Ongoing projects

This project aims at characterising heat and mass transport driven by convection severely confined sub-surface systems.

Image: Atamaca Desert, Chile

In the Spotlight 

After the splash, raindrops can become sandballs

When we think about soil erosion, we usually picture the dramatic moment a raindrop hits the ground and splashes sediment everywhere. Our work says: that’s only the opening act.

We found that on dry, sloped, granular soil, a raindrop can keep going after impact. Instead of vanishing or just splattering, it can start rolling downhill and scooping up grains along the way—like a tiny liquid snowball. As it rolls, it turns into a dense, gritty “sandball” that grows heavier and heavier until it becomes so loaded with sediment that it effectively jams into a semi-solid blob. In other words, we watched a single drop go from “splat” to “mini bulldozer.”

We also discovered that these sandballs don’t all look the same. By carefully controlling the drop conditions, we observed two stable, repeatable “signature shapes”:

• Peanut-like sandballs, which we link to hydrodynamic instabilities (the fluid flow inside and around the rolling drop starts shaping it in distinctive ways).

• Donut-like (toroidal) sandballs, which form under extreme sediment loading, when grains mechanically lock together and force a ring-like structure.

Our takeaway is simple: raindrop erosion isn’t just about splash at impact. There’s a surprisingly efficient post-impact phase—rolling, gathering, and growing—that can magnify how much soil a single drop moves. And because sandballs are essentially tiny moving packets of dense suspension and grain jamming, we think the same physics could matter well beyond erosion, from bioengineering and pharmaceuticals to the strange and wonderful world of snow physics.

Learn more in “Sandball genesis from raindrops. Trottet, B., Noto, D., Jerolmack, D. J. & Ulloa, H. N. 2025 Proceedings of the National Academy of Sciences USA. DOI:10.1073/pnas.251939212“. Also, watch mesmerizing videos here.

Additional short readings about our article are listed below:

Science: Watch these raindrops turn into rolling ‘sandballs’

Phys.org: Raindrops form ‘sandballs’ as they roll downhill, contributing more to erosion than previously thought

infobae: El impacto oculto de la lluvia en la erosion: como se forman bolas de arena que arrastran sedimentos (spanish)

err.ee: Liivased veepiisad kulutavad mägesid (estonian)

hayka: Физики увидели, как капли дождя становятся мощным природным скульптором (russian) 

Transition regimes in confined thermal convection

Thermal convection governs essential natural and engineered systems, including oceans, lakes, hydrothermal systems, and heat exchangers. All these systems are geometrically confined, and so are convective fluid motions. But, what controls the actual confinement, and how does it affect convective dynamics? Here, we introduce the “degree of confinement,” a universal parameter that characterizes the effect of lateral control on thermal convection, a metric encapsulating the fluid properties, system geometry, and thermal forcing. We demonstrate via laboratory experiments that local and global dynamics are ultimately determined by the degree of confinement that thermal plumes—the fundamental manifestation of convection—experience at their genesis. These experimental results were made possible with a new experimental method that allows you to reconstruct spatial temperature structure from the convective velocity field (Noto et al. 2023 Exp. Fluids). Our study bridges the paradigmatic problem of “Rayleigh–Bénard convection” with natural and engineering thermo-fluid systems of varying degrees of confinement. Furthermore, it provides experimental support of the so-called Hele-Shaw model, introduced by Letelier et al. (2019 J. Fluid Mech.) and provides a physical interpretation for geometrically-controlled optimal heat transfer reported by previous numerical studies (Chong et al. 2015 Phys. Rev. Lett.). Plus, we show how the emergence of very localized three-dimensional rolls — controlled by thermal plumes that are thinner than the Hele-Shaw cell’s gap — plays a major role in the global heat transfer rate. Last, we provide a phase map that shows you how to shift a confined convective system from Darcy regime to Hele-Shaw (convective) regime, and from the Hele-Shaw regime to 3D convection. This work was led by Daisuke Noto (University of Pennsylvania) in collaboration with Juvenal Letelier (Universidad de Chile).

Learn more in “Plume-scale confinement on thermal convection. Noto, D., Letelier, J. A. & Ulloa, H. N. 2024 Proceedings of the National Academy of Sciences USA. DOI:10.1073/pnas.2403699121“. Also, watch beautiful videos here.

Convective modes drive littoral-pelagic exchange of dissolved gases

The debate surrounding the impact of the nearshore zone on lake gas dynamics persists in the scientific community, primarily due to the lack of precise measurement methods for lateral gas movement. Existing models relying on diffusive horizontal transport fail to account for anomalies in gas concentrations within lake ecosystems. Our field experiments conducted in a nutrient-rich lake unveil the significant role of daily convective horizontal circulation in driving vast littoral-pelagic advective gas fluxes, far surpassing conventional estimates. These findings not only underscore the capacity of lateral fluxes to profoundly redistribute gases across entire basins but also shed light on concentration anomalies observed in lakes worldwide. Furthermore, our study challenges the prevailing notion of nocturnal littoral-to-pelagic gas exchange by revealing a continuous bi-directional transport facilitated by convective circulation, fundamentally reshaping our understanding of aquatic systems. Integrating this pivotal mechanism into gas budget frameworks is imperative to advance our comprehension and management of lake ecosystems.

Learn more in “Lake surface cooling drives littoral-pelagic exchange of dissolved gases. Doda, T., Ramón, C., Ulloa, H.N., Brennwald, M., Kiefer, R., Perga, M-E, Wüest, A., Schubert, C. & Bouffard, D. 2024 Science Advances. 10, DOI:10.1126/sciadv.adi0617“. Also, watch YouTube video

Tracking zooplankton swimming at intermediate Reynolds numbers

Deepening our understanding of animals’ collective motions represents a multidisciplinary goal. Yet, quantifying the motions of hundreds of animals in the laboratory and nature posits a fundamental challenge for digital image processing: How do we track each object out of the crowd while allowing them to move freely in a three-dimensional (3D) domain? Here, we present a simple tracking strategy to reconstruct 3D trajectories with the aid of a mirror, even if moving objects experience occlusion. We explain the method using synthetically generated datasets and apply it to measure collective motions of phototactic zooplankton, Daphnia magna, swimming in a lab-scale aquarium at intermediate Reynolds numbers, 1 < Re < 13. The method enables measuring statistics of characteristic features of D. magna swarm, including sinking velocities and flapping frequencies. Beyond the lab-scale animal tracking, we foresee further implementations of the method to study wild animals freely behaving in 3D environments irrespective of their species.

Learn more in “Simple tracking of occluded self-propelled organisms. Noto, D. & Ulloa, H.N. 2024 Meas. Sci. Technol. 35 035705, DOI:10.1088/1361-6501/ad1813“.

Stratified Horizontal Convection

Large stratified fluids, like oceans, atmospheres and lakes, do not heat uniformly at their surface. The latter creates horizontal temperature and density gradients that drive a localized overturning motion near their surface, known as stratified horizontal convection (SHC). We want to understand and characterise this large-scale flow structure via physical parameters. In this new research, we used lab experiments and theory to learn how heat and mass move within these types of fluid environments. By looking at the mechanical energy available in the system, we introduced the Péclet number, which encapsulates the stabilizing effect of stratification and the destabilizing effect of differential heating. We discovered that the Péclet number tells us about the small and big fluid motions, how much energy the fluid has, and how the flow happens in three dimensions. We remark that SHC stands apart from the traditional horizontal convection process due to a distinct feature: active overturning occurs only within a specific region of the fluid column rather than spanning its entirety. This work was developed in collaboration with colleagues from Hokkaido University and the Japanese Agency for Marine-Earth Science and Technology.

Learn more in “Stratified Horizontal Convection. Noto, D., Ulloa, H.N., Yanagisawa, T., & Tasaka, Y. Journal of Fluid Mechanics, A1. DOI:10.1017/jfm.2023.625“.