About

Paulo E. Arratia is a Professor and the Eduardo D. Glandt Distinguished Scholar in the Department of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania. He serves as the Faculty Director of the Undergraduate Research Office at Penn Engineering. His primary academic affiliation is with the Chemical and Biomolecular Engineering department, and he is involved in research activities within the Mechanical Engineering and Applied Mechanics department. Further details about his specific research focus, background, and key contributions are not provided in the page text.

Research topics

• Materials science
• Physics
• Mechanics
• Composite material
• Nanotechnology
• Chemistry
• Chemical engineering
• Thermodynamics
• Geology
• Chemical physics
• Computer Science
• Engineering
• Meteorology
• Organic chemistry
• Environmental science
• Polymer chemistry
• Classical mechanics
• Aerospace engineering
• Marine engineering
• Statistical physics
• Condensed matter physics

Selected publications

• Emergence of Purely Elasto-Plastic Turbulence in Shear Flows

  ArXiv.org · 2026-01-13

  articleOpen accessSenior author

  We observe the emergence of a distinct, elasticity-driven flow state in a yield-stress fluid in the absence of inertia. Numerical simulations show that this elasto-plastic turbulent state is characterized by a broad spectrum of fluctuations in velocity and stress. Results show a non-monotonic relationship between the volume fraction of the unyielded flow and plasticity. Surprisingly, we find that above a critical value of plasticity, the system can fluidize. Our results reveal the complex interplay between elasticity and plasticity in simple shear flows, indicating that plasticity can enhance rather than hinder momentum transport.

  PublisherOA PDF

• Data-driven particle dynamics: Structure-preserving coarse-graining for emergent behavior in nonequilibrium systems

  Proceedings of the National Academy of Sciences · 2026-05-13

  articleOpen access

  Multiscale systems are ubiquitous in science and technology, but are notoriously challenging to simulate as short spatiotemporal scales must be appropriately linked to emergent bulk physics. When expensive high-dimensional dynamical systems are coarse-grained into low-dimensional models, the entropic loss of information leads to emergent physics which are dissipative, history-dependent, and stochastic. To machine learn coarse-grained dynamics from time-series observations of particle trajectories, we propose a framework using the metriplectic bracket formalism that preserves these properties by construction; most notably, the framework guarantees discrete notions of the first and second laws of thermodynamics, conservation of momentum, and a discrete fluctuation-dissipation balance crucial for capturing nonequilibrium statistics. We introduce the mathematical framework abstractly before specializing to a particle discretization. As labels are generally unavailable for entropic state variables, we introduce a self-supervised learning strategy to identify emergent structural variables. We validate the method on benchmark systems and demonstrate its utility on two challenging examples: 1) coarse-graining star polymers at challenging levels of coarse-graining while preserving nonequilibrium statistics, and 2) learning models from high-speed video of colloidal suspensions that capture coupling between local rearrangement events and emergent stochastic dynamics. We provide open-source implementations in both PyTorch and LAMMPS, enabling large-scale inference and extensibility to diverse particle-based systems.

  PublisherDOI

• Emergence of Purely Elasto-Plastic Turbulence in Shear Flows

  arXiv (Cornell University) · 2026-01-13

  preprintOpen accessSenior author

  We observe the emergence of a distinct, elasticity-driven flow state in a yield-stress fluid in the absence of inertia. Numerical simulations show that this elasto-plastic turbulent state is characterized by a broad spectrum of fluctuations in velocity and stress. Results show a non-monotonic relationship between the volume fraction of the unyielded flow and plasticity. Surprisingly, we find that above a critical value of plasticity, the system can fluidize. Our results reveal the complex interplay between elasticity and plasticity in simple shear flows, indicating that plasticity can enhance rather than hinder momentum transport.

  PublisherDOI

• Coupled jet coordination and physical arrangement in salp-inspired multi-robot swimming

  Bioinspiration & Biomimetics · 2025-10-15 · 1 citations

  articleOpen accessCorresponding

  Salps are underwater invertebrates considered to be among the world's most energy-efficient examples of jet propulsion. They can swim as solitary individuals or as physically connected colonies, coordinating their jets to produce collective movement. Inspired by salps, we developed the SALP (Salp-inspired Approach to Low-energy Propulsion) system, where individual SALP robots can be physically connected into a multi-SALP group, and we investigate the coupled effects of physical arrangement and jet coordination on the swimming performance and energy efficiency of a two-SALP system. We conduct free swimming tests to evaluate locomotion performance metrics and find that the two-SALP system, when properly coordinated, is able to swim with 15.7% higher speed and 11.3% lower cost of transport than the single SALP. Supporting flow characterization experiments using particle image velocimetry reveal vortex ring structures emanating from robot SALP nozzles. The data suggest that propulsion performance is affected by the spatial arrangement of the vortex ring structure. In particular, we find that SALP systems that produce a parallel vortex ring arrangement produce less vortex circulation and impulse than an in-series vortex ring arrangement. Overall, the SALP system is a useful platform for exploring salp-inspired multi-jet locomotion strategies, enabling decoupling of physical and control parameters to expose underlying locomotion physics in ways that are difficult with the biological salp. These insights advance our understanding of multi-jet locomotion and support the development of more energy-efficient jet-propelled underwater robots in the future.

  PublisherDOI

• Geomimicry: Emergent Dynamics in Earth-Mediated Complex Materials

  ArXiv.org · 2025-11-01

  preprintOpen accessSenior author

  Soils and sediments are soft, amorphous materials with complex microstructures and mechanical properties, that are also building blocks for industrial materials such as concrete. These Earth-mediated materials evolve under prolonged environmental pressures like mechanical stress, chemical gradients, and biological activity. Here, we introduce geomimicry, a new paradigm for designing sustainable materials by learning from the emergent and adaptive dynamics of Earth-mediated matter. Drawing a parallel to biomimicry, we posit that these geomaterials follow evolutionary design rules, adapting their structure and function in response to persistent natural forces through locally evolved interactions and compositions. Our central argument is that by decoding these rules - primarily through understanding the emergence of novel exotic properties from multiscale interactions between heterogenous components - we can engineer a new class of adaptive, sustainable matter. We propose two complementary approaches here. The top-down approach looks to nature to identify building blocks and map them to functional groups defined by their mechanical behaviors, and then examine how environmental training tunes interactions among these groups. The bottom up approach seeks to leverage and test this framework, building earth materials one component at a time under fluctuating environmental stresses that guide assembly of complex and out-of-equilibrium materials. The goal is to create materials with programmed functionalities, such as erosion resistance or self-healing capabilities. Geomimicry offers a pathway to re-imagine climate-resilient soils and precision agriculture to new insights into planetary terraforming, fundamentally shifting the focus from static compositions to dynamic, evolving systems that are mediated via their environment.

  PublisherOA PDFDOI

• Quantification of Flagellar Gait Changes with Combined Shape Mode Analysis and Swimming Simulations

  arXiv (Cornell University) · 2025-01-02

  preprintOpen access

  Many different microswimmers propel themselves using flagella that beat periodically. The shape of the flagellar beat and swimming speed have been observed to change with fluid rheology. We quantify changes in the flagellar waveforms of Chlamydomonas reinhardtii in response to changes in fluid viscosity using (1) shape mode analysis and (2) a full swimmer simulation to analyze how shape changes affect the swimming speed and to explore the dimensionality of the shape space. By decomposing the gait into the time-independent mean shape and the time-varying stroke, we find that the flagellar mean shape substantially changes in response to viscosity, while the changes in the time-varying stroke are more subtle. Using the swimmer simulation, we quantify how the swimming speed is affected by the dimensionality of the flagellar shape reconstruction, and we show that the observed change in swimming speed with viscosity is explained by the variations in mean flagellar shape and beat frequency, while the changes in swimming speed from the different time-varying strokes are on the scale of variation between cells.

  PublisherOA PDFDOI

• Quantification of flagellar gait changes with combined shape mode analysis and swimming simulations

  Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences · 2025-09-11 · 1 citations

  articleOpen access

  Many different microswimmers propel themselves using flagella that beat periodically. The shape of the flagellar beat and swimming speed have been observed to change with fluid rheology. We quantify changes in the flagellar waveforms of Chlamydomonas reinhardtii in response to changes in fluid viscosity using (i) shape mode analysis and (ii) a full swimmer simulation to analyse how shape changes affect the swimming speed and to explore the dimensionality of the shape space. By decomposing the gait into the time-independent mean shape and the time-varying stroke, we find that the flagellar mean shape substantially changes in response to viscosity, while the changes in the time-varying stroke are more subtle. Using the swimmer simulation, we quantify how the swimming speed is affected by the dimensionality of the flagellar shape reconstruction, and we show that the observed change in swimming speed with viscosity is explained by the variations in mean flagellar shape and beat frequency, while the changes in swimming speed from the different time-varying strokes are on the scale of variation between cells. This article is part of the theme issue ‘Biological fluid dynamics: emerging directions’.

  PublisherDOI

• Equation of motion for taut-line buzzers

  Physical Review Applied · 2024-07-03 · 1 citations

  preprintOpen access

  Equations of motion are developed for the oscillatory rotation of a disk suspended between twisted strings kept under tension by a hanging mass, to which additional forces may be applied. In the absence of forcing, damped harmonic oscillations are observed to decay with an exponential time envelope for two different string types. This is consistent with damping caused by string viscosity, rather than air turbulence, and may be quantified in terms of a quality factor. To test the proposed equation of motion and model for viscous damping within the string, we measure both the natural oscillation frequency and the quality factor for widely varied values of string length, string radius, disk moment of inertia, and hanging mass. The data are found to scale in good accord with predictions. A variation where rotational kinetic energy is converted back and forth to spring potential energy is also discussed.

  PublisherOA PDFDOI

• An Alternative Scaling for Roughness Transitions in Turbulent Flows: The Role of the Internal Boundary Layer

  arXiv (Cornell University) · 2024-11-13

  preprintOpen accessSenior author

  When turbulent boundary layer flows encounter abrupt roughness changes, an Internal Boundary Layer (IBL) forms. Equilibrium theory breaks down in the nonequilibrium IBL, which may extend O(10) km for natural atmospheric flows. Here, we find that the IBL possesses a characteristic time-scale associated with the IBL height, $δ_i$. We show that $δ_i$ and the edge velocity set the scales of the mean and defect velocity profiles within the IBL, for simulation and experimental data covering a multitude of roughness transition types. We present a nontrivial extension of equilibrium theory to the dynamically adjusting IBL, which can be useful for modeling a range of environmental and industrial flows.

  PublisherOA PDFDOI

• Soft matter mechanics of baseball’s Rubbing Mud

  Proceedings of the National Academy of Sciences · 2024-11-04 · 5 citations

  articleOpen access

  Researchers looking for sustainable materials with optimal mechanical properties may draw inspiration from a baseball tradition. For nearly 100 y, a mysterious mud harvested from an undisclosed river site in New Jersey (USA) has been the agent of choice in the USA's Major League Baseball for "de-glossing" new baseballs. It is unclear, however, what makes this "Rubbing Mud" work. Here, we perform a multiscale investigation of the rheology and tribology of this mud material under baseball-relevant conditions and identify three mechanisms by which the mud alters the surface properties of the baseball. First, the mud creates a more uniform baseball surface by filling in pores in the leather; this is possible because of its relatively high cohesion (clays and organics) making the material remarkably shear thinning. Second, the residue of cohesive particles coating the baseball effectively doubles contact adhesion. Third, a sparse population of angular sand grains are bonded to the baseball by clay-sized particles, leaving a studded surface that enhances friction. The proportions of cohesive, frictional, and viscous elements in Rubbing Mud conspire to create a soft material with an unusual mix of properties, that could find other applications in the development of sustainable geomaterials. Our improved understanding of the flow and friction of natural muds may also find use in modeling natural hazards such as mudslides and for locomotion in muddy environments.

  PublisherDOI

Recent grants

• The Effects of Viscoelasticity on Filament Thinning & Drop Breakup in Microfluidic Devices: Single Molecule Experiments

  NSF · $300k · 2009–2012

• RUI: Particle Dynamics: Swimming Cells and Sheared Particulate Materials

  NSF · $360k · 2011–2016

• Investigating the Unsteady Rheology and Evolving Microstructure of Suspensions of Swimming Microorganism

  NSF · $336k · 2014–2019

• Viscoelastic Fluids in Parallel Shear Flows at low re: Instabilities, Bifurcations & Single Molecule Experiments

  NSF · $300k · 2013–2017

• Transport and Dynamics of Swimming Microorganisms in Time-Periodic Flows

  NSF · $421k · 2017–2022

Frequent coauthors

• D. J. Jerolmack

  54 shared

• J. P. Gollub

  Haverford College

  41 shared

• Nathan C. Keim

  Pennsylvania State University

  40 shared

• Ranjiangshang Ran

  Emory University

  37 shared

• Boyang Qin

  Peking University

  37 shared

• Arvind Gopinath

  33 shared

• Bryan O. Torres Maldonado

  29 shared

• D. J. Durian

  27 shared

Labs

• Penn Complex Fluids LabPI

Education

• Ph.D., Mechanical Engineering

  University of California, Berkeley

  1993

• M.S., Mechanical Engineering

  University of California, Berkeley

  1989

• B.S., Mechanical Engineering

  University of California, Berkeley

  1987

Awards & honors

• Eduardo D. Glandt Distinguished Scholar