“Painting with Numbers”:
Spraying and Air-Assisted Atomization
of Complex Fluids
Gareth H. McKinley (with Bavand Keshavarz)
Hatsopoulos Microfluids Laboratory
Department
of Mechanical Engineering,
MIT, 77 Massachusetts Avenue, Cambridge MA 02139
The extensional
rheological properties of dilute polymer solutions play a dominant role in many
industrially-important free-surface processes such as air-blast atomization and
rotary atomization. These high deformation rate processes are important in the
dispensing of diverse materials such as paints, fertilizer sprays and delivery
of airborne drugs. Similar hydrodynamic conditions also govern physiological
processes such as sneezing and airborne disease transmission. In this talk I
will explore the physics behind atomization of complex fluids using model polymer
solutions, several industrial paint formulations as well as human mucin. Although
the viscosity and surface tension of the polymeric fluids are close to those of
the underlying Newtonian solvent, both the mean droplet size and the droplet
size distribution change considerably. To understand why non-Newtonian fluids
differ so dramatically, one must recognize that the hydrodynamics of capillarity-driven
breakup and atomization are governed by an independent material function – the transient
extensional viscosity. To probe the
response of dilute polymeric solutions at realistic timescales and deformation
rates we use a recently developed instrument, the Rayleigh Ohnesorge Jet Elongational Rheometer (ROJER) [1]. Analyzing
the evolution in the jet diameter before break-up enables meaningful measurement
of fluid relaxation times down to values as small as 50 µs, and these values
can be directly correlated with differences in spray-size distributions. High-speed flow visualization images show that
this behavior arises from the non-linear dynamics close to the break-up point
which are dominated by an elasto-capillary force balance within the thinning
ligaments that sets the magnitude of the extensional viscosity in a complex
non-Newtonian fluid. This balance
between elasticity and capillarity on small length scales also leads to a very
broad and universal droplet size
distribution that can be described by a single parameter Gamma distribution [2].
We will also briefly explore how such
ideas can be extended to understand more advanced processes such as rotary
atomization that is used extensively in the automotive industry [3].
[1] Keshavarz B, Sharma V, Houze E.C., Koerner M.R., Moore J.R., Cotts P.M., Threlfall-Holmes, P., McKinley, G.H. Studying the Effects of Elongational Properties on Atomization of Weakly Viscoelastic Solutions using Rayleigh Ohnesorge Jetting Extensional Rheometry (ROJER). J. Non-Newt Fluid Mech. (2015), 222, 171-189. DOI: http://dx.doi.org/10.1016/j.jnnfm.2014.11.004
[2] Keshavarz, B., Houze, E.C., Moore, J.R., Koerner, M.R., McKinley, G.H. Ligament-Mediated Fragmentation of Viscoelastic Liquids, Phys. Rev. Lett. (2016), 117 154502. DOI: http:/dx.doi.org/10.1103/PhysRevLett.117.154502
[3] Keshavarz, B., Houze, E.C., Moore,
J.,R., Koerner, M.R., McKinley, G.H., Rotary Atomization of Newtonian and Viscoelastic
Fluids, Phys. Rev. Fluids, (2020), 5 033601. DOI: https://doi.org/10.1103/PhysRevFluids.5.033601
|
|
To register for the Complex
Fluids Seminar Series announcements by E-mail, please send a plain text
e-mail message to <mjrdomo (at) math.ubc.ca> with the following
content:
subscribe fluid-mech-seminar To unsubscribe, send: unsubscribe fluid-mech-seminar |
