Award Abstract # 2025643
ISS: A new paradigm for explaining catastrophic post-wildfire mudflows: transport phenomena and gravity-driven aggregation dynamics of hydrophobic particle-air-water mixtures

NSF Org: CBET
Division of Chemical, Bioengineering, Environmental, and Transport Systems
Recipient: UNIVERSITY OF CALIFORNIA, SAN DIEGO
Initial Amendment Date: July 29, 2020
Latest Amendment Date: July 29, 2020
Award Number: 2025643
Award Instrument: Standard Grant
Program Manager: Shahab Shojaei-Zadeh
sshojaei@nsf.gov
 (703)292-8045
CBET
 Division of Chemical, Bioengineering, Environmental, and Transport Systems
ENG
 Directorate for Engineering
Start Date: October 1, 2020
End Date: September 30, 2024 (Estimated)
Total Intended Award Amount: $400,000.00
Total Awarded Amount to Date: $400,000.00
Funds Obligated to Date: FY 2020 = $400,000.00
History of Investigator:
  • Ingrid Tomac (Principal Investigator)
    itomac@ucsd.edu
Recipient Sponsored Research Office: University of California-San Diego
9500 GILMAN DR
LA JOLLA
CA  US  92093-0021
(858)534-4896
Sponsor Congressional District: 50
Primary Place of Performance: University of California San Diego
9500 Gilman Drive
La Jolla
CA  US  92093-0085
Primary Place of Performance
Congressional District:
50
Unique Entity Identifier (UEI): UYTTZT6G9DT1
Parent UEI:
NSF Program(s): PMP-Particul&MultiphaseProcess,
Special Initiatives
Primary Program Source: 01002021DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s):
Program Element Code(s): 141500, 164200
Award Agency Code: 4900
Fund Agency Code: 4900
Assistance Listing Number(s): 47.041

ABSTRACT

This NSF-CASIS project will conduct a series of experiments on board the International Space Station (ISS) and on Earth to understand the role of gravity in the dynamics of mudflows. It is well established that rainfall triggers mudflows on recently burned slopes. After wildfires, the surficial burned soil is water-repellent or hydrophobic, preventing rain infiltration and leading to sudden and rapid mudflows. Post-wildfire gravity-driven mudflows are unpredictable, occur suddenly, and travel rapidly downhill, turning into debris flows and mobilizing large and heavy boulders. In January 2018 in Montecito, California, an intense 15-minute burst turned into a devastating debris flow which caused 21 deaths, led to $421 million in damages, and closed key transit corridors. The experiments will examine how the attachment of hydrophobic soil particles to air bubbles leads to the formation of aggregates that may give rise to the unusual flow behaviors observed in mudflows. Particle-air-water mixtures form interesting structures (bubbles, pipes and clusters) whose shapes are primarily governed by a balance between gravity and the attractive forces between air bubbles and water-repellent particles. The experiments on the ISS and on Earth will use a model system consisting of sand particles that have been made hydrophobic through a chemical treatment, air and water. After mixing, the material will flow through a plexiglas channel, and particle motions and evolution of aggregates will be imaged and correlated with characteristics of the overall flow. By comparing experiments on the ISS with those on Earth, the role of gravity in aggregate formation and flow behavior will be understood. The results of this study will help understand how mudslides are affected by rainfall intensity and duration and could lead to better early-warning systems and risk evaluation. The research team will include students, especially those from underrepresented groups, and the project will support educational activities to high-school students in local communities.

The goal of this project is to run experiments on Earth and in microgravity conditions to correlate mudflow composition with flow and transport characteristics on a micromechanical level. An understanding of the role of gravity on microstructural changes in flowing air-water-particle mixtures and, in particular, on the formation of particle-bubble agglomerates is crucial for predicting the rheological behavior of mudflows. Experiments will focus on how mudflow shear behavior depends on relative amounts of water, trapped air, and particles of various sizes. Experiments on Earth will identify how the mixture composition affect flow behavior and will delineate critical parameter ranges to be tested on board the ISS. Microgravity experiments will study the dynamics of hydrophobic particle attachment to air bubbles and the consequences of agglomeration on mixture flow and transport. Results will be used to derive governing equations that can describe the flow behavior of the mixtures, including effects of mixture rheology on the flow. Understanding the processes of mudslide initiation with respect to rainfall intensity and duration will lead to a more accurate predictive capability for the onset and development of mudslides that could mitigate catastrophic damage.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

PROJECT OUTCOMES REPORT

Disclaimer

This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

This project quantified the internal matrix properties of post-wildfire debris flows, accounting for different availability of hydrophobic soil particles and subsequent air entrapment mechanisms. Due to different surface wettability induced by the deposition of burned organic matter on soil grain surfaces, hydrophobic sand particles attach to air bubbles in water during mixing and entrap air via a liquid marble mechanism. Although much effort has been dedicated to understanding individual liquid marbles, multi-particle systems driven by gravity, fluid drag, and collisions have not yet been fully understood. As a result, the postfire debris flows adversely and extremely impact communities and infrastructures since they can flow very fast. The stability of liquid marbles in water depends on the force balance on the air bubble-particle boundary, which defines the internal structure of the mixture. Forces that separate or attach hydrophobic particles to an air bubble in water depend on gravity and bubble-particle-relative velocity, which change during flow and transport and are affected by collisions and sand concentrations. Experimental and theoretical approaches quantify the air entrapment under different sand-water volumetric concentrations effects of mixing speed, duration, and particle size on the final mixture's internal structure. Relatively large volumes of air get entrapped early into debris flows, but its content changes over time due to degassing and breaking down large liquid marbles. Empirical estimation of density reductions due to air entrapment in the mixture during the mixing process is developed. Post-wildfire debris flows' internal structure plays an important role during the shearing process. Different rheological models have been used to estimate or simulate the shear behavior of such a complex mixture. Common practices include the Newtonian model, the Bingham model, the Herschel-Bulkley model, the quadratic model, the Mohr-Coulomb frictional model, the Coulomb-Viscous model, the Velly model, and other nonlinear models. However, existing models ignore the entrapped air role in the rheological behavior of such a complex mixture. Furthermore, the Bagnold number helps to define different flow regimes when the mixture is under the shearing process with a wide range of shear rates. New empirical, experimental rheological relationships are proposed based on the role of the air bubble entrapment ratio and the volumetric concentration of liquid marble. 

 


Last Modified: 02/10/2025
Modified by: Ingrid Tomac

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