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                Acoustophoresis - A New Separation Concept
                   ultrasound separates chemical species
                Langley Research Center, Hampton, Virginia

                 courteously contributed by Mr. Jack Veach

    A concept under  development may expand the technology of chemical
    separation to includ ultrasonic-radiation pressure.

    When an ultrasonic wave passes through a medium, it carries energy
    and momentum; the loss of energy  by  the wave is accompanied by a
    transfer of momentum to the substance that carries the energy.

    For example, since  molecular  chains  have  different   nonlinear
    properties and absorb ultrasonic energy through such mechanisms as
    resonance relaxation, the  forces  on  these  chains depend on the
    frequency of the sound.

    Therefore, by selecting a specific frequency, one can "tune in" to
    a selected chemical property - acoustic  absorption - and separate
    chemical species (with different absorption coefficients) that may
    be impossible to separate by other means.

    For the separation of particles, the choice of acoustic wavelength
    will change the  acoustic scattering process and  thus  the  force
    imparted to the suspended particles.

    As the frequency  is varied from low to high, the larger particles
    (those with higher  scattering cross  section)  will  scatter  the
    sound first, followed by the smaller particles.

    Thus, the larger suspended particles will be swept from the liquid
    FIRST by the transferred momentum.

    As shown in the figure, the feed source supplies the liquid medium
    containing the desired species in mixture with other species.  The
    liquid is fed into the separation container.

    An ultrasonic transducer connected to an ultrasonic  driver  sends
    an ultrasonic wave  into  the  liquid,  exerting  on  the  desired
    species an acoustic-radiation force that DEPENDS ON THE ABSORPTION
    of the acoustic wave and on nonlinear interactions.

    Thus, the propagation  results   in  a  separation  based  on  the
    absorption (or scattering) of the acoustic wave.

    The separated species  are  removed sequentially  by  a  pump  and
    placed in different compartments (a,b,c,d).

    If the absorption  in  the different species is nearly equal, then
    acoustic streaming may mix the liquid, preventing separation.

    To minimize that effect by counteracting  the  streaming, a second
    transducer is driven by an antistreaming device.   In  effect, the
    second acoustic wave  can  be  tuned to a frequency different from




    that of the first, and to a different amplitude, thereby producing
    a high-resolution "shearing"  of  the  liquid  into  its  separate
    species.

    The acoustophoresis concept    can    utilize    not   only   bulk
    compressional waves but  also  surface  waves  or  boundary  waves
    between a solid (or liquid) container wall and the subject
    liquid.

    The free surface  of  the  subject liquid acts as a waveguide that
    contains the input acoustic energy.

    This work was done by Joseph S. Heyman of Langley Research Center.

    This invention is owned by NASA, and a patent application has been
    filed.  Inquiries concerning nonexclusive or exclusive license for
    its commercial development  should  be  addressed  to  the  Patent
    Counsel, Langley Research Center.  Refer to LAR-13388.

                 Taken from NASA Tech Briefs, January 1990