Hassan Masoud

Controlled release of nanoparticles and macromolecules from responsive microgel capsules.

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.

Analytical solution for Stokes flow inside an evaporating sessile drop: Spherical and cylindrical cap shapes

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....

Designing maneuverable micro-swimmers actuated by responsive gel†

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.

Harnessing synthetic cilia to regulate motion of microparticles

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.

Hydrodynamic schooling of flapping swimmers

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.

Modeling magnetic microcapsules that crawl in microchannels

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.

Analytical solution for inviscid flow inside an evaporating sessile drop.

Inviscid flow within an evaporating sessile drop is analyzed. The field equation E;{2}psi=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.

Drag and diffusion coefficients of a spherical particle attached to a fluid–fluid interface

Explicit analytical expressions for the drag and diffusion coefficients of a spherical particle attached to the interface between two immiscible fluids are constructed for the case of a small viscosity ratio between the fluid phases. The model is designed to explicitly account 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 a geometric perturbation approach, which is based on the deviation of the contact angle from a 90{\deg}-value. By testing the model against experimental and numerical data from the literature, good agreement is found within the entire range of contact angles below 90{\deg}. As an advantage of the method reported, the drag and diffusion coefficients can be calculated up to second order in the perturbation parameter, while it is sufficient to know the velocity and pressure fields only up to first order. Extensions to other particle shapes with known velocity and pressure fields are straightforward.

Efficient Flapping Flight Using Flexible Wings Oscillating at Resonance

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.

Fluid flows with interactive boundaries

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. This special issue of the European Journal of Computational Mechanics is dedicated to selected studies that use theory and numerical simulation to investigate the interaction of flowing fluid with complex and dynamically changing boundaries. The invited contributions cover a wide variety of topics ranging from mucociliary transport, fish locomotion, hydrodynamics of self-propelled particles, and particle motion in acoustic fields to deformation of elastic capsules in shear flows, fluttering of piezoelectric plates, dynamics of wave energy converters, and droplet coalescence on microstructured substrates. However, the selection is not meant to be comprehensive, but rather illustrative of applications of fluid flows with moving boundaries and theoretical and numerical methods used to advance the field. In the following, we outline the main objectives and findings of the articles. Guo and Kanso examine the performance of beating cilia in healthy and diseased states. Arranged periodically, the cilia are immersed in a two-layer system consisting of a nearly viscous fluid (periciliary layer) at the bottom and a viscoelastic fluid (mucus layer) on the top. An immersed boundary method is employed to resolve the interaction of the cilia with the host fluids. For the sake of simplicity, only one-way coupling is considered, as the kinematics of the cilia beating is assumed to remain unchanged. The simulations are used to examine the effect of the relative thickness of periciliary and mucus layers on the efficiency of mucus transport by cilia. The results indicate that a depletion of periciliary layer (observed in diseased systems) impedes the rate of mucus removal. Sprinkle et al. simulate the swimming of a fin-plate model of the so-called median/ paired fin fish using a constraint-based immersed body method. The authors investigate the implications of the diagonal fin insertion morphology and motility advantages offered by maintaining a rigid body while only undulating the fins. They quantify the energy expenditure per unit distance travelled for different geometries and find that the swimmers that keep a part of their body rigid have a mechanical advantage via minimising the cost of transport. Gaojin and Ardekani present a two-dimensional numerical study of undulatory swimming near a solid boundary. The authors consider swimming in Newtonian, shear-thinning