• Engineers describe how fluid suspensions

    From ScienceDaily@1:317/3 to All on Thu Apr 7 22:30:40 2022
    Engineers describe how fluid suspensions exhibit different behaviors at different scales

    Date:
    April 7, 2022
    Source:
    University of California - Santa Barbara
    Summary:
    Honey is already a pretty thick liquid, but let it begin to
    crystalize and it can become downright clumpy. The sugar crystals in
    suspension seem to increase its viscosity. This phenomenon occurs
    throughout the natural and constructed world: From mudflows to
    paint, suspensions of particles tend to behave like viscous fluids.



    FULL STORY ========================================================================== Honey is already a pretty thick liquid, but let it begin to crystalize
    and it can become downright clumpy. The sugar crystals in suspension
    seem to increase its viscosity. This phenomenon occurs throughout the
    natural and constructed world: From mudflows to paint, suspensions of
    particles tend to behave like viscous fluids.


    ========================================================================== Engineers use this to their advantage by modeling the macroscopic
    properties of a suspension based on the size and concentration of its particles. However, this approximation breaks down at a certain scale. UC
    Santa Barbara's Virgile Thie'venaz and Alban Sauret sought to determine
    when and how.

    They discovered that particles don't spread out evenly once a suspension
    drops below a certain length scale, such as when the fluid pinches in
    to form the neck of a droplet. Eventually, there will be a thin region
    without any particles that behave like a pure liquid. The findings,
    published in the Proceedings of the National Academy of Sciences,
    highlight the limit of approximations and have many potential applications
    in industrial settings.

    Viscosity quantifies the internal friction between layers of a fluid. In
    a viscous liquid, one layer exerts more drag on its neighbor, producing a thicker fluid that is more resistant to deformation and flow. Particles
    in a suspension behave in a similar manner. A particle is more likely
    to move when its neighbors move, which increases the fluid's effective viscosity. Higher concentrations bring particles closer together,
    strengthening the effect. "So as long as you look at the suspension
    from far away, it's just a more viscous liquid," explained Thie'venaz,
    a postdoctoral researcher in the mechanical engineering department.

    In droplet experiments, Thie'venaz and Sauret observed that suspensions
    will stretch like a viscous liquid down to a certain thickness,
    after which it becomes possible to pull the particles away from each
    other. This creates regions with varying concentrations that behave differently. Eventually, a region won't contain any particles and will
    act like a pure fluid. After this, the effective viscosity simplifies
    to that of the pure liquid.

    Engineers have compiled a lot of data to calibrate the effective
    viscosity of suspensions with particle size and concentration at large
    scales. Thie'venaz and Sauret's challenge was to figure out at what scale
    the approximations classically used to model suspensions began to unravel.

    With more experimentation, the authors determined that this threshold
    also varies with particle size and concentration. A suspension will
    transition from acting like a viscous fluid to behaving more like a heterogeneous mixture at scales on par with the size of the particles.

    Interestingly, smaller particles seem to have a proportionately stronger effect. "Relative to the particle size, the threshold is much larger
    for small particles at a given concentration," said Sauret, an assistant professor of mechanical engineering.

    For instance, a suspension with a 30% concentration of 140 micrometer
    particles may behave smoothly down to scales of 600mym, or about four
    times the particles' diameter. But a suspension with 20mym particles at
    the same concentration may display this effect down to 250mym: a smaller
    scale overall, but more than 12 times the particles' diameter.

    Predicting the behavior of a suspension has major applications in manufacturing. A process may require manipulating films or creating tiny droplets, and technicians need to be able to predict the properties of
    these systems. For dip-coated parts, properly manipulating the particles
    in a film can be the difference between a finished product and an absolute mess, Sauret explained.

    Spray coating provides an even clearer illustration of this phenomenon. A
    pure liquid, like a varnish, will behave differently than a suspension,
    like paint, when spray coating a product. When compared to a pure liquid
    with the same effective viscosity, a suspension will break up earlier
    with fewer, larger droplets. The researchers' next task is determining
    how the number and size of droplets depends on parameters like speed,
    particle concentration and particle size.

    Approximating suspensions as viscous liquids works well, but only at
    certain scales. "At some point that's going to fail," Sauret said. "And
    we need to be able to say, 'at this point you cannot use this approach,
    and instead you need to use a different method."

    ========================================================================== Story Source: Materials provided by
    University_of_California_-_Santa_Barbara. Original written by Harrison
    Tasoff. Note: Content may be edited for style and length.


    ========================================================================== Related Multimedia:
    * A_droplet_of_silicone_oil_pinches_of_from_fluids_with_different
    concentrations_of_140_mym_particles ========================================================================== Journal Reference:
    1. Virgile Thie'venaz, Alban Sauret. The onset of heterogeneity in the
    pinch-off of suspension drops. Proceedings of the National Academy
    of Sciences, 2022; 119 (13) DOI: 10.1073/pnas.2120893119 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/04/220407141840.htm

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