface tension determination.
Measurement of surface tension using excitation of n=3 oscillation mode
A method applying a frequency-swept signal to an electrode to excite a levitated sample into the n=3 mode of oscillation, capturing a plurality of images each associated with an applied frequency, quantifying sample resonance using a projection method of Legendre polynomials, and determining the measured resonance frequency based on maximum response deviation.
Multi-mode surface tension determination
Determining surface tension based on measured resonance frequencies at multiple oscillation modes including n=2, n=3, and n=4 modes. Surface tension can be determined for each mode at least partly using the respective measured resonance frequency and averaged for enhanced accuracy.
Use of predicted resonance frequency derived from physical parameters
Utilizing a predetermined resonance frequency predicted by a formula relating frequency to spherical radius, density, and surface tension of the sample to guide the frequency sweep range during excitation of the sample.
Sinusoidal signal excitation and liquid sample preparation
Applying a sinusoidal signal with fixed amplitude swept over the range of frequencies for excitation and melting the levitated sample into a liquid state using a heating laser to maintain substantially constant temperature during measurement.
System for electrostatic levitation and resonance measurement
A system comprising multiple electrodes, position sensors, a camera device, and at least one computing device executing instructions to levitate the sample using feedback-controlled voltages, apply excitation signals to induce n=3 mode oscillation, capture and associate images with applied frequencies, quantify sample resonance employing Legendre polynomial projections, and determine the resonance frequency.
Additional system features
Controlled melting of the sample using a heating laser with temperature monitoring, use of ultraviolet lamp for imparting charge, and operation of position sensors and computing devices to maintain levitation and perform image analysis for resonance measurement.
The claims cover a method and system for measuring surface tension by exciting multiple oscillation modes (especially n=3) of an electrostatically levitated sample, capturing synchronized imaging, employing Legendre polynomial based resonance quantification, and determining surface tension from measured resonance frequencies. The claims emphasize frequency-swept sinusoidal excitation, predicted resonance frequency guidance, and feedback-controlled levitation.
Stated Advantages
The method allows accurate surface tension measurement of high viscosity samples without difficulties found in pulse-decay techniques.
Electrostatic levitation offers improved sphericity compared to other levitation methods, enhancing measurement reliability.
The projection method of Legendre polynomials provides robust and general quantification of resonance applicable to multiple oscillation modes, including higher modes beyond what prior diameter-based methods allow.
The method overcomes challenges like control system perturbations and nonlinear mode interactions that affect conventional pulse-decay surface tension measurements.
The technique enables self-consistent benchmarking across multiple oscillation modes, improving measurement accuracy and confidence.
Documented Applications
Measurement of surface tension of liquid metals and alloys, including materials like Zirconium, Inconel 625, and Rhodium at high temperatures.
Measurement of thermophysical properties of materials difficult to process by other means, such as high-temperature or reactive liquids.
Application in containerless processing environments, particularly for research requiring non-contact measurement of surface tension.
Use in space exploration and manufacturing contexts, including experiments on the International Space Station leveraging low gravity conditions.
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