Yuren Tian

Deformation and location of an acoustically levitated liquid drop

A theoretical method to determine the location and static deformation of an acoustically levitated liquid drop in air is presented. The interaction between drop and sound field, involving nonspherical acoustic scattering and drop volume variation, is the crux of this analysis, which is valid for drops with aspect ratio as large as 2. Numerical calculations are presented of drop shape and location as functions of sound pressure, surface tension, and drop volume in both gravity (1g) and gravity‐free (0g) environments. The numerical results agree well with our experimental measurements and those of other researchers.A theoretical method to determine the location and static deformation of an acoustically levitated liquid drop in air is presented. The interaction between drop and sound field, involving nonspherical acoustic scattering and drop volume variation, is the crux of this analysis, which is valid for drops with aspect ratio as large as 2. Numerical calculations are presented of drop shape and location as functions of sound pressure, surface tension, and drop volume in both gravity (1g) and gravity‐free (0g) environments. The numerical results agree well with our experimental measurements and those of other researchers.

INVESTIGATIONS OF LIQUID SURFACE RHEOLOGY OF SURFACTANT SOLUTIONS BY DROPLET SHAPE OSCILLATIONS : THEORY

A theoretical analysis is presented for the shape oscillations of drops suspended in air. For drops of surfactant solution, the oscillation frequency is basically determined by the surface tension; the free‐damping constant depends on the surface viscoelasticities. Different types of surfactant mass transfer at the droplet surface produce different surface rheological behaviors. Analytical approximate solutions for free‐oscillation frequency and damping constant are derived by a perturbation method as functions of surface compositional elasticity, surface dilatational viscosity, and surface shear viscosity. These solutions are verified with exact numerical solutions. The existence of a second oscillation mode due to surface elasticity is illustrated. The phase relationships between the external driving forces and the droplet shapes for forced oscillations are discussed. It is found that at the 90° phase shift, the driving frequency and the slope of the phase diagram are equivalent to the free‐oscillation ...

Direct observation of microbubble oscillations

The study of microbubble oscillations is important for the understanding of sonoluminescence phenomena. Bubble dynamic behavior is conventionally detected with light scattering techniques. This method generally gives little information about the shapes of a bubble during its oscillation. In order to observe the bubble shapes, a direct imaging system was set up. The shape of a bubble levitated in a liquid is magnified and displayed on a TV screen. The bubble is illuminated with an LED lamp which is strobed at a frequency slightly different from the driving sound field. This technique can slow the moving image of the bubble, allowing one to observe the shape of a bubble oscillating between 5 to 100 μm in diameter. Experiments show that when a bubble oscillates with sonoluminescence, it keeps a closely spherical shape during the entire oscillation period. However, with the increase of gas concentration in the host liquid, sonoluminescence disappears and higher mode shapes are developed during the bubble’s sh...

A novel multiple drop levitator for the study of drop arrays

Abstract A novel acousto-electric levitator has been developed for studying various phenomena associated with drop arrays. In order to stably levitate a group of drops, charged drops are generated in an acoustic field. The acoustic radiation force draws charged drops toward a certain location; the electrostatic repulsion among the drops prevents droplet coalescence and produces a nearly uniformly spaced drop array. Measurements with single droplets allow for tests of the influence of the acoustic field on the droplet evaporation rates. Interference effects among drops are observed by measuring their evaporation rates in a droplet array. Measured rates in these preliminary tests of the new apparatus are slightly lower than predicted theoretically.

Numerical simulation of superoscillations of a Triton-bearing drop in microgravity

Large-amplitude nonlinear oscillations of an axially symmetric water drop of volume 7.33 cm 3 , initial aspect ratio 3.4, with surfactant Triton X-100 of 1.4 x 10 -4 g ml -1 (1 CMC), in microgravity are compared with predictions of the boundary-integral method. The small shear viscosity of the bulk phase, as well as the surface dilatational viscosity and surface shear viscosity are considered. When a very specific set of material properties is assumed, numerical simulations of the drop oscillations are in good agreement with the experimental results of drop oscillations measured in space during the second United States Microgravity Laboratory, USML-2

Studies of acousto-electrically levitated drop and particle clusters and arrays

Research into the behavior of clusters and arrays of fluid or solid particles is made possible by acousto-electric levitation in air and charging of “seed” droplets (10–30 μm in diameter) which come together in 2-D clusters (with up to 300 droplets). Such clusters condense into larger drops (e.g., 50–300 μm) which form uniformly spaced 2-D arrays of monodispersed drops. Similar behavior has been observed for charged solid particles. This research has applications to studies of drop evaporation, combustion, nucleation, and materials synthesis.

Frequency of quadrupole mode oscillations of spheroidal liquid drops in an acoustic field.

Ultrasonic levitation of liquid drops in air has been used [Y. Tian et al., J. Colloid Interface Sci. (submitted)] as a technique for measurement of the rheological properties (viscosity, interfacial tension, elasticity) of liquid surfaces and interfaces. The unavoidable deformation and asymmetry caused by the intensity of the sound field needed to levitate millimeter‐sized water drops in air in a 1‐g environment gives rise to a change in the frequency of quadrupole mode shape oscillations from the spherical value. This complicates measurements of rheological properties. Thus the frequency change due to the interaction with the sound field has been modeled. Using a unique conservation of energy approach, the shift due strictly to geometrical deformation of arbitrary origin is derived. Then realistic values of deformation and corresponding radiation force for typical cases are considered, modifying the previously obtained frequency shift values. Results are compared to experimental values obtained for mill...

The study of evaporation of multicomponent drops using the acoustoelectric levitator

The evaporation of single drops and drop arrays of multicomponent liquids is investigated using a novel acoustoelectric levitator. By simultaneous levitation and charging of the drops, 2D drop arrays with the drop diameters varying from 0.2 to 1 mm can be formed. Also by controlling both the acoustic and electric field intensities, arrays with different drop sizes and spaces between drops can be created. This allows us to study the interference effects among drops in an evaporating dense spray. Mixtures of different liquids such as alkanes (heptane, dodecane, and hexadecane) are studied. The evaporation rates of those mixtures with different molar ratios of components and different numbers of drops in an array are measured. The drop size and spacing are carefully controlled and varied. A video camera is used to record the drop size change with time, and the diameters of drops are calculated from the pictures. The results are compared with a theoretical analysis based on the diffusion‐controlled evaporatio...

Acousto‐electric generation of charged clusters and drop arrays

Novel, compact instrumentation for studying the behavior of drop arrays and sprays, and of clusters of drops, permits fundamental research into the behavior of reacting and nonreacting fluid species. The new capability is made possible by simultaneous levitation and charging of ‘‘seed’’ droplets (10–30 m in diameter) which come together in 2‐D clusters. These clusters are interesting in their own right because of their crystalline and quasi‐crystalline forms, which depend on the acoustic and electric field parameters. By varying the electric and acoustic field intensities, one can cause the clusters to condense into larger drops (e.g., 50–300 m) which, because of their charge, form uniformly spaced 2‐D arrays of monodispersed drops. One or more layers of these 2‐D arrays can form in the acoustic standing wave. Such a configuration permits a wide range of fundamental studies of drop evaporation, combustion, and nucleation. The drops can be single or multicomponent. Therefore, fundamental materials studies ...

Oscillations of a water drop with surfactant Triton X‐100 in zero gravity

Large amplitude nonlinear oscillations of an axially symmetric water drop of 2.51‐cm diameter, initial aspect ratio 3.4, with surfactant Triton X‐100 of 0.5 mM, in zero gravity are studied by a boundary element method. Included in the analysis are surface‐shear and dilatational viscos‐ ity, under the assumption that the shear viscosity of the bulk phase is small. Numerical simulations of the drop oscillations are in good agreement with the experimental results of drop oscillations measured in space during the second United States Microgravity Laboratory, USML‐2. The evolution of the drop oscillations for both experiment and simulation is given. The simulation provides predictions for the values of both surface dilatational viscosity and shear viscosity of 0.20 sp and 0.10 sp, respectively. In addition, with the simulated data, the damping constants, frequencies, and decomposed oscillation modes are computed. [Work supported by NASA through JPL, Contract No. 958722.]