Cleaning and liquid contact with solids – Processes – Including application of electrical radiant or wave energy...
Reexamination Certificate
2000-01-21
2002-05-28
Stinson, Frankie L. (Department: 1746)
Cleaning and liquid contact with solids
Processes
Including application of electrical radiant or wave energy...
C134S184000, C134S902000, C310S322000, C422S020000, C422S128000
Reexamination Certificate
active
06395096
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to acoustic microcavitation, but more specifically, to methods and apparatuses for controlling acoustic coaxing induced microcavitation (ACIM) in a fluid medium to perform various industrial, scientific, or medical tasks.
Acoustic microcavitation, which is the inducement of micron or sub-micron size bubbles in a liquid or fluid medium that survive a few microseconds or less, is to be contrasted with ultrasonic, megasonic, and cyrogenic aerosol cleaning methods. Microcavitation has been used on a limited scale or conceived for use in microparticle or sub-nanometer particle detection in ultrapure liquids, submicron particle eviction from silicon wafers, deinking of recyclable paper, paint removal, surgical procedures, destructive and non-destruction testing and measuring, thin film processing applications, etc.
Previously, at least two transducers were required to initiate and maintain cavitation. Prior ACIM was induced using a low frequency, high intensity primary acoustic field and a higher frequency, low intensity coaxing acoustic field. To effect ACIM, the two fields were substantially simultaneously directed at a site of a workpiece or object. It was crucial that at least part of the high frequency acoustic waves in the fluid medium pass the desired ACIM site precisely when the tensile part of the low frequency waves was present at the site. In this arrangement, it sometimes became unwieldy to articulate two transducers of different frequencies to achieve the desired ACIM zone, stationary or moving, where the different acoustic fields were to be synchronized and collocated.
Therefore, a need has arisen to simplify ACIM apparatuses and techniques to make them more practical to apply to the various applications identified herein.
In general, cavitation is the formation of cavities or bubbles in a liquid where the ensuing bubble dynamics and energy concentration result in implosive collapse of bubbles that achieve unique and surprising results. In the design of mechanical systems, cavitation has known destructive effects and therefore, was avoided. Cavitation remains enigmatic today as it was when Lord Rayleigh first investigated cavitational erosion of propellers almost a century ago. Cavitation is a mature subject and an encylopedic collection of information on acoustic cavitation is compiled in “Acoustic Bubble” by Tim Leighton (1997). Hydrodynamic cavitation is discussed in “Cavitation and Multiphase Flow Phenomena” by Frederick Hammitt (1980). Whether induced acoustically or associated with hydrodynamic flows, the mechanics and effects of cavitation are essentially the same. Acoustic cavitation has was also exhaustively reviewed by Flynn (1964), Neppiras (1979), Apfel (1981) and Prosperetti (1986).
Consider, for example, a free bubble in the path of a sound wave. In response to the sound wave, the bubble expands and contracts, and the energy mechanically stored during expansion is released in a concentrated manner during implosive collapse of the bubble. Should the bubble grow to about two and a half times its nominal or equilibrium size during negative excursions of acoustic pressure, then during the following positive half cycle of pressure, its speed of collapse could become supersonic (Lauterborn, 1969) thereby releasing excess energy that catastrophically expands the bubble. Such almost single cycle violent events are called transient or inertial cavitation, and may explain the energetic manifestations of cavitation which, among other things, are useful for surface erosion or particle eviction.
Unlike dramatic bubble growth within a single acoustic cycle seen in transient or inertial cavitation, there exists a more gradual process, termed rectified diffusion. Under favorable conditions, a small bubble exposed to a continuous sound wave tends to grow in size if rectified diffusion is dominant. According to Henry's law, for a gas soluble in liquid, the equilibrium concentration of dissolved gas in the liquid is directly proportional to the partial pressure of the gas above the liquid surface, the constant of proportionality being a function of temperature. When the bubble expands, the pressure extant at the bubble's interior falls and gas diffuses into the bubble from the surrounding liquid. When the bubble contracts, the pressure in the interior increases and the gas diffuses into the solution of the surrounding liquid. The area available for diffusion, however, is larger in the expansion mode than in the contraction mode. Consequently, there is a net diffusion of the gas into the bubble from the surrounding liquid over a complete cycle, which causes bubble growth due to rectified diffusion.
However, a bubble can grow only up to a critical size—to a resonance radius determined by the frequency of the impressed sound wave. For small amplitude oscillations, a bubble acts like a simple linear oscillator of mass equal to the virtual mass of a pulsating sphere, which is three times the mass of displaced fluid. Stiffness is primarily given by the internal pressure of the bubble times the ratio of specific heats. Surface tension effects are, however, significant for small bubbles. Following Minnaert (1933) and ignoring surface tension, there is a simple relation for the resonance radius of air bubbles in water:
(Resonance radius in &mgr;m)×(insonification frequency in MHfz)=3.2
This relation is valid within 5% even for a bubble radius of about 10 &mgr;m. Bubble response becomes increasingly vigorous at the resonance radius, and is limited by damping mechanisms in the bubble environment—e.g., viscous damping, acoustic radiation damping, and thermal damping. A post-resonance bubble may exhibit nonlinear modes of oscillations or become transient if the applied acoustic pressure amplitude is adequately high.
The above discussion presupposes the presence of a free bubble in the path of a sound wave. Free bubbles, however, do not last long in a body of water. Larger ones are rapidly removed due to buoyancy and the smaller ones dissolve even in nearly saturated water. While a 10 &mgr;m air bubble rises in water at a terminal speed of 300 &mgr;m/s, it can survive for about five seconds before dissolving completely. Dissolution is driven essentially by the excess pressure inside the bubble due to the surface tension.
It is very difficult to cavitate clean liquids (Greenspan and Tschiegg, 1967). A pure liquid purged of particulate impurities and stored in a perfectly smooth container can attain its theoretical tensile strength before undergoing cavitation or fracture. Under ideal conditions, water can be as strong as aluminum. The tensile strength of water based on the homogeneous nucleation theory exceeds 1000 bars. In cavitation studies, tensile strength is often quoted in terms of negative pressures, and cavitation threshold is understood as the pressure amplitude at which the first occurrence of cavitation is detected. Observed strengths (thresholds) in practice, however, are very much lower, rarely exceeding a few bars for reasonably clean liquids. This is because there exists gas pockets within the liquid which provide the necessary seeding for cavitation to occur at lower pressures.
A gas or cavitation site is often stabilized in a crevice (Harvey et al., 1944), either in a container wall or on a fluid-borne particle. Incomplete wetting traps gas at the root of a sharp crevice, stabilizing it against dissolution. Unlike a free bubble, though, surface tension in this case acts on a meniscus which is concave towards the liquid. Over-pressuring the liquid for sufficient duration prior to insonification can force the meniscus further into the crevice thereby causing full wetting of the crevice, which then gives rise to increased cavitation thresholds.
Until recently most acoustically generated, cavitation employed for cleaning applications, primarily used standing waves generated in a bath of liquid in which objects to be cleaned were immersed. In such ultrasonic cleaners, acoustic frequencies used were typi
Kornakov M.
McIntyre Harbin & King
Stinson Frankie L.
Uncopiers, Inc.
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