(3i) Nature-Inspired Fluids and Elasticity: A New Route Toward Functional Soft Materials

Overview: Building on my cross-disciplinary training in soft matter physics, plant biology, and materials engineering, I propose to develop a new research program focused on the design of plant-inspired functional soft materials. This research will yield fundamental insights into transport processes in active and responsive systems, as well as engineering principles for the design of new multifunctional materials.

Background: Plants are often seen as passive systems, but in reality, they use many functional soft materials to dynamically respond to their environment. As engineers, we can learn a lot from the design of these materials. To bridge the different fields of soft matter physics and plant biology, I first pursued a Ph.D. in 2016 from Aix-Marseille University in France, where I worked as a Laboratory of Excellence fellow with Profs. Yoël Forterre, Geoffroy Guéna, and Éric Badel. My graduate work focused on poroelastic couplings in plants which I explored experimentally using both natural and synthetic biomimetic branches. Through this work, I was able to unravel a new physical mechanism to explain long distance signaling in plants, solving a long-standing conundrum in the plant community (PNAS, 2017). I then started my postdoctoral work with Prof. Sunghwan Jung at Virginia Tech, now at Cornell University. There, I studied the impact of plant leaf geometry on mechanical properties, and pinch-off acoustics of water-entry projectiles. In these works, I respectively showed that leaves—which are made of a petiole and a lamina—modify petiole mechanical properties to cope with wind-induced drag on lamina, and observed and modeled that pinch-off generates an acoustic wave and subsequent acoustic force on water-entry projectiles (Sci. Rep 2018, JFM 2018). Following this position, I moved to France to work with Prof. Philippe Marmottant at CNRS-Grenoble, where I worked on acoustically activated microswimmers and studied drying of biomimetic leaves. For the first time, I showed that we can remotely control microswimmers using acoustic streaming (Adv. Mat. Int. 2018). In my second project, I was able to experimentally observe and model the dynamics of a drying meniscus in microfluidic channels for applications in plants and rocks (JRSI 2019). I continued my postdoctoral work with Prof. Kaare Jensen at the Technical University of Denmark, studying seed imbibition and biomimetic poroelastic membranes. In these works, I (1) showed that seed imbibition is only controlled by physical parameters (PRE 2018), and (2) revealed how the deformation of a poroelastic membrane can induce different hole deformation regimes (PRL, under 2^nd round of review). I am now at Princeton University working with Prof. Sujit Datta on the elastocapillary dynamics of collapsible tubes and hydrogel swelling for applications in agriculture. In my first project, I uncovered how the fluid and elasticity couplings at the lung airway scale can be incorporated in the overall breathing dynamics and airway geometry at the lung scale to numerically model inhalation (JRSI, submitted). In my second work, I revealed that hydrogel swelling can be hindered by confinement, and developed a model that universally describes how hydrogel swelling, and possible restructuring of the surrounding medium, depends on hydrogel properties, medium properties, and external stresses (Sci. Adv., under review).

Together, these experiences provide me with knowledge of poroelasticity, fluid mechanics, acoustics, microfluidics, and hydrogel physics, giving me the tools to build experiments investigating the complex physics of plants and subsequent soft matter applications.

Research Interests: My previous research has revealed that plants are sophisticated hydraulic machines, using flow-controlled processes in place of vasculature, muscles, and nervous systems. Chemical engineering problems require similar flow control abilities to enhance transport, responsiveness, and mixing. Recent progress in soft matter has shown promise in passively providing these features because soft materials can respond sensitively and non-linearly to fluid, mechanical, or osmotic stresses. However, these advances present a unique challenge: highly deformable and mechanically non-linear materials are difficult to integrate efficiently into system designs. To address this challenge, my lab will use biomimetics to relate complex biological functions to simple physical properties, enabling us to engineer smarter functional soft materials. In particular, I will investigate how plants use couplings between flow, colloids, and soft materials to regulate nutrient distribution, stress responses, and even move themselves. I will then harvest these physics to design novel materials for passive control over batch flows, soft robotics, and drug delivery.

My research group, the Nature Inspired Fluids and Elasticity lab, will operate at the intersection of engineering, physics, and biology to tackle both fundamental and applied problems. We will initially focus on three areas:

• I will investigate how plants dynamically control water flow using poroelastic membranes, hydrogel coatings, colloidal flow, and liquid phase changes. Microfluidic “plant-on-a-chip” experiments will dramatically enhance our understanding of plant fluid dynamics in response to climate change. Uncovering these physics will help us design new techniques to control batch flows, utilize nanoparticle deposition to design healing materials, and protect piping from cavitation or freezing events.
• A key challenge in soft robotics is imparting mechanical sensing, but plants are already experts at providing such feedback passively. I will use a hydrogel-soil setup to investigate how plants sense mechanical constraints to guide growth (following Louf et al., PNAS 2017), or alternatively redistribute water between organisms in an ecosystem. This work will also enable the design of a new class of soft robots that can passively respond to their environments, providing safer robot-human interactions.
• Hydrogel origami has been used to design capsules for drug delivery, but current geometries are restricted to swelling. My recent work (Louf et al., in progress) has shown drying can be highly non-linear, providing a new avenue for sensitive origami design. This increased sensitivity can provide access to more complex geometries for use in new fields such as soft robotics or humidity actuators.

Teaching Interests: In my classroom, I aim to spark curiosity and help students build their own knowledge. I brought this philosophy to classrooms at several levels. Before starting my Ph.D., I taught physics and chemistry to over 200 middle school students. During my Ph.D. I took classes on academic pedagogy and applied what I learned through teaching assistantships in several graduate and undergraduate engineering courses. I further honed these skills at the University of Grenoble, where I was in charge of undergraduate algebra and calculus for a semester. In the U.S., I have given guest lectures during my postdocs at Virginia Tech and Princeton University, on “Physics applied to biology” and “Fluid and Elasticity Couplings for Biology.” Drawing on these experiences, I would feel comfortable teaching several undergraduate and graduate classes in the Chemical Engineering curriculum: Thermodynamics, Mechanics and Dynamics of soft matter, Mathematics in Engineering, and Mass, Momentum and Energy Transport. I will also seek to develop a new upper-level elective course on “Bio-inspired physics of soft materials” that draws together soft matter physics, plant biology, and materials engineering.

Successful Proposals: Andlinger E-ffiliate grant on “atmospheric water harvesting using Moisture-Absorbent Temperatue-Controled Hydrogels (MATCHes)”, 2019 ($150k); Laboratory of excellence Graduate fellowship on “poroelastic couplings in biomimetic branches: link with plants mechanoperception”, 2012 (€100k).

Proposals under consideration: NSF proposal on “Dynamic model network of the human lung” currently under review in the Biomechanics and Mechanobiology program ($200k); DARPA proposal on “Phase Optimal Water ExtRaction (POWER)” currently under review ($2M).

Building on these experiences securing funding, I will fund my research program by targeting key programs in the National Science Foundation (Biomechanics and Mechanobiology, Mechanics of Materials and Structures, Environmental Engineering, Fluid Dynamics, Particulate and Multiphase Processes, Division of Material Research), Department of Energy (Basic Energy Science and Biological and Environmental Research), Army Research Office (Polymer Chemistry, Materials Design), and Office of Naval Research (Bio-inspired Autonomous Systems, Biomaterials and Bionanotechnology).

Publications:

1. Louf, JF; Kratz, F; Datta, SS. “Elasto-capillary network model of inhalation”, Journal of the Royal Society Interface, under review
2. Louf, JF; Lu, N; O’Connell, M; Cho, HJ; Datta, SS. “Under pressure: Mechanics of swelling hydrogels under confinement,” Science Advances, under review
3. Louf, JF; Knoblauch, J; Jensen, K. “Bending and stretching of soft pores enable passive fluid flow control,” Physical Review Letters, under 2^nd round of review
4. Dollet, B.; Louf, JF; Alonzo, M.; Jensen, K.; Marmottant, P. “Drying of channels by evaporation through a permeable medium,” Journal of the Royal Society Interface, 2019, 16:20180690
5. Louf, JF; Nelson, L; Kang, H; Ntoh Song, P; Zehnbauer, T; Jung, S. “How wind drives the correlation between leaf shape and mechanical properties,” Scientific Reports, 2018, 8, 16314
6. Louf, JF; Zheng, Y; Kumar, A; Bohr, T; Gundlach, C; Harholt, J; Friis Poulsen, H; Jensen, KH. “Imbibition in plant seeds,” Physical Review E, 2018, 98, 042403
7. Louf, JF; Chang, B; Eshraghi, J; Mituniewicz, A; Vlachos, P; Jung, S. “Ripple dynamics of water entry after pinch off,” Journal of Fluid Mechanics, 2018, 850, 611-623
8. Louf, JF; Bertin, N; Dollet, B; Stephan, O; Marmottant, P. “Hovering Microswimmers Exhibit Ultra-Fast Motion to Navigate under acoustic forces,” Advanced Material Interfaces, 2018, 1800425
9. Louf, JF; Guéna, G; Badel, É; Forterre, Y. “A universal mechanism for hydraulic signals in plants,” Proceedings of the National Academy of Sciences, 2017, 114, pp 11034-11039