Publications

Mandre. Brachistochronous motion of a flat plate parallel to its surface immersed in a fluid. J. Fluid Mech., 939 A27 (2022).
PDF Publisher linkAbstract: We determine the globally minimum time đ needed to translate a thin submerged flat plate a given distance parallel to its surface within a work budget. The Reynolds number for the flow is assumed to be large so that the drag on the plate arises from skin friction in a thin viscous boundary layer. The minimum is determined computationally using a steepest descent, where an adjoint formulation is used to compute the gradients. ... (read more)
Fluid mechanics Optimization Boundary Layers 
Mandre. Workminimizing kinematics for small displacement of an infinitely long cylinder. J. Fluid Mech. 893 R4 (2020).
PDF Publisher linkAbstract: We consider the timedependent speed of an infinitely long cylinder that minimizes the net work done on the surrounding fluid to travel a given distance perpendicular to its axis in a fixed amount of time. The flow that develops is twodimensional. An analytical solution is possible using calculus of variations for the case that the distance travelled and the viscous boundary layer thickness that develops are much smaller than the circle radius. ... (read more)
Fluid mechanics Optimization Boundary Layers 
Venkadesan, Yawar, Eng, Dias, Singh, Tommasini, Haims, Bandi and Mandre. Stiffness of the human foot and evolution of the transverse arch. Nature 579, 97100 (2020).
PDF Publisher linkAbstract: The stiff human foot enables an efficient pushoff when walking or running, and was critical for the evolution of bipedalism. The uniquely arched morphology of the human midfoot is thought to stiffen it, whereas other primates have flat feet that bend severely in the midfoot. However, the relationship between midfoot geometry and stiffness remains debated in foot biomechanics, podiatry and palaeontology. These debates centre on the medial longitudinal arch and have not considered whether stiffness is affected by the second, transverse tarsal arch of the human foot. ... (read more)
Biomechanics Foot Evolution 
Dhawale, Mandre and Venkadesan. Dynamics and stability of running on rough terrains. R. Soc. Open Sci. 6: 181729 (2019).
PDF Publisher linkAbstract: Stability of running on rough terrain depends on the propagation of perturbations due to the ground. We consider stability within the sagittal plane and model the dynamics of running as a twodimensional body with alternating aerial and stance phases. Stance is modelled as a passive, impulsive collision followed by an active, impulsive pushoff that compensates for collisional losses. Such a runner has infinitely many strategies to maintain periodic gaits on flat ground. ... (read more)
Biomechanics Running 
Su, Miller, Mandre and Breuer. Confinement effects on energy harvesting by a heaving and pitching hydrofoil. J. Fluids Struct. 84, 233242 (2019).
PDF Publisher linkAbstract: Wall confinement effects on the energy harvesting performance by a flapping hydrofoil (aspect ratio 4.5) have been investigated in a circulating water flume at a Reynolds number of 50,000. Measurements of hydrodynamic forces are taken for three different confinement configurations (unconfined, onewall and twowall confinement) and a series of confinement levels for each configuration. Compared with the unconfined situation, a significant improvement of efficiency performance is obtained for strong twowall confinement due to the enhancement of the hydrodynamic forces, while only a modest increase is observed in the onewall confinement configuration. ... (read more)
Renewable energy Oscillating foils Hydrokinetic energy 
Akella, Singh, Mandre and Bandi. Dynamics of a camphoric acid boat at the airâwater interface. Phys. Lett. A 382(17), 11761180 (2018).
PDF Publisher linkAbstract: We report experiments on an agarose gel tablet loaded with camphoric acid (cboat) spontaneously set into motion by surface tension gradients on the water surface. We observe three distinct modes of cboat motion: harmonic mode where the cboat speed oscillates sinusoidally in time, a steady mode where the cboat maintains constant speed, and an intermittent mode where the cboat maintains nearzero speed between sudden jumps in speed. Whereas all three modes have been separately reported before in different systems, controlled release of Camphoric Acid (CA) from the agarose gel matrix allowed the observation of all the three modes in the same system. ... (read more)
Fluid Mechanics Surfactant 
Sane, Mandre and Kim. Surface tension of flowing soap films. J. Fluid Mech. 841, R2 (2018).
PDF Publisher linkAbstract: The surface tension of flowing soap films is measured with respect to the film thickness and the concentration of soap solution. We perform this measurement by measuring the curvature of the nylon wires that bound the soap film channel and use the measured curvature to parametrize the relation between the surface tension and the tension of the wire. We find that the surface tension of our soap films increases when the film is relatively thin or is made of soap solution of low concentration; otherwise, it approaches an asymptotic value of 30 mN/m. ... (read more)
Fluid Mechanics Surfactants 
Bandi, Akella, Singh, Singh and Mandre. Hydrodynamic signatures of stationary Marangonidriven surfactant transport. Phys. Rev. Lett. 119, 264501 (2017).
PDF Publisher linkAbstract: We experimentally study steady Marangonidriven surfactant transport on the interface of a deep water layer. Using hydrodynamic measurements, and without using any knowledge of the surfactant physicochemical properties, we show that sodium dodecyl sulphate and Tergitol 15S9 introduced in low concentrations result in a flow driven by adsorbed surfactant. At higher surfactant concentration, the flow is dominated by the dissolved surfactant. Using camphoric acid, whose properties are a priori unknown, we demonstrate this methodâs efficacy by showing its spreading is adsorption dominated. ... (read more)
Fluid Mechanics Surfactants 
Mandre. Axisymmetric spreading of surfactant from a point source. J. Fluid Mech. 832, 777792 (2017).
PDF Publisher linkAbstract: Guided by computation, we theoretically calculate the steady flow driven by the Marangoni stress due to a surfactant introduced on a fluid interface at a constant rate. Two separate extreme cases, where the surfactant dynamics is dominated by the adsorbed phase or the dissolved phase, are considered. We focus on the case where the size of the surfactant source is much smaller than the size of the fluid domain, and the resulting Marangoni stress overwhelms the viscous forces so that the flow is strongest in a boundary layer close to the interface. ... (read more)
Fluid Mechanics Surfactants 
Kim and Mandre. Marangoni elasticity of flowing soap films. Phys. Rev. Fluids 2, 082001(R) (2017).
PDF Publisher linkAbstract: We measure the Marangoni elasticity of a flowing soap film to be 22 mN/m irrespective of its width, thickness, flow speed, or the bulk soap concentration. We perform this measurement by generating an oblique shock in the soap film and measuring the shock angle, flow speed, and thickness. We postulate that the elasticity is constant because the film surface is crowded with soap molecules. Our method allows nondestructive measurement of flowing soap film elasticity and the value 22 mN/m is likely applicable to other similarly constructed flowing soap films. ... (read more)
Fluid Mechanics Surfactants 
Mukherjee, Mandre and Mahadevan. Controllable biomimetic birdsong. J. R. Soc. Interface.14: 20170002 (2017).
PDF Publisher linkAbstract: Birdsong is the product of the controlled generation of sound embodied in a neuromotor system. From a biophysical perspective, a natural question is that of the difficulty of producing birdsong. To address this, we built a biomimetic syrinx consisting of a stretched simple rubber tube through which air is blown, subject to localized mechanical squeezing with a linear actuator. A large static tension on the tube and small dynamic variations in the localized squeezing allow us to control transitions between three states: a quiescent state, a periodic state and a solitary wave state. ... (read more)
Biomechanics Birdsong 
Nguyen, Yu, Bandi, Venkadesan and Mandre. Curvatureinduced stiffening of fish fin. J. R. Soc. Interface. 14: 20170247.
PDF Publisher linkAbstract: How fish modulate their fin stiffness during locomotive manoeuvres remains unknown. We show that changing the fin’s curvature modulates its stiffness. Modelling the fin as bendable bony rays held together by a membrane, we deduce that fin curvature is manifested as a misalignment of the principal bending axes between neighbouring rays. An external force causes neighbouring rays to bend and splay apart, and thus stretches the membrane. This coupling between bending the rays and stretching the membrane underlies the increase in stiffness. ... (read more)
Biomechanics Fish fins 
Kim, Strom, Mandre, Breuer. Energy harvesting performance and flow structure of an oscillating hydrofoil with finite span. J. Fluids Struct. 70, 314326 (2017).
PDF Publisher linkAbstract: The energy harvesting performance and resulting flow structures of a hydrofoil oscillating in pitch and heave are studied experimentally in a water flume. The shape of a hydrofoil crosssection is shown to have negligible influence on the power generation for the geometries tested. It is found that contribution to efficiency from heaving motion increases with reduced frequency at optimal pitching amplitude. However, contribution to efficiency from pitching motion decreases with reduced frequency because the development of a leadingedge vortex during the stroke is delayed at the high reduced frequency. ... (read more)
Fluid Mechanics Renewable energy Oscillating foils Hydrokinetic energy 
Singh, Bandi, Mahadevan and Mandre. Linear stability analysis for monami in a submerged seagrass bed. J. Fluid Mech. 786, R1 (2016).
PDF Publisher linkAbstract: The onset of monami â the synchronous waving of seagrass beds driven by a steady flow â is modelled as a linear instability of the flow. Unlike previous works, our model considers the drag exerted by the grass in establishing the steady flow profile, and in damping out perturbations to it. We find two distinct modes of instability, which we label modes 1 and 2. Mode 1 is closely related to KelvinâHelmholtz instability modified by vegetation drag, whereas mode 2 is unrelated to KelvinâHelmholtz instability and arises from an interaction between the flow in the vegetated and unvegetated layers. ... (read more)
Fluid Mechanics Biomechanics Environment 
Kaplan, Wu, Mandre, Aizenberg and Mahadevan. Dynamics of evaporative colloidal patterning. Phys. Fluids 27, 092105 (2015).
PDF Publisher linkAbstract: Drying suspensions often leave behind complex patterns of particulates, as might be seen in the coffee stains on a table. Here, we consider the dynamics of periodic band or uniform solid film formation on a vertical plate suspended partially in a drying colloidal solution. Direct observations allow us to visualize the dynamics of band and film deposition, where both are made of multiple layers of close packed particles. We further see that there is a transition between banding and filming when the colloidal concentration is varied. ... (read more)
Fluid Mechanics Materials Science 
He, Nguyen and Mandre. Capillary interactions between nearby interfacial objects. Europhys. Lett. 102, 38001 (2013).
PDF Publisher linkAbstract: We develop a general method to study the capillary interactions between objects of arbitrary shape which float close to each other on an interface, a regime in which multipole expansion is not useful. The force is represented as a power series in the small distance between the objects, of which the leading order is finite. For objects with size a much larger than the capillary length lc, the force scales as (a/lc)1/2 and the prefactor depends on the mean radius of curvature R at the closest points. ... (read more)
Fluid Mechanics Capillary Cheerios Effect 
Kolinski, Rubinstein, Mandre, Brenner, Weitz and Mahadevan. Skating on a film of air: Drops impacting a surface. Phys. Rev. Lett. 108, 074503 (2012).
PDF Publisher linkAbstract: The commonly accepted description of drops impacting on a surface typically ignores the essential role of the air that is trapped between the impacting drop and the surface. Here we describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads. ... (read more)
Fluid Mechanics Droplet impact 
Mandre and Brenner. The mechanism of a splash on a dry solid surface. J. Fluid Mech. 690, 148172 (2012).
PDF Publisher linkAbstract: From rain storms to ink jet printing, it is ubiquitous that a highspeed liquid droplet creates a splash when it impacts on a dry solid surface. Yet, the fluid mechanical mechanism causing this splash is unknown. About fifty years ago it was discovered that corona splashes are preceded by the ejection of a thin fluid sheet very near the vicinity of the contact point. Here we present a firstprinciples description of the mechanism for sheet formation, the initial stages of which occur before the droplet physically contacts the surface. ... (read more)
Fluid Mechanics Droplet impact 
Wei, Mandre and Mahadevan. The branch with the furthest reach. Europhys. Lett. 97, 14005 (2012).
PDF Publisher linkAbstract: How should a given amount of material be moulded into a cantilevered beam clamped at one end, so that it will have the furthest horizontal reach? Here, we formulate and solve this variational problem for the optimal variation of the crosssection area of a heavy cantilevered beam with a given volume V, Young’s modulus E, and density Ï, subject to gravity g. We find that the crosssectional area should vary according a universal profile that is independent of material parameters, with both the length and maximum reachout distance of the branch that scale as $(EV/Ïg)^1/4$, with a universal selfsimilar shape at the tip with the area of crosssection $aâŒs^3$, s being the distance from the tip, consistent with earlier observations of tree branches, but with a different local interpretation than given before. ... (read more)
Biomechanics Optimization 
Schneider, Mandre and Brenner. Algorithm for microfluidic assembly line. Phys. Rev. Lett. 106, 094503 (2011).
PDF Publisher linkAbstract: Microfluidic technology has revolutionized the control of flows at small scales giving rise to new possibilities for assembling complex structures on the microscale. We analyze different possible algorithms for assembling arbitrary structures, and demonstrate that a sequential assembly algorithm can manufacture arbitrary 3D structures from identical constituents. We illustrate the algorithm by showing that a modified HeleShaw cell with 7 controlled flow rates can be designed to construct the entire English alphabet from particles that irreversibly stick to each other. ... (read more)
Fluid Mechanics Materials Science Microfluidics 
Mani, Mandre and Brenner. Events before droplet splashing on a solid surface. J. Fluid Mech. 647: Dedicated to Professor S. H. Davis on his 70th birthday, 163185 (2010).
PDF Publisher linkAbstract: A highvelocity (â1 m/s) impact between a liquid droplet (â1 mm) and a solid surface produces a splash. Classical observations traced the origin of this splash to a thin sheet of fluid ejected near the impact point, though the fluid mechanical mechanism leading to the sheet is not known. Mechanisms of sheet formation have heretofore relied on initial contact of the droplet and the surface. In this paper, we theoretically and numerically study the events within the time scale of about 1 ÎŒs over which the coupled dynamics between the gas and the droplet becomes important. ... (read more)
Fluid Mechanics Droplet impact 
Mandre and Mahadevan. A generalized theory of viscous and inviscid flutter. Proc. R. Soc. A.466141â156 (2009).
PDF Publisher linkAbstract: We present a unified theory of flutter in inviscid and viscous flows interacting with flexible structures based on the phenomenon of 1â:â1 resonance. We show this by treating four extreme cases corresponding to viscous and inviscid flows in confined and unconfined flows. To see the common mechanism clearly, we consider the limit when the frequencies of the first few elastic modes are closely clustered and small relative to the convective fluid time scale. ... (read more)
Fluid Mechanics Biomechanics Geophysics Fluidstructure interaction Flutter 
Mandre, Mani and Brenner. Precursors to splashing of a liquid droplet on a solid surface. Phys. Rev. Lett. 102, 134502 (2009).
PDF Publisher linkAbstract: A high velocity impact between a liquid droplet and a solid surface produces a splash. Classical work traced the origin of the splash to a thin sheet of fluid ejected near the impact point. Mechanisms of sheet formation have heretofore relied on initial contact of the droplet and the surface. We demonstrate that, neglecting intermolecular forces between the liquid and the solid, the liquid does not contact the solid, and instead spreads on a very thin air film. ... (read more)
Fluid Mechanics Droplet impact 
Bird, Mandre and Stone. Shorttime dynamics of partial wetting. Phys. Rev. Lett. 100, 234501 (2008).
PDF Publisher linkAbstract: When a liquid drop contacts a wettable surface, the liquid spreads over the solid to minimize the total surface energy. The first moments of spreading tend to be rapid. For example, a millimetersized water droplet will wet an area having the same diameter as the drop within a millisecond. For perfectly wetting systems, this spreading is inertially dominated. Here we identify that even in the presence of a contact line, the initial wetting is dominated by inertia rather than viscosity. ... (read more)
Fluid Mechanics Capillarity Wetting Materials Science 
Rust, Mandre and Balmforth. The feasibility of generating lowfrequency volcano seismicity by flow through a deformable channel. Geological Society, London, Special Publications, 307, 4556, 1 January 2008.
PDF Publisher linkAbstract: Oscillations generated by flow of magmatic or hydrothermal fluids through tabular channels in elastic rocks are a possible source of lowfrequency seismicity. We assess the conditions required to generate oscillations of approximately 1 Hz via hydrodynamic flow instabilities (roll waves), flowdestabilized standing waves set up on the elastic channel walls (wall modes), and unstable normal modes ringing in an adjacent fluid reservoir (clarinet modes). Stability criteria are based on physical and dimensional arguments, and discussion of destabilized elastic modes is supplemented with laboratory experiments of gas flow through a channel in a block of gelatine, and between a rigid plate and a rubber membrane. ... (read more)
Fluid Mechanics Fluidstructure interaction Geophysics 
Balmforth, Ghadge, Kettapun and Mandre. Bounds on doublediffusive convection. J. Fluid Mech. 569, 2950 (2006).
PDF Publisher linkAbstract: We consider doublediffusive convection between two parallel plates and compute bounds on the flux of the unstably stratified species using the background method. The bound on the heat flux for RayleighâBĂ©nard convection also serves as a bound on the doublediffusive problem (with the thermal Rayleigh number equal to that of the unstably stratified component). In order to incorporate a dependence of the bound on the stably stratified component, an additional constraint must be included, like that used by Joseph (Stability of Fluid Motion, 1976, Springer) to improve the energy stability analysis of this system. ... (read more)
Fluid Mechanics Geophysics Analysis Double diffusion 
Chen, Mandre and Feng. An experimental study of the coalescence between a drop and an interface in Newtonian and polymeric liquids. Phys. Fluids 18, 092103 (2006).
PDF Publisher linkAbstract: When a water drop falls onto an oilwater interface, the drop usually rests for some time before merging with the water underneath the interface. We report experiments on this process using water and oilbased Newtonian liquids and polymer solutions, with an emphasis on the nonNewtonian effects. We deduce that the drop surface is immobilized by contaminants preexisting in the fluids, and find that the rest time scales with the matrix viscosity for Newtonian fluids. ... (read more)
Fluid Mechanics Droplet coalescence Capillarity 
Chen, Mandre and Feng. Partial coalescence between a drop and a liquidliquid interface. Phys. Fluids 18, 051705 (2006).
PDF Publisher linkAbstract: This Letter reports experimental results for partial coalescence when a drop merges with an interface. We find an intermediate range of drop sizes in which the merger is not complete but a daughter drop is left behind. This phenomenon is governed primarily by inertia and interfacial tension, and three regimes can be further delineated depending on the roles of viscosity and gravity. Scaling relationships are developed for the drop size ratio and the coalescence time. ... (read more)
Fluid Mechanics Capillarity Droplet coalescence 
Balmforth and Mandre. Dynamics of roll waves. J. Fluid Mech. 514, 133 (2004).
PDF Publisher linkAbstract: Shallowwater equations with bottom drag and viscosity are used to study the dynamics of roll waves. First, we explore the effect of bottom topography on linear stability of turbulent flow over uneven surfaces. Lowamplitude topography is found to destabilize turbulent roll waves and lower the critical Froude number required for instability. At higher amplitude, the trend reverses and topography stabilizes roll waves. At intermediate topographic amplitude, instability can be created at much lower Froude numbers due to the development of hydraulic jumps in the equilibrium flow. ... (read more)
Fluid Mechanics Instability Waves 
Lichter, Roxin and Mandre. Mechanisms of liquid slip and solid surfaces. Phys. Rev. Lett. 93, 086001 (2004).
PDF Publisher linkAbstract: One of the oldest unresolved problems in fluid mechanics is the nature of liquid flow along solid surfaces. It is traditionally assumed that across the liquidsolid interface, liquid and solid speeds exactly match. However, recent observations document that on the molecular scale, liquids can slip relative to solids. We formulate a model in which the liquid dynamics are described by a stochastic differentialdifference equation, related to the FrenkelKontorova equation. The model, in agreement with molecular dynamics simulations, reveals that slip occurs via two mechanisms: localized defect propagation and concurrent slip of large domains. ... (read more)
Fluid Mechanics Molecular Dynamics Materials Science Liquid slip 
Ghosal and Mandre. A simple model illustrating the role of turbulence on phytoplankton blooms. J. Math. Bio. 46, 333â346 (2003).
PDF Publisher linkAbstract: The problem of the vertical distribution of phytoplankton is considered in the presence of gravitational settling, turbulent mixing, population growth due to cell division and a constant rate of loss due to predation and natural death. Nutrients are assumed to be plentiful so that the production rate depends only on the light available for photosynthesis. The nonlinear saturation of plankton growth is modeled by allowing the attenuation rate of light to be a linear function of the plankton density. ... (read more)
Biomechanics Fluid Mechanics