Amir A. Pahlavan

Predicting and controlling the transport of colloids in porous media is essential for a broad range of applications, from drug delivery to contaminant remediation. Chemical gradients are ubiquitous in these environments, arising from reactions, precipitation/dissolution or salinity contrasts, and can drive particle motion via diffusiophoresis. Yet our current understanding mostly comes from idealised settings with sharply imposed solute gradients, whereas in porous media, flow disorder enhances solute dispersion, and leads to diffuse solute fronts. This raises a central question: Does front dispersion suppress diffusiophoretic migration of colloids in dead-end pores, rendering the effect negligible at larger scales? We address this question using an idealised one-dimensional dead-end geometry. We derive an analytical model for the spatio-temporal evolution of colloids subjected to slowly varying solute fronts and validate it with numerical simulations and microfluidic experiments. Counterintuitively, we find that diffuseness of the solute front enhances removal from dead-end pores: although smoothing reduces instantaneous gradient magnitude, it extends the temporal extent of phoretic forcing, yielding a larger cumulative drift and higher clearance efficiency than sharp fronts. Our results highlight that solute dispersion does not weaken the phoretic migration of colloids from dead-end pores, pointing to the potential relevance of diffusiophoresis at larger scales, with implications for filtration, remediation and targeted delivery in porous media.

Chemical gradients are ubiquitous in porous media flows, from tidal salt gradients in aquifers to irrigation-driven gradients in soils and ionic gradients from metabolic activity in tissues. Although chemical gradients are known to drive diffusiophoretic migration of colloids, these nonequilibrium forces have largely been ignored in porous media flows. Under typical subsurface conditions, flow velocities within preferential pathways exceed phoretic velocities by orders of magnitude, suggesting that diffusiophoresis would be limited to stagnant pockets. Here, using microfluidic experiments, numerical simulations, and theoretical modeling, we show that even moderate solute gradients, typical of natural mixing, can markedly alter colloid transport. We uncover a previously overlooked effect: cross-streamline phoretic migration within preferential flow pathways, which changes macroscopic dispersion by orders of magnitude and suppresses the impact of geometric disorder on transport. Our findings challenge classical models of colloid transport, highlighting the broad implications of solute gradients for technological, biomedical, and environmental applications.

Active wetting extends classical wetting physics to living systems, in which cells and tissues spread by generating internal forces rather than relying solely on passive interfacial tensions. Unlike passive systems, which evolve toward thermodynamic and mechanical equilibrium by minimizing free energy, active systems remain far from equilibrium due to continuous energy input and dissipation. Their dynamics are sustained, adaptive, and responsive to chemical and mechanical cues in ways that depart fundamentally from passive behavior. In addition, active systems lack a unified energetic or variational principle to describe their evolution. What insights can be drawn from passive models and how these models might be generalized to account for activity remain open questions. Studying active wetting may thus reveal new principles of nonequilibrium dynamics at soft and living interfaces, and offer deeper understanding of key biological processes such as wound healing, cancer invasion, and biofilm growth.

Understanding particle-mediated interfacial processes in confined spaces undergoing deformation is important for many natural and industrial processes. Here, we combine laboratory experiments and theoretical analysis to investigate how particle dynamics shapes suspension-air interfacial pattern formation when the suspension is stretched. It is found that even slightly nonuniform particle concentration promotes wavy and finger-like morphologies, while particle perturbations lead to dendritic pattern with strong finger branching.

Contact lines, where fluid interfaces meet solid surfaces, pose a fundamental challenge to modeling fluid-fluid displacement in confined geometries, as they violate the classical no-slip boundary condition. Recent experiments reveal that contact-line motion in a capillary tube produces compact displacement at low flow rates and unstable fingering at high flow rates. We present a phase-field model with a novel formulation of the boundary wetting conditions. Our model captures the equilibrium configurations at arbitrary wettability, and also predicts dynamic configurations, including wetting transitions, thin-film formation and interface pinch-off, in quantitative agreement with experiments.

Solute gradients next to an interface drive a diffusioosmotic flow, the origin of which lies in the intermolecular interactions between the solute and the interface. These flows on the surface of colloids introduce an effective slip velocity, driving their diffusiophoretic migration. In confined environments, the interplay between diffusiophoresis and diffusioosmosis governs the motion of colloids. Previous studies have indeed demonstrated the quantitative modulation of phoretic migration by the osmotic flows. Here, we show that diffusioosmotic flows can lead to qualitatively distinct outcomes, reversing the direction of colloidal focusing expected from diffusiophoresis alone. Using microfluidic experiments in a T-junction, numerical simulations, and theoretical modeling, we explain our observations to be due to an interplay between diffusiophoretic migration of colloids toward the walls and their entrainment in a diffusioosmotic vortex. We show this focusing to be persistent for a range of salt types, salt gradients, and flow rates, and establish a criterion for its emergence. Our work sheds light on how boundaries modulate the solute-mediated transport of colloids in confined environments and how the colloidal trajectories can be utilized to infer the surface properties.

Bacteria are one of the oldest life forms on Earth, dating back to more than 3.5 billion years ago. They control the global cycling of carbon, nitrogen, and oxygen. They provide plants, fungi and other organisms with the necessary nutrients and elements. They help us digest our food, protect us against pathogens, and even affect our behavior. Microplastics, however, have disrupted the bacterial ecosystems across the globe, from the soil to the oceans. Microplastics are tiny plastic particles formed as a result of the breakdown of the consumer products and plastic waste. Due to their stability and persistence, they can travel long distances in the soil and subsurface environments, ultimately making their way to the water resources, rivers, and oceans. In this journey, they interact with bacteria and other micro/macro-organisms, become ingested or colonized, and act as carriers for contaminants and pathogens. How and whether bacteria adapt to these new microplastic-rich ecosystems are open questions with far-reaching implications for the health of our planet and us. Therefore, there is an urgent need for improving our fundamental understanding of bacterial interactions with the microplastics in complex environments. In this commentary, we focus on the nexus of bacteria, biofilms, and microplastics, also known as the “plastisphere”, and discuss the challenges and opportunities.

Fluid instabilities can be harnessed for facile self-assembly of patterned structures on the nano- and microscale. Evaporative self-assembly from drops is one simple technique that enables a range of patterning behaviors due to the multitude of fluid instabilities that arise due to the simultaneous existence of temperature and solutal gradients. However, the method suffers from limited controllability over patterns that can arise and their morphology. Here, we demonstrate that a range of distinct crystalline patterns including hexagonal arrays, branches, and sawtooth structures emerge from evaporation of water drops containing calcium sulfate on hydrophilic and superhydrophilic substrates. Different pattern regimes emerge as a function of contact line dynamics and evaporation rates, which dictate which fluid instabilities are most likely to emerge. The underlying physical mechanisms behind instability for controlled self-assembly involve Marangoni flows and forced wetting/dewetting. We also demonstrate that these patterns composed of water-soluble inorganic crystals can serve as sustainable and easily removable masks for applications in microscale fabrication.

Solute gradients next to an interface drive a diffusioosmotic flow, the origin of which lies in the intermolecular interactions between the solute and the interface. These flows on the surface of colloids introduce an effective slip velocity, driving their diffusiophoretic migration. In confined environments, we expect the interplay between diffusiophoretic migration and diffusioosmotic flows near the walls to govern the motion of colloids. These near-wall osmotic flows are, however, often considered weak and neglected. Here, using microfluidic experiments in a T-junction, numerical simulations, and theoretical modeling, we show that the interplay between osmotic and phoretic effects leads to unexpected outcomes: forming a colloidal vortex in the absence of inertial effects, and demixing and focusing of the colloids in the direction opposite to what is commonly expected from diffusiophoresis alone. We show these colloidal vortices to be persistent for a range of salt types, salt gradients, and flow rates, and establish a criterion for their emergence. Our work sheds light on how boundaries modulate the solute-mediated transport of colloids in confined environments.

We investigate interfacial instability in a lifting Hele-Shaw cell by experiments and theory. We characterize the unexplored transition from stable to unstable patterns under a wide range of controlling parameters. Surprisingly, we find that the perturbation growth rate-based criterion for the onset of instability from linear stability theory is too strict by over 3 orders of magnitude. To reconcile this striking discrepancy, we propose a new criterion based on perturbation amplitude, which is in excellent agreement with the experimental results. We further show that the fingering pattern evolves to produce a hierarchical fluid structure and derive a theoretical equation to predict the fingering evolution.

During mesenchymal migration, F-actin protrusion at the leading edge and actomyosin contraction determine the retrograde flow of F-actin within the lamella. The coupling of this flow to integrin-based adhesions determines the force transmitted to the extracellular matrix and the net motion of the cell. In tissues, motion may also arise from convection, driven by gradients in tissue-scale surface tensions and pressures. However, how migration coordinates with convection to determine the net motion of cellular ensembles is unclear. To explore this, we study the spreading of cell aggregates on adhesive micropatterns on compliant substrates. During spreading, a cell monolayer expands from the aggregate towards the adhesive boundary. However, cells are unable to stabilize the protrusion beyond the adhesive boundary, resulting in retraction of the protrusion and detachment of cells from the matrix. Subsequently, the cells move upwards and rearwards, yielding a bulk convective flow towards the centre of the aggregate. The process is cyclic, yielding a steady-state balance between outward (protrusive) migration along the surface, and 'retrograde' (contractile) flows above the surface. Modelling the cell aggregates as confined active droplets, we demonstrate that the interplay between surface tension-driven flows within the aggregate, radially outward monolayer flow and conservation of mass leads to an internal circulation.

The fluid-fluid interface is a complex environment for a floating object where the statics and dynamics may be governed by capillarity, gravity, inertia, and other external body forces. Yet, the alignment of these forces in intricate ways might result in beautiful pattern formation and self-assembly of these objects, as in the case of bubble rafts or colloidal particles. While interfacial self-assembly has been explored widely, controlled manipulation of floating objects, e.g. drops, at the fluid-fluid interface still remains a challenge largely unexplored. In this work, we reveal the self-assembly and manipulation of water drops floating at an oil-air interface. We show that the assembly occurs due to electrostatic interactions between the drops and their environment. We highlight the role of the boundary surrounding the system by showing that even drops with a net zero electric charge can self-assemble under certain conditions. Using experiments and theory, we show that the depth of the oil bath plays an important role in setting the distance between the self-assembled drops. Furthermore, we demonstrate ways to manipulate the drops actively and passively at the interface.

drops, at the fluid-fluid interface still remains a challenge largely unexplored. In this work, we reveal the self-assembly and manipulation of water drops floating at an oil-air interface. We show that the assembly occurs due to electrostatic interactions between the drops and their environment. We highlight the role of the boundary surrounding the system by showing that even drops with a net zero electric charge can self-assemble under certain conditions. Using experiments and theory, we show that the depth of the oil bath plays an important role in setting the distance between the self-assembled drops. Furthermore, we demonstrate ways to manipulate the drops actively and passively at the interface.

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The shapes of highly volatile oil-on-water droplets become strongly asymmetric when they are out of equilibrium. The unsaturated organic vapor atmosphere causes evaporation and leads to a strong Marangoni flow in the bath, unlike that previously seen in the literature. Inspecting these shapes experimentally on millisecond and submillimeter time and length scales and theoretically by scaling arguments, we confirm that Marangoni-driven convection in the subphase mechanically stresses the droplet edges to an extent that increases for organic droplets of smaller contact angle and accordingly smaller thickness. The viscous stress generated by the subphase overcomes the thermodynamic Laplace pressure. The oil droplets develop copious regularly spaced fingers, and these fingers develop spike-shaped and branched treelike structures. Unlike this behavior for single-component (surfactant-free) oil droplets, droplets composed of two miscible (surfactant-free) organic liquids develop a rim of the less volatile component along the droplet perimeter, from which jets of monodisperse smaller droplets eject periodically due to the Rayleigh-Plateau instability. When evaporation shrinks droplets to μm size, their shapes fluctuate chaotically, and ellipsoidal shapes rupture into smaller daughter droplets when subphase convection flow pulls them in opposite directions. The shape of the evaporating oil droplets is kneaded and sculpted by vigorous flow in the water subphase.

We present a theoretical study of viscous slug motion inside a microscopically rough capillary tube, where pronounced stick–slip motion can emerge at slow displacement rates. The mathematical description of this intermittent motion can be reduced to a system of ordinary differential equations, which also describe the motion of a pendulum inside a fluid-filled rotating drum. We use this analogy to show that the stick–slip motion transitions to steady sliding at high displacement rates. We characterize this crossover with a simple scaling relation and show that the crossover is accompanied by a shift in the dominant energy dissipation mechanisms within the system.

Droplets of alcohol-based formulations are common in applications from sanitizing sprays to printing inks. However, our understanding of the drying dynamics of these droplets on surfaces and the influence of ambient humidity is still very limited. Here, we report the drying dynamics of picoliter droplets of isopropyl alcohol deposited on a surface under controlled humidity. Condensation of water vapor in the ambient environment onto alcohol droplets leads to unexpectedly complex drying behavior. As relative humidity (RH) increases, we observed a variety of phenomena including enhanced spreading, nonmonotonic changes in the drying time, the formation of pancake-like shapes that suppress the coffee-ring effect, and the formation of water-rich films around an alcohol-rich drop. We developed a lubrication model that accounts for the coupling between the flow field within the drop, the shape of the drop, and the vapor concentration field. The model reproduces many of the experimentally observed morphological and dynamic features, revealing the presence of unusually large spatial compositional gradients within the evaporating droplet and surface-tension-gradient-driven flows arising from water condensation/evaporation at the surface of the droplet. One unexpected feature from the simulation is that water can evaporate and condense concurrently in different parts of the drop, providing fundamental insights that simpler models based on average fluxes lack. We further observed rim instabilities at higher RH that are well-described by a model based on the Rayleigh-Plateau instability. Our findings have implications for the testing and use of alcohol-based disinfectant sprays and printing inks.

A collection of the data and scripts used to produce figures and derive conclusions for: "Aerodynamic interactions of drops on parallel fibers." The DOI number for this paper is 10.1038/s41567-023-02159-4. It is available at the following URL: https://www.nature.com/articles/s41567-023-02159-4. Please see Repo_Contents.pdf for details.

A collection of the data and scripts used to produce figures and derive conclusions for: "Aerodynamic interactions of drops on parallel fibers." The DOI number for this paper is 10.1038/s41567-023-02159-4. It is available at the following URL: https://www.nature.com/articles/s41567-023-02159-4. Please see Repo_Contents.pdf for details.

sequestration to microfluidics. Due to the interactions between the fluids and the solid walls, fluid invasion undergoes a wetting transition from complete displacement at low displacement rates to leaving a film of the defending fluid on the confining surfaces at high displacement rates. While most real surfaces are rough, fundamental questions remain about the type of fluid-fluid displacement that can emerge in a confined, rough geometry. Here, we study immiscible displacement in a microfluidic device with a precisely controlled structured surface as an analogue for a rough fracture. We analyze the influence of the degree of surface roughness on the wetting transition and the formation of thin films of the defending liquid. We show experimentally, and rationalize theoretically, that roughness affects both the stability and dewetting dynamics of thin films, leading to distinct late-time morphologies of the undisplaced (trapped) fluid. Finally, we discuss the implications of our observations for geologic and technological applications.

For almost 200 y, the dominant approach to understand oil-on-water droplet shape and stability has been the thermodynamic expectation of minimized energy, yet parallel literature shows the prominence of Marangoni flow, an adaptive gradient of interfacial tension that produces convection rolls in the water. Our experiments, scaling arguments, and linear stability analysis show that the resulting Marangoni-driven high-Reynolds-number flow in shallow water overcomes radial symmetry of droplet shape otherwise enforced by the Laplace pressure. As a consequence, oil-on-water droplets are sheared to become polygons with distinct edges and corners. Moreover, subphase flows beneath individual droplets can inhibit the coalescence of adjacent droplets, leading to rich many-body dynamics that makes them look alive. The phenomenon of a "vortex halo" in the liquid subphase emerges as a hidden variable.