Hassan Masoud

Self-interacting flapping swimmers

We simulate the locomotion of an infinite array of wings that heave vertically with a prescribed sinusoidal motion and are free to translate in the horizontal direction. These simulations indicate that the wings can take advantage of their collectively generated wake flows. We find ranges of flapping frequency over which there are multiple stable states of locomotion, with one of these swimming states having both higher speeds and efficiencies than an isolated flapping and locomoting wing. These results may be important to understanding the role of hydrodynamic interactions in fish schooling and bird flocking.


Collective surfing of chemically active particles

We use a semi-implicit Fourier pseudo-spectral method to study the collective dynamics of particles bound to a 2d interface above a 3d fluid layer of finite depth. These particles are chemically active and produce a diffusing field that creates surface-tension gradients along the surface. The resultant Marangoni stresses create flows that carry the particles, possibly concentrating them. We calculate the linear stability condition of this system and demonstrated that the instability leads to the accumulation of active particles and examined its consequence on the 3d flow. In particular, we demonstrate the emergence of a vortex ring in the bulk of the fluid. Moreover, we show analytically that for sufficiently deep and shallow fluid layers the collective surfing of active particles is described by the so-called parabolic-elliptic Keller-Segel (KS) model in two dimensions. Originally conceived to describe the chemotactically-driven aggregation of slime molds, the KS model is one of the canonical models of mathematical biology. The KS model describes the spatial dynamics of a population of motile organisms that secrete a chemical that is self-attractive.


Inertialess Marangoni propulsion

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, Peclet, and Reynolds numbers. Our derivations are based on the Lorentz reciprocal theorem, which eliminates the need for developing the detailed flow field.


Rotation of porous ellipsoids in shear flows

We develop a domain integral technique to study 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 behavior of particles, and that the Jeffery's prediction of the angular velocity of impermeable ellipsoids in simple shear flows remains an excellent approximation, if not an exact one, for porous ellipsoids. Employing an appropriate scaling, we also derive approximate expressions for torque exerted on ellipses and spheroids rotating in a quiescent fluid. Findings of our work 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.


Friction of elastomers on smooth and grooved surfaces

We study the frictional behavior of dry elastomers in relative motion against adhesive and non-adhesive surfaces. We consider smooth substrates and those with triangular, forward and backward sawooth asperities. We show that the friction experienced on non-adhesive surfaces is the lowest for a surface with forward sawtooth roughness, while the surface with backward roughness leads to the highest friction. For adhesive surfaces, our simulations reveal that the friction force is independent of the surface geometry when the sliding velocity is sufficiently slow. We also examine the effects of sliding velocity, temperature, and normal load on the hysteretic and adhesive friction of our network. In particular, our simulations predict a bell-shaped curve for the dependence of the friction coefficient on the sliding velocity. We demonstrate that the velocity at which the friction is maximum depends on the system temperature for the hysteretic friction and is a function of the ratio between the thermal and adhesion energy for the adhesive friction.


Bioinspired micro-swimmers powered by responsive gels

We design a synthetic micro-swimmer that not only self-propels, but also navigates in a low-Reynolds-number environment. Our simple swimmer consists of a cubic gel body with two rectangular stiff flaps attached to its opposite sides and a stimuli-sensitive flexible flap at the body front end. The responsive gel undergoes periodic expansions and contractions, which can be experimentally triggered by an oscillatory chemical reaction or by oscillating magnetic and electric fields. Periodic volumetric changes of the body lead to the time-irreversible beating motion of the propulsive flaps which propel the micro-swimmer through the inertialess fluid. We demonstrate that our swimmer can successfully turn in the desired direction following the application of an external stimulus to the responsive steering flap. In this scenario, the steering flap bends and flutters around its curved profile resulting in a rotating torque that deflects the swimmer trajectory.


Controlled release of solutes from responsive microgel capsules

We examine 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.


Patterned microchannels to separate colloid-polymer mixtures

We use dissipative particle dynamics (DPD) to design nano-structured surfaces able to selectively regulate interactions between microchannel walls and flowing colloid-polymer suspensions. We show that 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. Furthermore, we demonstrate that surfaces decorated with tilted posts can discriminate nanoscopic entities with regard to their shape and, thus, can be utilized for separating colloid-polymer mixtures.


Transport through random polymer networks

We employ a hybrid computational method to examine the permeation and hindered diffusion through mechanically loaded (anisotropic) random polymer networks. We use bond-bending lattice spring model (LSM) to capture the deformation of random networks of interconnected filaments and we exploit dissipative particle dynamics (DPD) to explicitly model the viscous fluid and its interactions with the network and diffusive objects. Our simulations reveal that the network transport properties are independent of the network internal structure and are solely function of the network porosity and degree of anisotropy. Furthermore, our results indicate that the network permeability in any direction, under any load, can be related to the network degree of alignment represented by a second order orientation tensor.


Flapping flight with flexible wings

We use fully-coupled three-dimensional simulations to examine the 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 resonant oscillations of elastic wings drastically enhance aerodynamic lift, thrust, and effciency. 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 effcient microscale flying vehicles.


Magnetically-driven synthetic microcapsules

Using computer simulations and theory, we examine how to design magnetically-responsive synthetic microcapsules that are able to move in a steady manner in microfluidic channels. To model the magnetic capsules propelled in fluid-filled microchannels, we employ a hybrid computational method for fluid-structure interactions. This method integrates the lattice Boltzmann model for the fluid dynamics and the lattice spring model for the micromechanics of solids. The results indicate that such mobile fluid-filled containers could find application in lab-on-chip systems for controlled delivery of minute amounts of fluidic samples to highly specific locations. The compliant microcapsules represent certain biological cells and polymeric capsules.


Flow and particle deposition inside an evaporating sessile drop

We analytically solve the Stokes and potential flows within evaporating sessile drops of cylindrical and spherical cap shapes. Solutions are obtained by employing the method of separation of variables and eigenfunction expansion in the toroidal coordinate system. We also examine the deposition patterns resulting from the flow within evaporating colloidal drops by numerically integrating Langevin equations for the suspended particles. Our calculations reveale that the free surface of the drop plays a major role in defining the final deposition pattern.

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