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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