Research

From fundamental studies in physics, material science and biology, to applications in engineering, the “interface” between two continuous phases includes complex molecular and particulate structures. The mechanical, chemical, thermal and transport properties of such compound interfaces are crucial to the macroscopic response of many systems. Therefore, controlling and optimizing various interfacial transport phenomena remains a canonical problem with strong intellectual interest and broad industrial impacts. We are harnessing various interfacial transport dynamics in complex fluids, as a synergetic strategy to manipulate the physical and chemical properties for engineering soft materials.
This page gives a preview of some of our group's highlighted research. For more information on our group's research, feel free to check our publications page and/or feel free to contact us! We are always interested in discussing interesting ideas and pursuing new collaborations.
Bubble bursting dynamics in nature and engineering

Effect of surface viscoelasticity on top jet drops produce by bursting bubbles
Abstract: Jet drops resulting from bubble bursting at a liquid surface play a key role in various mass transfer processes across the interface, including sea spray aerosol generation and pathogen transmission. However, the impact of structurally compound interfaces, characterized by complex surface rheology introduced by surface-active contaminants, on the jet drop ejection still remains unclear. Here, we experimentally investigate the influence of surface viscoelasticity on the size and velocity of the top jet drops from surface bubble bursting, examining both pure protein and mixed protein–surfactant solutions. We document that for bubble bursting at a pure-protein-laden surface where surface elasticity dominates, the increase in Ec, i.e. the interfacial elastocapillary number as the ratio between the effects of interfacial elasticity and capillarity, efficiently increases the radius and decreases the velocity of the top jet drop, ultimately inhibiting the jet drop ejection.
Publication: Yang, Z., Barbhai, S., Ji, B., Feng, J*., 2024. Soft Matter, 20, 4868.

Daughter oil droplet entrainment by oil-coated bubble bursting
Abstract: Compound bubbles with a liquid coating in another continuous immiscible bulk phase are ubiquitous in a wide range of natural and industrial processes. Their formation, rise and ultimate bursting at the air–liquid interface play crucial roles in the transport and fate of natural organic matter and contaminants. However, the dynamics of compound bubbles has not received considerable attention until recently. Here, we investigate the entrainment of daughter oil droplets in bulk water produced by a bursting oil-coated bubble. We propose a scaling analysis for the daughter oil droplet size that well captures the experimental results for a wide range of oil coating fractions and Ohnesorge numbers of the bulk liquid. Our findings may advance the fundamental understanding of compound bubble bursting and provide guidance and modelling constraints for bubble-mediated contaminant transport in liquids.
Publication: Yang, Z., Ji, B., Feng, J*., 2023. Journal of Fluid Mechanics, 977, A10

Daughter bubble entrainment via bubble bursting at a viscoelastic surface
Abstract: Bubble bursting at liquid surfaces is ubiquitous and plays a key role for the mass transfer across interfaces, impacting global climate and human health. Here, we document an unexpected phenomenon that when a bubble bursts at a viscoelastic surface of a bovine serum albumin solution, a daughter bubble is entrapped with no subsequent jet drop ejection, contrary to the counterpart experimentally observed at a Newtonian surface. We show that the strong surface dilatational elastic stress from the viscoelastic surface retards the cavity collapse and efficiently damps out the precursor waves, and thus facilitates the dominant wave focusing above the cavity nadir. The onset of daughter bubble entrainment is well predicted by an interfacial elastocapillary number comparing the effects of surface dilatational elasticity and surface tension. Our work highlights the important role of surface rheology on free surface flows, and may find important implications in bubble dynamics with a contaminated interface exhibiting complex surface rheology.
Publication: Ji, B., Yang, Z., Ewoldt, R.H., Feng, J*., 2023. Physical Review Letters, 131, 104002.

Enhanced singular jets in oil-coated bubble bursting
Abstract: Bubble-bursting aerosols have a key role in mass and momentum transfer across interfaces. Previous studies report that the bursting of a millimetre-sized bare bubble at an aqueous surface produces jet drops with a typical size on the order of 100 μm. Here we show that jet drops can be as small as a few micrometres when the bursting bubble is coated by a thin oil layer. The faster and smaller jet drops result from the singular dynamics of the oil-coated cavity collapse. The air–oil–water compound interface offers a distinct damping mechanism to smooth out the precursor capillary waves during cavity collapse, leading to a more efficient focusing of the dominant wave and thus allowing singular jets over a much wider parameter space than that of a bare bubble. We develop a theoretical explanation for the parameter limits of the singular jet regime by considering the interplay between inertia, surface tension and viscous effects. Contaminated bubbles are widely observed, therefore previously unrecognized fast and small contaminant-laden jet drops may contribute to the aerosolization and airborne transmission of bulk substances.
Publication: 1) Yang, Z., Ji, B., Ault. J., Feng, J* 2023. Nature Physics, 19, 884–890. 2) Ji, B., Yang, Z., Feng, J*., 2021. Nature Communications, 12, 1-10.

Water-to-air transfer of Nano/Microsized particulates: Enrichment effect in bubble bursting jet drop
Abstract: Bubbles dispersed in liquids are widely present in many natural and industrial processes and play a key role in mediating mass transfer during their lifetime from formation to rising to bursting. In particular, nano/microsized particulates and organisms present in the bulk water can be highly enriched in the jet drops ejected during bubble bursting, impacting global climate and public health. However, the detailed mechanism of this enrichment remains obscure with the enrichment factor being difficult to predict. Here, we experimentally investigate the enrichment of nano/microsized particles in bubble bursting jet drops and highlight the underlying hydrodynamic mechanism, combining the effects of bubble scavenge and bursting on the transport of particles. Scaling laws for the enrichment factor are subsequently proposed that describe both our and prior experimental results reasonably well. Our study may provide new insights for water-to-air transfer of bulk particulates such as microbes related to bubble bursting.
Publication: Ji, B., Singh., A., Feng, J*., 2022. Nano Letters, 22, 5626–5634.
Lipid membrane dynamics in non-equilibrium biological environment

Vesicle dynamics under light-induced oxidation
Abstract: Exposure of cell membranes to reactive oxygen species can cause oxidation of membrane lipids. Oxidized lipids undergo drastic conformational changes, compromising the mechanical integrity of the membrane and causing cell death. For giant unilamellar vesicles, a classic cell mimetic system, a range of mechanical responses under oxidative assault has been observed including formation of nanopores, transient micron-sized pores, and total sudden catastrophic collapse (i.e., explosion). However, the physical mechanism regarding how lipid oxidation causes vesicles to explode remains elusive. Here, with light-induced asymmetric oxidation experiments, the role of spontaneous curvature on vesicle instability and its link to the conformational changes of oxidized lipid products is systematically investigated. A comprehensive membrane model is proposed for pore-opening dynamics incorporating spontaneous curvature and membrane curling, which captures the experimental observations well. The kinetics of lipid oxidation are further characterized and how light-induced asymmetric oxidation generates spontaneous curvature in a non-monotonic temporal manner is rationalized. Using the framework, a phase diagram with an analytical criterion to predict transient pore formation or catastrophic vesicle collapse is provided. The work can shed light on understanding biomembrane stability under oxidative assault and strategizing release dynamics of vesicle-based drug delivery systems.
Publication: Kumar, V., Pak, O., Feng, J*., 2024. Advanced Science, 2400504.

Pore dynamics of lipid vesicles under light-induced osmotic stress
Abstract: Understanding the physical mechanisms governing the response of lipid vesicles under an osmotic imbalance is crucial not only for advancing our knowledge of osmoregulation in living cells but also for guiding the design of biomedical vesicular systems. When placed under osmotic stress, lipid vesicles exhibit a variety of responses, from simple engorgement, to swelling with eventual pore formation, to the only recently observed irreversible explosion triggered by photoreactions. Here, we present a unifying model that incorporates all of these dynamic responses by elucidating the associated energy landscape of vesicle outcomes. We demonstrate the essential, yet previously unrecognized, role of the spontaneous curvature in determining vesicle responses under extreme osmotic stress. We utilize numerical experiments to construct phase diagrams of pore dynamics, which are consistent with the experimental observations, and we further discuss the impacts of compositional lipid properties. Our work not only advances a fundamental understanding of vesicle response in nonequilibrium environments, but also extends the possibility for precise design of vesicle systems regarding controlled release of therapeutic substances in biomedical applications.
Publication: 1) Kumar, V., Pak, O., Feng, J*., 2022. Physical Review Applied, 17. 2) Kumar, V., Shin S., Feng, J.*, 2020. Soft Matter, 16, 8904
Vesicle delivery by diffusiophoresis in dead-end pore geometries
Abstract: Passive targeted drug delivery to solid tumors is driven by the permeation of drug carriers through porous vasculature. Due to a dominant transport mechanism for the entry and migration of the drug carriers being diffusion, the passive drug delivery is relatively slow and ineffective in delivering large carriers to the desired location. Here, we propose a method for delivering liposomes into the interstitium at orders of magnitude faster than the diffusion. Using microfluidic model tumor microenvironment, we show that by exchanging the solutes of the interstitial fluid, the liposomes can respond to the change in the chemistry of the surrounding fluid, thereby penetrating deep into the confined pore space at an accelerated transport rate. In addition, by further exchanging the environment with a hypotonic solution, the delivered liposomes can expel their inner content continuously via periodic osmotic bursting, allowing controlled release of encapsulated molecules in hard-to-reach spaces. Our study suggests an active delivery strategy to enhance the permeation of therapeutic molecules into the interstitium.
Publication: 1) Shin, S., Down, V.S., Feng, J., 2019. Physical Review Applied, 12, 024014. 2) Doan, V., Chun, S., Feng, J., Shin, S., 2021. Nano Letters, 21, 7625-7630.
Multiphase flows involving capillary effects and microscale fluid-structure interaction in environmental and biomedical applications

Transport of an elongated confined bubble moving in non-Newtonian fluids
Abstract: The motion of a long gas bubble in a confined capillary tube is ubiquitous in a wide range of engineering and biological applications. While the understanding of the deposited thin viscous film near the tube wall in Newtonian fluids is well developed, the deposition dynamics in commonly encountered non-Newtonian fluids remains much less studied. Here, we investigate the dynamics of a confined bubble moving in shear-thinning fluids with systematic experiments. The thickness of the deposited liquid film, the bubble speed and the bubble front/rear menisci are measured, which are further rationalized with the recent theoretical studies based on appropriate rheological models. Compared with Newtonian fluids, the film thickness decreases for both the carboxymethyl cellulose and Carbopol solutions when the shear-thinning effect dominates. We show that the film thickness follows the scaling law from Aussillous & Quéré with an effective capillary number Cae, considering the characteristic shear rate in the film. The shear-thinning effect also influences the bubble speed and delays the transition to the parabolic region in the bubble front and rear menisci. Our study may advance the fundamental understandings and engineering guidelines for coating processes involving thin-film flows and non-Newtonian fluids.
Publication: 1) Chun, S., Ji, B., Yang, Z., Kumar, V., Feng, J*, 2022. Journal of Fluid Mechanics, 953, A12. 2) Chun, S., Yang, Z., Feng, J*., 2024. Droplet, e121.

Flow rate-pressure drop relations for non-Newtonian fluids in deformable configurations: theory and experiments
Abstract: We provide an experimental framework to measure the flow rate–pressure drop relation for Newtonian and shear-thinning fluids in two common deformable configurations: (i) a rectangular channel and (ii) an axisymmetric tube. Using the Carreau model to describe the shear-dependent viscosity, we identify the key dimensionless rheological number Cu, which characterizes shear thinning, and we show that our experiments lie within the power-law regime of shear rates. To rationalize the experimental data, we derive the flow rate–pressure drop relation taking into account the two-way-coupled fluid-structure interaction between the flow and its compliant confining boundaries. We thus identify the second key dimensionless number 𝛼, which characterizes the compliance of the conduit. We then compare the theoretical flow rate–pressure drop relation to our experimental measurements, finding excellent agreement between the two. We further contrast our results for shear-thinning and Newtonian fluids to highlight the influence of Cu on the flow rate–pressure drop relation. .
Publication: Chun, S., Boyko, E., Christov, I. C., Feng, J*., 2024. Physical Review Fluids, 9, 043302.
Polymer self-assembly and nanoparticles for therapeutic delivery and diagnostic
With significant attention focused on nanoscience in recent years, nanomaterial-based drug delivery has been propelled to the forefront by researchers from both academia and industry. However, a lot of new developed therapeutics for diseases cannot be moved into clinical trials since the new compounds are highly hydrophobic and difficult to dose. Using the Bubble Bursting and Flash Nanoprecipitation Platforms based on rapid mixing and polymer self-assembly, our lab is developing a versatile, low-cost and scalable protocol to prepare difficult-to-deliver therapeutics into polymeric nanomedicines. Our lab is working closely with people from pharmaceutical chemistry for various therapies and bioimaging agents, which are difficult to delivery in traditional formulations. In addition, with microfluidics, our lab is capable of generating composite microparticles encapsulating nanoparticles with polymer scaffolds to provide an additional barrier for sustained release. Our lab is also interested in studying how the drug-loaded polymeric nanoparticles transport through the human body. The wide compatibility for various materials and the multimodal imaging properties of the nanoparticles by our platforms will enable us to better observe the dynamics for optimum characteristics for enhanced delivery. Representative publications are:

Transport of an elongated confinedbubble moving in non-Newtonian fluids confined by a circular channel
Abstract: Abamectin has been widely used as a biopesticide to control pests, such as pinewood and root-knot nematodes. However, the low aqueous solubility and poor photostability of abamectin have largely hindered the delivery and bioavailability of conventional formulations, causing a significant rise in costs as well as limiting the large-scale application in crop production. Here, we report the formulation of abamectin nanopesticides for extended photostability and sustained release, using a continuous and scalable technique called Flash NanoPrecipitation (FNP). Cost-effective and biocompatible stabilizers, hypromellose acetate succinate (HPMCAS) and lecithin, were used in the formulation with optimized mass ratios for nanoparticle stability. The encapsulation efficiency of abamectin in nanoparticles was higher than 90% with a mass loading capacity up to 50%. The optimized formulations not only showed improved photostability compared with free abamectin but also achieved more sustained release behaviors in a controlled manner. These results suggest that the abamectin nanoparticles formulated by FNP can improve the performance of the aqueous solubility, photostability, and controlled release of abamectin. In addition, our work highlights the potential of FNP for the mass production of low-cost abamectin nanopesticides with affordable materials, which is essential for the large-scale application of nanopesticides in food and agriculture.
Publication: Chun, S., Feng, J.*, 2021. ACS Applied Nano Materials, 4, 1228–1234.

Translational Formulation of Nanoparticle Therapeutics from Laboratory Discovery to Clinical Scale
Abstract: Abamectin has been widely used as a biopesticide to control pests, such as pinewood and root-knot nematodes. However, the low aqueous solubility and poor photostability of abamectin have largely hindered the delivery and bioavailability of conventional formulations, causing a significant rise in costs as well as limiting the large-scale application in crop production. Here, we report the formulation of abamectin nanopesticides for extended photostability and sustained release, using a continuous and scalable technique called Flash NanoPrecipitation (FNP). Cost-effective and biocompatible stabilizers, hypromellose acetate succinate (HPMCAS) and lecithin, were used in the formulation with optimized mass ratios for nanoparticle stability. The encapsulation efficiency of abamectin in nanoparticles was higher than 90% with a mass loading capacity up to 50%. The optimized formulations not only showed improved photostability compared with free abamectin but also achieved more sustained release behaviors in a controlled manner. These results suggest that the abamectin nanoparticles formulated by FNP can improve the performance of the aqueous solubility, photostability, and controlled release of abamectin. In addition, our work highlights the potential of FNP for the mass production of low-cost abamectin nanopesticides with affordable materials, which is essential for the large-scale application of nanopesticides in food and agriculture.
Publication: 1) Feng, J., Markwalter C.E., Tian C., Armstrong, M. and Prud’homme, R.K., 2019. Journal of Translational Medicine, 17, 200. 2) Feng, J., Zhang, Y., McManus, S.A., Qian, R., Ristroph, K.D., Ramachandruni, H., Gong, K., White, C.E. Rawal, A. and Prud’homme, R.K., 2019. Soft Matter, 15, 2400-2410.