Publications

Updated on 12/22/2021

Inception of forest fire front: model experiments and theory

Authors:

A.L. Yarin, W. Zhang, E. Zussman

Abstract:

To quantify how inter-source spacing (“spatial step”), inclination angle (slope), and wind affect flame-propagation velocity, model experiments were conducted in the present work. Propagation velocities and flame morphologies were quantified and visualized. The measured dependence of velocity on wind speed, slope, and source spacing is then used to formulate and illustrate a theory for forest-fire flame-front propagation affected by wind over flat and hilly terrains. The results demonstrate dramatic effect of wind on flame propagation and the overall configuration of the flame front, as well as a significant difference between highly volatile and flammable versus less flammable fuels.

The model with continuous flame front developed in the present work approximates the cases where the tree-to-tree distances and the characteristic flame transfer times between trees are much smaller and shorter, respectively, than the sizes and evolution times of forest fire. The model implies that the flame propagation is sustained by volatiles rapidly released by plants in fire, which sustain fire propagation serving as a fuel mixing in a thin flame front with oxidizer from the ambient air like in diffusion flames. It is also implied that the flame front can be considered as continuous on its scale which is much larger than the tree-to-tree distances (that means the so-called diffusion approximation in the mathematical sense). The theoretical predictions reveal that the flame front configuration is strongly affected by landscape topography and wind, much more than by the initial ignition configuration.

It should be emphasized that (i) the model lab-scale experiments here only elucidate on the physical level the main effects which are to be expected from the large-scale experiments: that flame propagation velocity strongly depends on fuel volatility, spacing between sources, terrain inclination, and wind presence. (ii) The large-scale predictive capability of the quasi-physical theoretical/numerical model developed here is still to be verified by fitting the two lumped parameters of the model to large-scale data. The latter is currently hardly possible because the available published data on distillation stage of forest fires are scarce and typically lacking many important details in their totality, like slopes and wind speed, the type of wood involved, the volatile and water content in the wood, etc. (iii) Accordingly, the present numerical results, albeit some of them were obtained for a real kilometer-scale landscape, are illustrative in nature. Still, in future widening of a detailed experimental data bank and its interpretation by such methods as Machine Learning and AI will allow a reliable and detailed verification of the present quasi-physical theoretical/numerical approach. That will yield potentially impactful insights into occurrence of forest fires around urban areas as well as of urban fires, with the present quasi-physical model being used for monitoring, advanced planning and mitigation.

International Journal of Heat and Mass Transfer, 262, 128379 (2026)

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Capillary self-thinning of threads of polyelectrolyte solutions with axial electric field

Authors:

P. Martin, G. Vasilyev, E. Zussman, A.L. Yarin

Abstract:

A theory of solutions of charged polyelectrolyte macromolecules treating them as electric dipoles is proposed. In a thin thread of polyelectrolyte solution sustained between two disks—whether wettable or non-wettable—these dipoles are reoriented by an axial electric field, aligning themselves with the field direction. This alignment causes significant axial elastic stresses and affects capillary self-thinning dynamics of the thread, slowing the process and potentially arresting it entirely, and even leading to oscillatory regimes. Accordingly, the evolution of the thread radius deviates significantly from the exponential decay characteristic of solutions of flexible polymer macromolecules and the linear decay characteristic of Newtonian fluids. At relatively high electric field strengths, in a thread where elastic stresses become dominant, an oscillatory regime emerges. Here, the cross-sectional radius grows and oscillates in time. We attribute this phenomenon to a case of Hadamard instability. The theory is supported by experimental data acquired in this work.

PNAS (Proceedings of the National Academy of Sciences of the United States of America) 122(43), e2422879122 (2025).

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Rheological behavior of brain tissue: Experiments versus theory and forensic applications

Authors:

V. Kosmerl, S.A. Bentil, A.L. Yarin

Abstract:

Experimental data describing the uniaxial compression and relaxation of brain tissue is compared to the predictions from a rheological model developed by Yarin and Kosmerl (2023). A qualitative agreement between the model and experiments with swine brain tissue is confirmed, and the uniformly-valid values (i.e., valid in all rheometric experiments without any change) of the rheological parameters are established. These are the values of the following four parameters: G (the shear modulus), κ (the bulk modulus), α (the dimensionless degree of hyperelasticity), and θ (the viscoelastic relaxation time). In addition, the present rheological model with the established rheological parameters is incorporated into a dynamic model of bullet penetration into brain tissue after a short-range shooting, when muzzle gases and/or air fill the bullet channel leading to its widening, wave propagation, fragmentation and backspatter of brain tissue. This problem is of significant interest in forensic science because there is a urgent need to provide physics-informed models to reconstruct and analyze crime scenes.

Phys. Fluids 36, 051904 (2024)

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Drop impact onto a substrate wetted by another liquid: Corona detachment from the wall film

Authors:

Stumpf, I.V. Roisman, A.L. Yarin, C. Tropea

Abstract:

Drop impact onto a thin liquid film of another liquid is observed and characterized using a highspeed video system. A new mode of splash – a complete, simultaneous corona detachment -has been observed, which is the result of the lamella breakup near the wall film. The abrupt outward and upward displacement of the lamella leads to an extreme stretching of the corona wall, resulting in its rapid thinning and a rupture. This rupture triggers propagating Taylor-Culick rims, which rapidly spread, meet and thus undercut simultaneously the entire corona, resulting in its detachment. Special experiments with the spreading corona impingement onto a fixed needle, supplement the physical evidence of the above-mentioned mechanism. A self-consistent theory of the observed phenomena is proposed and compared with experiments, exhibiting good agreement.

Journal of Fluid Mechanics, 956, A10, (2023)

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Electrospun membranes filtering 100 nm particles from air flow by means of the van der Waals and Coulomb forces

Authors:
Chen, J. Wu, A.L. Yarin

Abstract:

Nonwoven fibrous filter membranes are widely used in filtration because of their low cost. They are less effective in intercepting airborne particles of the order of 100 nm, which is of the SARS-CoV-2 (COVID-19) virus’s size.  Many diseases, including COVID-19, predominantly spread by droplets released by breathing, coughing, sneezing, or medical procedures. It was shown that the smallest droplets can evaporate in air before settling, thus, making viruses airborne and easily penetrating even the best masks and filters. As a result, air-filtering membranes, which are capable of effectively interception of ~100 nm nanoparticles are highly desirable. A traditional way to improve filtration efficiency by overlapping several layers of nonwoven fabrics increases the required pressure drop, and thus, should be avoided as much as possible. Here, we propose and demonstrate an innovated approach to enhance performance of filtration membranes based on (i) a dramatic reduction in the fiber size, and (ii) metal coating of the fibers. The first component of this approach allows one to incorporate a novel physical mechanism of filtration, the short-range van der Waals forces, whereas the second one adds the long-range electric Coulomb forces if the oncoming nanoparticles are pre-charged and the metal-plated membrane grounded. In the present work, the ~ 100 nm Aluminum nanoparticles are filtered as a model of commensurate airborne single COVID-19 viruses, and Platinum is used as the sputter-coated material for the fiber coating. The resulting filtration efficiency enhanced by the electric Coulomb forces alone is increased by the factor of 1.77, while the filtration efficiency additionally facilitated by the van der Walls forces increased by the factor of 2.44. In comparison to the filter membranes with ~500 nm fibers without the electric forces involved, the van-der-Waals-electric filter membrane with fibers ~90 nm is 2.24 1.77=3.96 times more effective. The quality factor of a membrane which combines the van der Waals and Coulomb forces is 10.6 psi-1, which almost three times that of a comparable membrane without the electric Coulomb force (with only van der Waals forces being used).

Journal of Membrane Science, 644, 120138 (2022)

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Blood backspatter interaction with propellant gases

Authors

Gen Li, Nathaniel Sliefert, James B. Michael, Alexander L. Yarin

Abstract

The theoretical results of the present work reveal a significant interaction of the oncoming vortex ring of propellant muzzle gases with backward blood spatter. It is shown that there is even possibility that a blood droplet from the backspatter will fully turn around by a powerful vortex ring and land behind a victim. Such a predicted outcome is confirmed by experimental data of fully reversed drop trajectories observed in the experiments conducted in the second part of this work [N. Sliefert, G. Li, J. B. Michael, A. L. Yarin, “Experimental and numerical study of blood backspatter interaction with propellant gases,” Phys. Fluids 33, 043319 (2021)]. A parametric study is conducted here to investigate the totality of the outcomes of the vortex ring interaction with the backward blood spatter and the corresponding deflections and landing locations of blood drops. Furthermore, a secondary vortex ring is introduced here to reveal a continuous effect of the propellant gas.

 

 

Phys of Fluids 33, 043318 (2021)

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Reopening dentistry after COVID-19

Authors

Jevon Plog, Jingwei Wu, Yasmin J Dias, Farzad Mashayek, Lyndon F Cooper, Alexander L Yarin

Abstract

The aerosol transmissibility of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has impacted the delivery of health care and essentially stopped the provision of medical and dental therapies. Dentistry uses rotary, ultrasonic, and laser-based instruments that produce water-based aerosols in the daily, routine treatment of patients. Abundant aerosols are generated, which reach health care workers and other patients. Viruses, including SARS-CoV-2 virus and related coronavirus disease (COVID-19) pandemic, continued expansion throughout the USA and the world. The virus is spread by both droplet (visible drops) and aerosol (practically invisible drops) transmission. The generation of aerosols in dentistry-an unavoidable part of most dental treatments-creates a high-risk situation. The US Centers for Disease Control and The Occupational Safety and Health Administration consider dental procedures to be of “highest risk” in the potential spreading of SARS-CoV-2 and other respiratory viruses. There are several ways to reduce or eliminate the virus: (i) cease or postpone dentistry (public and personal health risk), (ii) screen patients immediately prior to dental treatment (by appropriate testing, if any), (iii) block/remove the virus containing aerosol by engineering controls together with stringent personal protective equipment use. The present work takes a novel, fourth approach. By altering the physical response of water to the rotary or ultrasonic forces that are used in dentistry, the generation of aerosol particles and the distance any aerosol may spread beyond the point of generation can be markedly suppressed or completely eliminated in comparison to water for both the ultrasonic scaler and dental handpiece.

 

 

 

Physics of Fluids 32, 083111 (2020)

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Electrospun membranes filtering 100 nm particles from air flow

Authors

Kailin Chen, Jingwei Wu,  Alexander L. Yarin

Abstract
Nonwoven fibrous filter membranes are widely used in filtration because of their low cost. They are less effective in intercepting airborne particles of the order of 100 nm, which is of the SARS-CoV-2 (COVID-19) virus’s size. Many diseases, including COVID-19, predominantly spread by droplets released by breathing, coughing, sneezing, or medical procedures. It was shown that the smallest droplets can evaporate in air before settling, thus, making viruses airborne and easily penetrating even the best masks and filters. As a result, air-filtering membranes, which are capable of effective interception of ∼100 nm nanoparticles are highly desirable. A traditional way to improve filtration efficiency by overlapping several layers of nonwoven fabrics increases the required pressure drop, and thus, should be avoided as much as possible. Here, we propose and demonstrate an innovative approach to enhance performance of filtration membranes based on (i) a dramatic reduction in the fiber size, and (ii) metal coating of the fibers. The first component of this approach allows one to incorporate a novel physical mechanism of filtration, the short-range van der Waals forces, whereas the second one adds the long-range electric Coulomb forces if the oncoming nanoparticles are pre-charged and the metal-plated membrane grounded. In the present work, the ∼100 nm aluminum nanoparticles are filtered as a model of commensurate airborne single COVID-19 viruses, and Platinum is used as the sputter-coated material for the fiber coating. The resulting filtration efficiency enhanced by the electric Coulomb forces alone is increased by the factor of 1.77, while the filtration efficiency additionally facilitated by the van der Waals forces increased by the factor of 2.44. In comparison to the filter membranes with ∼500 nm fibers without the electric forces involved, the van-der-Waals-electric filter membrane with fibers ∼90 nm is 2.24 1.77 = 3.96 times more effective. The quality factor of a membrane which combines the van der Waals and Coulomb forces is 10.6 psi−1, which is almost three times that of a comparable membrane without the electric Coulomb force (with only van der Waals forces being used).

 

 

Journal of Membrane Science (2021): 120138.

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Evolution of Drops under the Surface Tension and Electric Field

Authors

Rafael Granda, Jevon Plog, Gen Li, Vitaliy Yurkiv, Farzad Mashayek, Alexander L. Yarin

Abstract
The creeping-flow theory describing evolution and steady-state shape of two-dimensional ionic-conductor drops under the action of surface tension and the subcritical (in terms of the electric Bond number) electric field imposed in the substrate plane is developed. On the other hand, the experimental data are acquired for drops impacted or softly deposited on dielectric surfaces of different wettability and subjected to an in-plane subcritical electric field. Even though the experimental situation involves viscous friction of drops with the substrates and wettability-driven motion of the contact line, the comparison to the theory reveals that it can accurately describe the steady-state drop shape on a non-wettable substrate. In the latter case, the drop is sufficiently raised above the substrate, which diminishes the three-dimensional effects, making the two-dimensional description (lacking the no-slip condition at the substrate and wettability-driven motion of the contact line) relevant. Accordingly, it is demonstrated how the subcritical electric field deforms the initially circular drops until an elongated steady-state configuration is reached. In particular, the surface tension tends to round off the non-circular drops stretched by the electric Maxwell stresses imposed by the electrodes. A more pronounced substrate wettability leads to more elongated steady-state configurations observed experimentally than those predicted by the two-dimensional theory. The latter cases reveal significant three-dimensional effects in the electrically driven drop stretching. In the supercritical electric fields (corresponding to the supercritical electric Bond numbers), the electrical stretching of drops predicted by the present linearized two-dimensional theory results in splitting into two separate droplets. This scenario is corroborated by the predictions of the fully nonlinear results for similar electrically stretched bubbles in the creeping-flow regime available in the literature as well as by the present experimental results on a substrate with slip.

 

 

Langmuir 37.39 (2021): 11429-11446.

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