Ch. Brücker

Nano-newton drag sensor based on flexible micro-pillars

A new simple and low-cost sensor concept to measure drag forces on fixed particles in the near-wall region with detectable forces down to pico-newton with a novel microoptomechanical system (MOMS) based on a flexible micro-pillar is presented. The cylindrical pillar with a diameter of a few microns is manufactured from an elastomer such that it is very flexible and easily deflected by the fluid forces or supplementary forces acting on flow obstacles attached to the tip. The pillar-tip bending is detected optically using a highly magnifying optical system. A feasibility study was carried out in a plate–cone rheometer with an air bubble of diameter 140 µm attached to the pillars tip to show that the micro-pillar sensor is capable of detecting net particle drag forces. The Reynolds number based on the bubble diameter Rebubble was varied in the range of 0.1–15. The calibration of the pure sensor structure shows linear behaviour of the pillar. The experimental results of the bubble drag in plane shear flow showed good agreement with theoretical predictions. The optics and the sensor used in this experiment allow a reliable detection of forces down to a minimum of about 5–10 nN. Using microscopic optics even smaller forces are detectable. The sensor geometry can be varied in a wide range, making it possible to measure forces of order down to nano-newtons on particles possessing a geometric length of a few µm to several hundred µm. Therefore, the method fits well into existing drag measurement concepts closing a gap between optical tweezers and recent atomic force cantilevers. Furthermore, the new sensor concept possesses very low intrusive interference. It has to be emphasized that unlike cantilever methods, the new micro-pillar sensor allows measurements of the two wall-parallel drag components. Using arrays of sensors, drag forces on multiple particles in a plane can be measured.

The role of ventilation frequency in airway reopening

Inhomogeneously compliant lungs need special treatment during ventilation as they are often affected by respiratory insufficiency which is frequently caused by a regional collapse of the airways. To treat respiratory insufficiency atelectatic areas have to be recruited. Beside conventional mechanical ventilation, high-frequency oscillatory ventilation (HFOV) is an efficient method for airway reopening. Using a transparent in-vitro model of the human lung the influence of varying frequencies on the reopening behavior of atelectatic regions is investigated for volume controlled ventilation. The experiments show that higher ventilation frequencies at constant tidal volume enhance the probability of successful reopening of collapsed lung regions and thus, lead to a more homogeneous distribution of air within the lung. This effect can be attributed (i) to larger flow velocities and thus larger pressure losses in the free pathways as the ventilation frequency increases and (ii) to higher inertia effects. In consequence, the static pressure in the branches above the atelectatic regions increases until it reaches a level at which recruitment is achieved.

Evidence of rare backflow and skin-friction critical points in near-wall turbulence using micropillar imaging

The recent discovery of rare backflow events in turbulent boundary layer flows based on the analysis of simulation data has again raised the need of experimental visualizations of wall-shear stress fields in unsteady flows. The localization of critical points, which are thought to strongly correlate with large-scale events in the log-layer, is of importance. Up to now, there is no experimental proof of these rare events and their topological patterns. Their existence in a turbulent boundary-layer flow along a flat plate is shown herein by means of imaging with 2D arrays of flexible micropillars attached at the wall.

Interaction of flexible surface hairs with near-wall turbulence

The interaction of near-wall turbulence with hairy surfaces is investigated in a turbulent boundary layer flow along a flat plate in an oil channel at Re = 1.2 × 10⁶. The plate is covered locally with a dense carpet of elastomeric micro-hairs (length L = 1 mm, length in viscous units L( + ) = 30) which are arranged in a regular grid (60 × 30 hairs with a streamwise spacing Δx( + )≈15 and a spanwise spacing Δy( + )≈30). Instead of the micro-structures used in previous studies for sensory applications, the surface hairs are considerably larger and much more densely distributed with a spacing of S/D < 5 such that they interact with each other by flow coupling. The non-fluctuating mean part of the flow forces a substantial pre-bending in the streamwise direction (reconfiguration). As a consequence, the hairs align with the streamwise direction, thus imposing anisotropic damping characteristics with regard to flow fluctuations in streamwise and spanwise or wall-normal directions. Near-wall high-frequency disturbances excited by the passage of turbulent sweeps are dampened over their course along the carpet. The cooperative action of the hairs leads to an energy transfer from small-scale motion to larger scales, thus increasing the coherence of the motion pattern in streamwise and spanwise directions. As a consequence of the specific arrangement of the micro-hairs in streamwise columns a reduced spanwise meandering and stabilization of the streamwise velocity streaks is achieved by promoting varicose waves and inhibiting sinusoidal waves. Streak stabilization is known to be a major contributor to turbulent drag reduction. Thus it is concluded that hairy surfaces may be of benefit for turbulent drag reduction as hypothesized by Bartenwerfer and Bechert (1991 Z. Flugwiss. Weltraumforsch. 15 19-26).

Visualizing Flow Partitioning in a Model of the Upper Human Lung Airways

The convective transport of fluid within the human upper airways is investigated in a transparent model of the tracheobronchial tree. Oscillatory flow through the branching network with six generations was studied at varying Reynolds numbers between 400 and 2600 and Womersley numbers from 5.5 to 12.3 in the trachea representing clinical conditions during high frequency oscillatory ventilation. The flow partitioning within the model was visualized using advection of neutrally buoyant tracer particles, which were illuminated by short light pulses and recorded by a high speed camera. Integration of the particle locations for a large number of cycles provides the probability distribution of particles passing certain branches within the bifurcating network, and thus, the dispersion of particles in the airways. The results show the different characteristics of flow partitioning at varying Womersley and Reynolds numbers.

Fluid transport via pneumatically actuated waves on a ciliated wall

To manipulate fluids actively a pneumatically actuated micro membrane device is developed to generate a directed transversal fluid transport in a liquid layer next to the wall. The biomimetic approach is based on the principle of cilia-type arrays that generate a mean flow by travelling wave activation. Rows of long flaps, which mimic the comb row of a ctenophore, are positioned off-centre along a row of cavities. Each cavity is covered by a flexible membrane that supports the flaps. The membranes with the flaps on top are deflected by applying a well-defined pressure profile to the cavities under the membranes such that an individual beat can be generated for each flap. Flow visualization experiments were carried out under the conditions of travelling waves. The results show a mean velocity profile that resembles that of a wall-jet. Mixing effects with increased retention times of the fluid occur in the vicinity of the membrane surfaces.

A multiple-segment long-distance microscope for flow visualization and measurements*

In this article, we present a new imaging technique for multiple-segment microscopy at long working distances. In many applications, images of small structures might have to be taken simultaneously at widely separated points. At present, this is a problem because with a single lens it is possible to either take an image of one structure with high resolution or image the area that includes all the structures onto the CCD chip of the camera but with a strongly reduced resolution of the single structure. We have used a micro-lens array in combination with an aperture array to blank out the space between widely distributed spots and to highly magnify the areas of interest onto one chip. An application of this technique to wall shear stress measurements via imaging of wall-mounted flexible micro-pillars is demonstrated.