1. Editorial: Fluid flows with interactive boundaries
    H. Masoud and A. M. Ardekani, European Journal of Computational Mechanics 26, 1-3 (2017)
  2. One of the most fascinating and non-intuitive class of problems in fluid mechanics are those involving the interplay between dynamic boundaries and fluid flows. The coupling between the dynamics of the fluid and the kinematics and mechanics of the boundaries in these situations often gives rise to unexpected behaviours that are of paramount importance in many engineering, biological and biomedical contexts. The study of problems involving fluid flows with interactive boundaries have been the focus of the field in the past two decades, to the extent that many examples of such problems have frequently appeared on the cover of recent editions of fluid mechanics journals and textbooks. This relatively new and exciting frontier in fluid mechanics is also highly interdisciplinary and has attracted many engineers and scientists alike.


  1. Reverse Marangoni surfing
    V. Vandadi, S. Jafari Kang, and H. Masoud, Journal of Fluid Mechanics 811, 612-621 (2017)
  2. We theoretically study the surfing motion of chemically and thermally active particles located at a flat liquid-gas interface that sits above a liquid layer of finite depth. The particles' activity creates and maintains a surface tension gradient resulting in the auto-surfing. It is intuitively perceived that Marangoni surfers propel towards the direction with a higher surface tension. Remarkably, we find that the surfers may propel in the lower surface tension direction depending on their geometry and proximity to the bottom of the liquid layer. In particular, our analytical calculations for Stokes flow and diffusion-dominated scalar fields (i.e. chemical concentration and temperature fields) indicate that spherical particles undergo reverse Marangoni propulsion under confinement whereas disk-shaped surfers always move in the expected direction. We extend our results by proposing an approximate formula for the propulsion speed of oblate spheroidal particles based on the speeds of spheres and disks.




  1. Alternative mechanism for coffee-ring deposition based on active role of free surface
    S. Jafari Kang, V. Vandadi, J. D. Felske, and H. Masoud, Physical Review E 94, 063104 (2016)
    (Highlighted in Materials Today, Nevada Today, Phys.org, Science Daily, Membrane Quarterly, and Nanowerk)
  2. When a colloidal sessile droplet dries on a substrate, the particles suspended in it usually deposit in a ring-like pattern. This phenomenon is commonly referred to as the "coffee-ring" effect. One paradigm for why this occurs is as a consequence of the solutes being transported towards the pinned contact line by the flow inside the drop, which is induced by surface evaporation. From this perspective, the role of the liquid-gas interface in shaping the deposition pattern is somewhat minimized. Here, we propose an alternative mechanism for the coffee-ring deposition. It is based on the bulk flow within the drop transporting particles to the interface, where they are captured by the receding free surface and subsequently transported along the interface until they are deposited near the contact line. That the interface captures the solutes as the evaporation proceeds is supported by a Lagrangian tracing of particles advected by the flow field within the droplet. We model the interfacial adsorption and transport of particles as a one-dimensional advection-generation process in toroidal coordinates and show that the theory reproduces ring-shaped depositions. Using this model, deposition patterns on both hydrophilic and hydrophobic surfaces are examined, in which the evaporation is modelled as being either diffusive or uniform over the surface.



  1. Reciprocal theorem for convective heat and mass transfer from a particle in Stokes and potential flows
    V. Vandadi, S. Jafari Kang, and H. Masoud, Physical Review Fluids (Rapid Communications) 1, 022001(R) (2016)
  2. In the study of convective heat and mass transfer from a particle, key quantities of interest are usually the average rate of transfer and mean distribution of the scalar (i.e. temperature or concentration) at the particle surface. Calculating these quantities using conventional equations requires the detailed knowledge of the scalar field, which is available predominantly for problems involving uniform scalar and flux boundary conditions. Here, we derive a reciprocal relation between two diffusing scalars that are advected by oppositely driven Stokes or potential flows whose streamline configurations are identical. This relation leads to alternative expressions for the aforementioned average quantities based on the solution of the scalar field for uniform surface conditions. We exemplify our results via two applications: (i) heat transfer from a sphere with non-uniform boundary conditions in Stokes flow at small Peclet numbers, and (ii) extension of Brenner's theorem for the invariance of heat transfer rate to flow reversal.



  1. Oscillatory Marangoni flows with inertia
    O. Shardt, H. Masoud, and H. A. Stone, Journal of Fluid Mechanics 803, 94-118 (2016)
  2. When the surface of a liquid has a non-uniform distribution of a surfactant that lowers surface tension, the resulting variation in surface tension drives a flow that spreads the surfactant towards a uniform distribution. We study the spreading dynamics of an insoluble and non-diffusing surfactant on an initially motionless liquid. We derive solutions for the evolution over time of sinusoidal variations in surfactant concentration with a small initial amplitude relative to the average concentration. In this limit, the coupled flow and surfactant transport equations are linear. In contrast to exponential decay when the inertia of the flow is negligible, the solution for unsteady Stokes flow exhibits oscillations when inertia is sufficient to spread the surfactant beyond a uniform distribution. This oscillatory behaviour exhibits two properties that distinguish it from that of a simple harmonic oscillator: the amplitude changes sign at most three times, and the decay at late times follows a power law with an exponent of −3/2. As the surface oscillates, the structure of the subsurface flow alternates between one and two rows of counter-rotating vortices, starting with one row and ending with two during the late-time monotonic decay. We also examine numerically the evolution of the surfactant distribution when the system is nonlinear due to a large initial amplitude.



  1. Drag and diffusion coefficients of a spherical particle attached to a fluid-fluid interface
    A. Dörr, S. Hardt, H. Masoud, and H. A. Stone, Journal of Fluid Mechanics 790, 607-618 (2016)
  2. Explicit analytical expressions for the drag and diffusion coefficients of a spherical particle attached to the flat interface between two immiscible fluids are constructed for the case of a vanishing viscosity ratio between the fluid phases. The model is designed to account explicitly for the dependence on the contact angle between the two fluids and the solid surface. The Lorentz reciprocal theorem is applied in the context of geometric perturbations from the limiting cases of 90o and 180o contact angles. The model agrees well with the experimental and numerical data from the literature. Also, an advantage of the method utilized is that the drag and diffusion coefficients can be calculated up to one order higher in the perturbation parameter than the known velocity and pressure fields. Extensions to other particle shapes with known velocity and pressure fields are straightforward.



  1. Hydrodynamic schooling of flapping swimmers
    A. Becker*, H. Masoud*, J. Newbolt, M. J. Shelley, and L. Ristroph, Nature Communications 6, 8514 (2015)
    (*Authors of equal contribution)
    (Highlighted in National Science Foundation News, APS Physics Central Podcast, Science Daily, and Futurity)
  2. Fish schools and bird flocks are fascinating examples of collective behaviours in which many individuals generate and interact with complex flows. Motivated by animal groups on the move, here we explore how the locomotion of many bodies emerges from their flow-mediated interactions. Through experiments and simulations of arrays of flapping wings that propel within a collective wake, we discover distinct modes characterized by the group swimming speed and the spatial phase shift between trajectories of neighbouring wings. For identical flapping motions, slow and fast modes coexist and correspond to constructive and destructive wing–wake interactions. Simulations show that swimming in a group can enhance speed and save power, and we capture the key phenomena in a mathematical model based on memory or the storage and recollection of information in the flow field. These results also show that fluid dynamic interactions alone are sufficient to generate coherent collective locomotion, and thus might suggest new ways to characterize the role of flows in animal groups.






  1. Mobility of membrane-trapped particles
    H. A. Stone and H. Masoud, Journal of Fluid Mechanics 781, 494-505 (2015)

    Rheological and transport studies of model thin films and membranes, often inspired by biological systems, make use of translational or rotational motion or diffusion of particles trapped in the surface film. Here, we consider the translational mobility of spherical and oblate spheroidal particles protruding into the surrounding subphase liquid. Both the subphase and surface film contribute to the resistance experienced by the particle, which is calculated as a function of the degree of protrusion as well as the viscosity contrast between the surface film and the surrounding fluid. The calculations are based on a combination of a perturbation expansion involving the particle shape and the Lorentz reciprocal theorem.




  1. Collective surfing of chemically active particles
    H. Masoud and M. J. Shelley, Physical Review Letters 112, 128304 (2014)
    (Highlighted as PRL Editors' Suggestion)

    We study theoretically the collective dynamics of immotile particles bound to a 2D surface atop a 3D fluid layer. These particles are chemically active and produce a chemical concentration field that creates surface-tension gradients along the surface. The resultant Marangoni stresses create flows that carry the particles, possibly concentrating them. For a 3D diffusion-dominated concentration field and Stokesian fluid we show that the surface dynamics of active particle density can be determined using nonlocal 2D surface operators. Remarkably, we also show that for both deep or shallow fluid layers this surface dynamics reduces to the 2D Keller-Segel model for the collective chemotactic aggregation of slime mold colonies. Mathematical analysis has established that the Keller-Segel model can yield finite-time, finite- mass concentration singularities. We show that such singular behavior occurs in our finite-depth system, and study the associated 3D flow structures.


  1. A reciprocal theorem for Marangoni propulsion
    H. Masoud and H. A. Stone, Journal of Fluid Mechanics (Rapids) 741, R4 (2014)

    We study the Marangoni propulsion of a spheroidal particle located at a liquid–gas interface. The particle asymmetrically releases an insoluble surface-active agent and so creates and maintains a surface tension gradient leading to the self-propulsion. Assuming that the surface tension has a linear dependence on the concentration of the released agent, we derive closed-form expressions for the translational speed of the particle in the limit of small capillary, Péclet and Reynolds numbers. Our derivations are based on the Lorentz reciprocal theorem, which eliminates the need to develop the detailed flow field.


  1. On the rotation of porous ellipsoids in simple shear flows
    H. Masoud, H. A. Stone, and M. J. Shelley, Journal of Fluid Mechanics (Rapids) 733, R6 (2013)

    We study theoretically the dynamics of porous ellipsoids rotating in simple shear flows. We use the Brinkman–Debye–Bueche (BDB) model to simulate flow within and through particles and solve the coupled Stokes–BDB equations to calculate the overall flow field and the rotation rate of porous ellipsoids. Our results show that the permeability has little effect on the rotational behaviour of particles, and that Jeffery’s prediction of the angular velocity of impermeable ellipsoids in simple shear flows (Proc. R. Soc. Lond. A, vol. 102, 1922, pp. 161–179) remains an excellent approximation, if not an exact one, for porous ellipsoids. Employing an appropriate scaling, we also present approximate expressions for the torque exerted on ellipses and spheroids rotating in a quiescent fluid. Our findings can serve as the basis for developing a suspension theory for non-spherical porous particles, or for understanding the orientational diffusion of permeable ellipses and spheroids.

  1. Designing maneuverable micro-swimmers actuated by responsive gel
    H. Masoud, B. I. Bingham and A. Alexeev, Soft Matter 8, 8944 (2012)
    (Highlighted in Georgia Tech's Homepage, IEEE Computer Society News, ASME Nanotechnology Institute News, Soft Matter World Newsletter, Human Health and Science, Gizmag, Phys.Org, Georgia Tech College of Engineering, Science Daily, MedGadget, Communications of the ACM, and Newswise)

    We use computational modeling to design a self-propelling micro-swimmer that can navigate in a low-Reynolds-number environment. Our simple swimmer consists of a responsive gel body with two propulsive flaps attached to its opposite sides and a stimuli-sensitive steering flap at the swimmer front end. The responsive gel body undergoes periodic expansions and contractions leading to a time-irreversible beating motion of the propulsive flaps, which propels the micro-swimmer. We examine the effects of body elasticity and flap geometry on the locomotion of the swimmer and show how they can be tailored to optimize the swimmer propulsion. We also probe how the swimmer trajectory can be controllably changed using the steering flap that bends when exposed to an external stimulus. We demonstrate that the steering occurs due to two effects: steering flap bending and periodic beating. Furthermore, our simulations reveal that the turning direction can be regulated by changing the stimulus strength.



  1. Controlled release of nanoparticles and macromolecules from responsive microgel capsules
    H. Masoud and A. Alexeev, ACS Nano 6, 212 (2012)

    Using a mesoscale computational model, we probe the release of nanoparticles and linear macromolecules from hollow microgel capsules that swell and deswell in response to external stimuli. Our simulations reveal that responsive microcapsules can be effectively utilized for steady and pulsatile release of encapsulated solutes. Swollen gel capsules allow steady, diffusive release of nanoparticles and polymer chains, whereas gel deswelling causes burst-like discharge of solutes driven by an outward flow of the solvent enclosed within a shrinking capsule. We demonstrate that this hydrodynamic release can be regulated by introducing rigid microscopic rods in the capsule interior. Thus, our findings disclose an efficient approach for controlled release from stimuli-responsive microcarriers that could be useful for designing advanced drug delivery systems.



  1. Efficient flapping flight using flexible wings oscillating at resonance
    H. Masoud and A. Alexeev, In Natural Locomotion in Fluids and on Surfaces, Edited by S. Childress, A. E. Hosoi, W. W. Schultz, and Z. J. Wang, pp. 235-245, Springer, New York (2012)

    We use fully-coupled three-dimensional computer simulations to examine aerodynamics of elastic wings oscillating at resonance. Wings are modeled as planar elastic plates plunging sinusoidally at a low Reynolds number. The wings are tilted from horizontal, thereby generating asymmetric flow patterns and non-zero net aerodynamic forces. Our simulations reveal that resonance oscillations of elastic wings drastically enhance aerodynamic lift, thrust, and efficiency. We show that flexible wings driven at resonance by a simple harmonic stroke generate lift comparable to that of small insects that employ a significantly more complicated stroke kinematics. The results of our simulations point to the feasibility of using flexible resonant wings with a simple stroke for designing efficient microscale flying vehicles.


  1. Harnessing synthetic cilia to regulate motion of microparticles
    H. Masoud and A. Alexeev, Soft Matter 7, 8702 (2011)
    (Invited "Highlight" article)

    Functional synthetic cilia lining solid surfaces could potentially yield a unique approach for regulating transport processes at interfaces. We use computer simulations to probe how non-motile and actuated cilia can be harnessed to control the motion of microscopic particles suspended in a Newtonian fluid. We show that biomimetic cilia can be arranged to create hydrodynamic currents that can either direct particles towards the ciliated surface or expel them away, thereby modifying the effective interactions between solid surfaces and particulates. In addition to revealing new approaches for regulating the microscale particle transport, our findings point to a new strategy for creating functional materials that employ active and responsive synthetic cilia.



  1. Selective control of surface properties using hydrodynamic interactions
    H. Masoud and A. Alexeev, Chemical Communications 47, 472 (2011)
    (Invited paper for Emerging Investigators themed issue)
    (Highlighted in the Virtual Journal of Nanoscale Science & Technology 22, 25, 2010)

    Using computational modeling, we design nano-structured surfaces able to selectively regulate interactions between microchannel walls and flowing colloid–polymer suspensions. Depending on the geometry of nanoscopic posts lining internal channel surfaces, suspended nanoparticles and polymeric chains can be either hydrodynamically attracted to channel walls or repelled to the bulk fluid.



  1. Permeability and diffusion through mechanically deformed random polymer networks
    H. Masoud and A. Alexeev, Macromolecules 43, 10117 (2010)

    We develop a hybrid computational method to examine the permeation and hindered diffusion through mechanically loaded (anisotropic) polymer networks. We use the bond-bending lattice spring model to capture the micromechanics of random networks of interconnected filaments coupled with the dissipative particle dynamics to explicitly model the viscous fluid and diffusive objects. Our simulations reveal that the transport properties are independent of the network internal organization and are solely function of the network porosity and degree of anisotropy due to a mechanical deformation. Furthermore, our results indicate that the network permeability under load can be estimated based on the network alignment that is characterized by a second order orientation tensor. Our findings have implications for designing drug delivery agents, paper manufacturing, tissue engineering, and understanding the function of biological systems.



  1. Resonance of flexible flapping wings at low Reynolds number
    H. Masoud and A. Alexeev, Physical Review E 81, 056304 (2010)
    (Highlighted in Georgia Tech's Homepage, National Science Foundation News, US News & World Report, The Tech File, Georgia Tech College of Engineering, Innovations Report, ScienceMagNews, Gizmag, and Space Daily)

    Using three-dimensional computer simulations, we examine hovering aerodynamics of flexible planar wings oscillating at resonance. We model flexible wings as tilted elastic plates whose sinusoidal plunging motion is imposed at the plate root. Our simulations reveal that large-amplitude resonance oscillations of elastic wings drastically enhance aerodynamic lift and efficiency of low-Reynolds-number plunging. Driven by a simple sinusoidal stroke, flexible wings at resonance generate a hovering force comparable to that of small insects that employ a very efficient but much more complicated stroke kinematics. Our results indicate the feasibility of using flexible wings driven by a simple harmonic stroke for designing efficient microscale flying machines.


  1. Modeling magnetic microcapsules that crawl in microchannels
    H. Masoud and A. Alexeev, Soft Matter 6, 794 (2010)
    (Invited paper for Emerging Themes in Soft Matter: Responsive and Active Soft Materials issue)
    (Highlighted in the Virtual Journal of Nanoscale Science & Technology 21, 9, 2010)

    Using computational modeling, we probe how to design microfluidic systems in which magnetic microcapsules could autonomously crawl along channel walls. The polymeric microcapsules are fluid-filled elastic shells with embedded superparamagnetic nanoparticles and, thereby, can be controlled by external magnetic fields. We show that when a magnetic force circulates normal to a sticky microchannel wall, capsules can propel in a steady, autonomous manner. The unidirectional capsule propulsion is facilitated by hydrodynamic interactions between the capsules and channel walls, and is most effective when the magnitudes of magnetic and adhesive forces are equal to each other. Furthermore, the propulsion efficiency is greater for compliant capsules. Our findings could be useful for designing novel microfluidic devices where mobile magnetic microcapsules could be harnessed as microscale transport vehicles.



  1. Analytical solution for Stokes flow inside an evaporating sessile drop: Spherical and cylindrical cap shapes
    H. Masoud and J. D. Felske, Physics of Fluids 21, 042102 (2009)

    Exact analytical solutions are derived for the Stokes flows within evaporating sessile drops of spherical and cylindrical cap shapes. The results are valid for all contact angles. Solutions are obtained for arbitrary evaporative flux distributions along the free surface as long as the flux is bounded at the contact line. Specific results and computations are presented for evaporation corresponding to uniform flux and to purely diffusive gas phase transport into an infinite ambient. Wetting and nonwetting contact angles are considered with the flow patterns in each case being illustrated. For the spherical cap with evaporation controlled by vapor phase diffusion, when the contact angle lies in the range 0 ≤ θc < π/2, the mass flux of vapor becomes singular at the contact line. This condition requires modification when solving for the liquid-phase transport. Droplets in all of the above categories are considered for the following two cases: the contact lines are either pinned or free to move during evaporation. The present viscousflow behavior is compared to the inviscid flow behavior previously reported. It is seen that the streamlines for viscousflow lie farther from the substrate than the corresponding inviscid ones.


  1. Analytical solution for inviscid flow inside an evaporating sessile drop
    H. Masoud and J. D. Felske, Physical Review E 79, 016301 (2009)

    Inviscid flow within an evaporating sessile drop is analyzed. The field equation E2ψ = 0 is solved for the stream function. The exact analytical solution is obtained for arbitrary contact angle and distribution of evaporative flux along the free boundary. Specific results and computations are presented for evaporation corresponding to both uniform flux and purely diffusive gas phase transport into an infinite ambient. Wetting and nonwetting contact angles are considered, with flow patterns in each case being illustrated. The limiting behaviors of small contact angle and droplets of hemispherical shape are treated. All of the above categories are considered for the cases of droplets whose contact lines are either pinned or free to move during evaporation.