Paulo E. Arratia
University of Pennsylvania
Chemical and Biochemical Engineer
Fellow, Elected 2025
Prof. Arratia’s research focuses on understanding the mechanisms governing the rheology and flow behavior of complex fluids in physical and
biological systems. In dense suspensions, he has shown the emergence of a “reversible plastic”
regime near yielding in which a sample undergoing cyclic shear recovers its initial configuration
but through a different, dissipative path. He has developed relationships between sample
microstructure, bulk rheology, and memory by exploring the utility of order parameters to identify
structural signatures associated with particle rearrangements. Prof. Arratia showed that the flow of
a viscoelastic fluid in a cross-slot geometry give rise to two different instabilities even at low Re:
(i) a (bi-stable) symmetric-breaking instability, in which the flow remains, and (ii) an unstable
flow instability as the Weissenberg number is increased beyond a critical value. In simple parallel
shear flows, such as channels and pipes, he has demonstrated that viscoelasticity can drive the flow
unstable via a subcritical instability. This is analogous to the Newtonian laminar-turbulence
scenario in pipe flows except that the governing parameter is the Weissenberg number.
In biological systems, Prof. Arratia has pioneered experimental studies on the effects of fluid
rheology on the motility behavior of living microorganisms. He has shown that fluid elasticity, as
well as other fluid rheological properties, can significantly affect the propulsion speed, kinematics,
and biomechanics of worm nematodes, bacteria, and algae. He has shown how fluid elasticity can
significantly hinder the swimming speed of an undulatory swimmer but enhance the speed of
flagellated bacteria; his experiments have show that one can use fluid elasticity and shear-thinning
to break the so-called “scallop theorem” and obtain net motion of externally actuated particles. His
research group has recently shown that shear-thinning viscosity can enhance the rheotactic
behavior of bacteria. In physiological flows, Prof. Arratia has shown that blood plasma is not
Newtonian under strong extensional flows, that fluid elastic stresses can order trains of red blood
cells in straight channels, and that saliva from xerostomia patients has significantly greater
extensional viscosity relative to healthy patients.
Finally, Prof. Arratia has long served The Society of Rheology, most recently by editing the
Rheology Bulletin publication for its members.