Drag Force on a Starting Plate Scales with the Square Root of Acceleration.

We report results on the instantaneous drag force on plates that are accelerated in a direction normal to the plate surface, which show that this force scales with the square root of the acceleration. This is associated with the generation and advection of vorticity at the plate surface. A new scaling law is presented for the drag force on accelerating plates, based on the history force for unsteady flow. This scaling avoids previous inconsistencies in using added mass forces in the description of forces on accelerating plates.

Nanoscale contact line visualization based on Total Internal Reflection Fluorescence Microscopy.

We describe a novel measurement method to study the contact line of a droplet at nanoscale level. The method is based on Total Internal Reflection Fluorescence Microscopy (TIRFM), which uses an evanescent excitation field produced by total internal reflection of light. The evanescent field depends on the angle of the incident light and has an exponential intensity decay, characterized by the penetration depth. The penetration depth is determined by imaging a fluorescent particle probe that is traversed using an Atomic Force Microscopy (AFM) setup. The result confirms the exponential behavior of the evanescent field intensity, and the value of the penetration depth also corresponds with the value predicted based on the optical configuration. By using the intensity distribution of a fluorescent dye and the value for the penetration depth of the evanescent wave, it is possible to reconstruct the interface of a partial wetting droplet. The reconstructed interface based on TIRFM is in good agreement with the interface obtained from two reference measurements: non-disturbing AFM-imaging and conventional contact angle measurement. The latter lacks spatial resolution, while the former is limited to particular droplets. This new non-contact measurement does not suffer from these drawbacks, making it a very useful tool to study the fundamental wetting behavior of both stationary and dynamic interfaces.

Improving rowing performance by adjusting oar blade size and angle.

The principal aim of the work presented here is to investigate and demonstrate that a forward tilted rowing blade would result in a more efficient and effective motion of the blade through the water that would result in a higher boat speed when an equal input power is provided. A 1:5 scaled rowing boat is used to determine the performance of rowing blades with different sizes and blade angles. This is used to validate the results of a previous study where the optimal blade angle of 15 ∘ with respect to the oar shaft was determined ( 1). The input power and speed of the rowing boat can be compared between original and modified oar blades. Measurements in a towing tank demonstrate that a modified rowing blade result in faster rowing by 0.4% at the same input power. Maintaining the same stroke rate, the improvement of the blade efficiency is compensated by using a 4-6% increased blade area to yield the same input power.

A comparison of the value of viscosity for several water models using Poiseuille flow in a nano-channel.

The viscosity-temperature relation is determined for the water models SPC/E, TIP4P, TIP4P/Ew, and TIP4P/2005 by considering Poiseuille flow inside a nano-channel using molecular dynamics. The viscosity is determined by fitting the resulting velocity profile (away from the walls) to the continuum solution for a Newtonian fluid and then compared to experimental values. The results show that the TIP4P/2005 model gives the best prediction of the viscosity for the complete range of temperatures for liquid water, and thus it is the preferred water model of these considered here for simulations where the magnitude of viscosity is crucial. On the other hand, with the TIP4P model, the viscosity is severely underpredicted, and overall the model performed worst, whereas the SPC/E and TIP4P/Ew models perform moderately.

Asymmetric cupula displacement due to endolymph vortex in the human semicircular canal.

The vestibular system in the inner ear senses angular head manoeuvres by endolymph fluid which deforms a gelatinous sensory structure (the cupula). We constructed computer models that include both the endolymph flow (using CFD modelling), the cupula deformation (using FEM modelling), and the interaction between both (using fluid-structure interaction modelling). In the wide utricle, we observe an endolymph vortex. In the initial time steps, both the displacement of the cupula and its restorative forces are still small. As a result, the endolymph vortex causes the cupula to deform asymmetrically in an S-shape. The asymmetric deflection increases the cupula strain near the crista and, as a result, enhances the sensitivity of the vestibular system. Throughout the head manoeuvre, the maximal cupula strain is located at the centre of the crista. The hair cells at the crista centre supply irregularly spiking afferents, which are more sensitive than the afferents from the periphery. Hence, the location of the maximal strain at the crista may also increase the sensitivity of the semicircular canal, but this remains to be tested. The cupula overshoots its relaxed position in a simulation of the Dix-Hallpike head manoeuvre (3 s in total). A much faster head manoeuvre of 0.222 s showed to be too short to cause substantial cupula overshoot, because the cupula time scale of both models (estimated to be 3.3 s) is an order of magnitude larger than the duration of this manoeuvre.

Effects of a Fence on Pollutant Dispersion in a Boundary Layer Exposed to a Rural-to-Urban Transition.

Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL.

Influence of virtual images on the signal-to-noise ratio in digital in-line particle holography.

A theoretical analysis describing the dependence of the signal-tonoise ratio (SNR) on the number of pixels and the number of particles is presented for in-line digital particle holography. The validity of the theory is verified by means of numerical simulation. Based on the theory we present a practical performance benchmark for digital holographic systems. Using this benchmark we improve the performance of an experimental holographic system by a factor three. We demonstrate that the ability to quantitatively analyze the system performance allows for a more systematic way of designing, optimizing, and comparing digital holographic systems.

Cilia-driven particle and fluid transport over mucus-free mice tracheae.

To date, there is only a fragmentary understanding of the fundamental mechanisms of airway mucociliary transport. Application of the latest measurement techniques can aid in deciphering the complex interplay between ciliary beat and airway surface liquid (ASL) transport. In the present study, direct, quasi-simultaneous measurements of the cilia-induced fluid and bead transport were performed to gain a better insight into both transport mechanisms. In this study cilia-induced periciliary liquid (PCL) transport is measured by means of micro Particle Image Velocimetry (µPIV) with neutrally buoyant tracers. Particle Tracking Velocimetry (PTV) with heavier polystyrene-ferrite beads is performed to simulate particle transport. Contrary to recent literature, in which the presence of mucus was deemed necessary to maintain periciliary liquid (PCL) transport, effective particle and fluid transport was measured in our experiments in the absence of mucus. In response to muscarine or ATP stimulation, maximum fluid transport rates of 250 µm/s at 15 µm distance to the tracheal epithelia were measured while bead transport rates over the epithelia surfaces reached 200 µm/s. We estimated that the mean bead transport is dominated by viscous drag compared to inertial fluid forces. Furthermore, mean bead transport velocities appear to be two orders of magnitude larger compared to bead sedimentation velocities. Therefore, beads are expected to closely follow the mean PCL flow in non-ciliated epithelium regions. Based on our results, we have shown that PCL transport can be directly driven by the cilia beat and that the PCL motion may be capable of driving bead transport by fluid drag.

Interfaces and inhomogeneous turbulence.

A Euromech colloquium, on interfacial processes and inhomogeneous turbulence, was held in London on 28-30 June 2010. Papers were presented describing and analysing the influence of interfaces that separate turbulent/non-turbulent regions, between regions of contrasting fluid properties, or at the edge of boundaries. This paper describes a summary of the work presented, giving a snapshot of the current progress in this area, along with discussions about future research directions.

Experimental investigation of the flow induced by artificial cilia.

The fluid transport produced by rectangular shaped, magnetically actuated artificial cilia of 70 µm length and 20 µm width was determined by means of phase-locked Micro Particle Image Velocimetry (µPIV) measurements in a closed microfluidic chamber. The phase-averaged flow produced by the artificial cilia reached up to 130 µm s(-1) with an actuation cycle frequency of 10 Hz. Analysis of the measured flow data indicate that the present system is capable of achieving volume flow rates of V[combining dot above](cilia) = 14 ± 4 µl min(-1) in a micro channel of 0.5 × 5 mm(2) cross-sectional area when no back pressure is built up. This corresponds to an effective pressure gradient of 6 ± 1 Pa m(-1), which equals a pressure difference of 0.6 ± 0.1 mPa over a distance of 100 µm between two rows of cilia. These results were derived analytically from the measured velocity profile by treating the cilia as a thin boundary layer. While the cilia produce phase-averaged velocities of the order of O(10(2)µm s(-1)), time-resolved measurements showed that the flow field reverses two times during one actuation cycle inducing instantaneous velocities of up to approximately 2 mm s(-1). This shows that the flow field is dominated by fluid oscillations and flow rates are expected to increase if the beating motion of the cilia is further improved.

Magnetically-actuated artificial cilia for microfluidic propulsion.

In this paper we quantitatively analyse the performance of magnetically-driven artificial cilia for lab-on-a-chip applications. The artificial cilia are fabricated using thin polymer films with embedded magnetic nano-particles and their deformation is studied under different external magnetic fields and flows. A coupled magneto-mechanical solid-fluid model that accurately captures the interaction between the magnetic field, cilia and fluid is used to simulate the cilia motion. The elastic and magnetic properties of the cilia are obtained by fitting the results of the computational model to the experimental data. The performance of the artificial cilia with a non-uniform cross-section is characterised using the numerical model for two channel configurations that are of practical importance: an open-loop and a closed-loop channel. We predict that the flow and pressure head generated by the artificial cilia can be as high as 18 microlitres per minute and 3 mm of water, respectively. We also study the effect of metachronal waves on the flow generated and show that the fluid propelled increases drastically compared to synchronously beating cilia, and is unidirectional. This increase is significant even when the phase difference between adjacent cilia is small. The obtained results provide guidelines for the optimal design of magnetically-driven artificial cilia for microfluidic propulsion.

Observation of hydrophobic-like behavior in geometrically patterned hydrophilic microchannels.

We present our observation of meta-hydrophobicity, where geometrically patterned surfaces make hydrophilic microchannels exhibit hydrophobic-like behaviors. We analyze the wetting-induced energy decrease that results from the surface geometries and experimentally demonstrate how those geometries can modulate the dynamics of capillary-driven wetting and evaporation-driven drying of microfluidic systems. Our results also show that the modulated wetting dynamics can be employed to generate regulated patterns of microbubbles.

Measurements of the wall shear stress distribution in the outflow tract of an embryonic chicken heart.

In order to study the role of blood-tissue interaction in the developing chicken embryo heart, detailed information about the haemodynamic forces is needed. In this study, we present the first in vivo measurements of the three-dimensional distribution of wall shear stress (WSS) in the outflow tract (OFT) of an embryonic chicken heart. The data are obtained in a two-step process: first, the three-dimensional flow fields are measured during the cardiac cycle using scanning microscopic particle image velocimetry; second, the location of the wall and the WSS are determined by post-processing flow velocity data (finding velocity gradients at locations where the flow approaches zero). The results are a three-dimensional reconstruction of the geometry, with a spatial resolution of 15-20 microm, and provides detailed information about the WSS in the OFT. The most significant error is the location of the wall, which results in an estimate of the uncertainty in the WSS values of 20 per cent.

Mechanics of the turbulent-nonturbulent interface of a jet.

We report the results of an experimental investigation of the mechanics and transport processes at the bounding interface between the turbulent and nonturbulent regions of flow in a turbulent jet, which shows the existence of a finite jump in the tangential velocity at the interface. This is associated with small-scale eddying motion at the outward propagating interface (nibbling) by which irrotational fluid becomes turbulent, and this implies that large-scale engulfment is not the dominant entrainment process. Interpretation of the jump as a singular structure yields an essential and significant contribution to the mean shear in the jet mixing region. Finally, our observations provide a justification for Prandtl's original hypothesis of a constant eddy viscosity in the nonturbulent outer jet region.

Holographic simultaneous readout polarization multiplexing based on photoinduced anisotropy in bacteriorhodopsin.

Bacteriorhodopsin (bR) is a reversible photochromic protein that can be used as a holographic medium. The dichroic absorption of the bR molecule is polarization dependent, thereby allowing for the recording of polarization holograms. The properties of polarization holograms can be used to multiplex two independent images in a single bR film. A new technique and associated polarization-multiplexing scheme are demonstrated that allow for simultaneous readout of two orthogonally polarized images while achieving a high normalized diffraction efficiency for each of the individual images.