A numerical study of swimming particle motion and nutrient transport is conducted for a semidilute to dense suspension in a thin film. The steady squirmer model is used to represent the motion of living cells in suspension with the nutrient uptake by swimming particles modelled using a first-order kinetic equation representing the absorption process that occurs locally at the particle surface. An analysis of the dynamics of the neutral squirmers inside the film shows that the vertical motion is reduced significantly. The mean nutrient uptake for both isolated and populations of swimmers decreases for increasing swimming speeds when nutrient advection becomes relevant as less time is left for the nutrient to diffuse to the surface. This finding is in contrast to the case where the uptake is modelled by imposing a constant nutrient concentration at the cell surface and the mass flux results to be an increasing monotonic function of the swimming speed. In comparison to non-motile particles, the cell motion has a negligible influence on nutrient uptake at lower particle absorption rates since the process is rate limited. At higher absorption rates, the swimming motion results in a large increase in the nutrient uptake that is attributed to the movement of particles and increased mixing in the fluid. As the volume fraction of swimming particles increases, the squirmers consume slightly less nutrients and require more power for the same swimming motion. Despite this increase in energy consumption, the results clearly demonstrate that the gain in nutrient uptake make swimming a winning strategy for micro-organism survival also in relatively dense suspensions.

We report results of a series of detailed experiments designed to unveil the dynamics of a particle of radius $a$ moving in high-frequency, low-Reynolds-number oscillatory flow. The fundamental parameters in the problem are the Strouhal ($\hbox{\it Sl}$) and the particle Reynolds numbers ($\hbox{\it Re}_p$), as well as the fluid-to-particle density ratio $\alpha$. The experiments were designed to cover a range of $\hbox{\it Sl} \hbox{\it Re}_p$ from 0.015 to 5 while keeping $\hbox{\it Re}_p < 0.5$ and $\hbox{\it Sl} > 1$. The primary objective of the experiments is to investigate stationary history effects associated with the Basset drag, which are maximized when the viscous time scale $a^2/\nu$ is of the same order of the flow time scale $9/\Omega$, where $9$ is a geometrical factor for the sphere, $\nu$ is the kinematic viscosity and $\Omega$ is the angular frequency of the background flow. The theoretically determined behaviour of stationary history effects is confirmed unequivocally by the experiments, which also validate the fractional derivative behaviour (of order $1/2$) of the history drag for the range of parameters under study.

We explore the capacity of a flexible flap to increase mixing in a microchannel for a flap Reynolds number Ref ranging from 0.3–80. The fictitious-domain (DLM) method is used to model the fluid and solid interactions. The momentum equations for the fluid and solid are solved individually using the finite-volume and finite-difference methods. The equations are coupled using distributed Lagrange multipliers. The stress in the solid is derived from the nonlinear beam equations. Fluid mixing is quantified by solving the mass transport equation for a solute with low molecular diffusivity and calculating a global mixing fraction M. The flap is actuated using a distributed follower force along the length of the flap. The results show that mixing is enhanced for larger flap displacements and for dimensionless frequencies Sl between 1 and 2. Optimal mixing occurs when the flap length is 2/3 the microchannel height. The influence of the hydrodynamic force on the beam bending motion enhances the mixing process. Under optimal conditions the flap behaves as a rapid mixing device where 80% of the long time mixing fraction is reached during an initial time interval of 3.8 s.

No abstract

No abstract

Interactions between oil droplets and marine particle aggregates, such as marine snow, may affect the behavior of oil spills. Marine snow is known to scavenge fine particles from the water column, and has the potential to scavenge oil droplets in the same manner. To determine the degree to which such a process is important in the evolution of oil spills, we quantify the collision of oil droplets and marine aggregates using existing collision rate equations. Results show that interaction of drops and aggregates can substantially influence the drop size distribution, but like all such processes this result is sensitive to the local concentration of oil and aggregates. The analysis also shows that as the size distribution of oil droplets shifts toward larger droplets, a greater fraction of the total oil volume collides with marine aggregates. This result is robust to a variety of different assumptions in the collision model. Results also show that there is not always a dominant collision mechanism. For example, when droplets and aggregates are both close to 10 lm in radius, shear and differential settling contribute nearly equally to the collision rate. This overlap suggests that further research on the interaction of shear and differential settling could be useful.

Polymer actuators based on Gold/PolyPyrrole bilayers were microfabricated and their properties tested for flow promoting in the microdomain. When implemented in microchannels these actuators behaved as efficient micromixers for both, flow-through and stagnant conditions. Particle tracking experiments and numerical simulations of cross-sectional domains verified the capacity of these devices to promote complex, high velocity flows with chaotic advection properties in microscopic environments. Thinner devices could be actuated at higher frequencies than thicker devices, up to 10 Hz for 10 nm thick Gold layers with voltages not over 0.6 V (vs. Ag/AgCl), which led to enhanced flow generation properties. The results herein demonstrate that these actuators are practical candidates for fluid manipulation in the microdomain (for applications such as micromixing and pumping, and possibly even for propelling of swimming microdevices).

No abstract