Swimming by reciprocal motion at low Reynolds number

Biological microorganisms swim with flagella and cilia that execute nonreciprocal motions for low Reynolds number (Re) propulsion in viscous fluids. This symmetry requirement is a consequence of Purcell's scallop theorem, which complicates the actuation scheme needed by microswimmers. However, most biomedically important fluids are non-Newtonian where the scallop theorem no longer holds. It should therefore be possible to realize a microswimmer that moves with reciprocal periodic body-shape changes in non-Newtonian fluids. Here we report a symmetric 'micro-scallop', a single-hinge microswimmer that can propel in shear thickening and shear thinning (non-Newtonian) fluids by reciprocal motion at low Re. Excellent agreement between our measurements and both numerical and analytical theoretical predictions indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. This reciprocal swimming mechanism opens new possibilities in designing biomedical microdevices that can propel by a simple actuation scheme in non-Newtonian biological fluids.

Microdevices for Locomotion in Complex Biological Fluids

Tian Qiu

Locomotion is an essential feature for survival of many organisms, and it is also a technical requirement for untethered biomedical microdevices to operate inside the human body for targeted drug delivery and minimally invasive surgery. The overall goal of this thesis is to develop microdevices that are capable of actively propelling themselves through and performing tasks in complex biological fluids. Two major challenges are encountered: the complex fluidic environment and the small length scale. The first part of the thesis addresses the first challenge: the diverse rheological properties of biological fluids. Most biological fluids are non-Newtonian, and to move in these fluids, the propulsion scheme is highly dependent on the size of the device: 1) When it is comparable or smaller than the mesh size of the biopolymeric network, then the device experiences purely viscous drag, thus the schemes for propulsion in Newtonian fluids will still be effective. To study the active microrheology of porous biological fluids, a magnetic tweezer set-up is constructed to pull magnetic beads of varying size through the porcine vitreous. It is found that for unhindered propulsion through the vitreous, nanodevices with a cross-section of less than 500 nm are needed. 2) When the size of the device is larger than the fluidic mesh size, non-Newtonian properties of the fluid become important and can be utilized for propulsion. We design the first symmetric micro-swimmer actuated by reciprocal motion and demonstrate its propulsion in biologically-relevant fluids. A larger scale low Reynolds number swimmer model is also built and excellent agreement between measurements and theory indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. It opens new possibilities of using a simple actuation scheme for propulsion in non-Newtonian biological fluids. The second challenge is the small size of the device which severely limits the choice of available miniaturized actuators. We explore a novel wireless ultrasonic actuation scheme and a prototype is tested for application in the urinary tract. In order to test the prototype, a general technique is first developed to build a phantom of the human kidney. Based on high resolution medical imaging data, a three dimensional (3D) model is constructed using 3D printing and polymer casting techniques. The phantom not only faithfully reproduces the anatomical structural details, but also mimics the mechanical properties of the real tissue. To address the challenge of powering untethered microdevices, a novel bubble array streaming surface (BASS) actuator is developed. Under ultrasonic excitation, the oscillation of micro gas bubbles results in acoustic streaming and provides a propulsive force that drives the device. Ultrasonic actuators with different bubble sizes are fabricated, and individual driving frequency and propulsive force are measured. The actuator operates as an end-effector of a miniaturized endoscope, which has a cross-sectional side length of only 1 mm, thinner than the endoscopes currently in use. The tip of the end-effector is equipped with a miniaturized camera, its orientation is controlled wirelessly by the actuator, and active cystoscopy is successfully performed inside a rabbit bladder. Miniaturized medical instruments and micro-robots will benefit from the wireless actuation schemes demonstrated herein.

EPFL2016

Dam Break of Newtonian Fluids and Granular Suspensions

Nicolas Andreini

The objective of this thesis was to increase our understanding of two-phase geophysical flows (e.g. debris flows) by providing velocity profiles in idealized laboratory avalanches. To that end, we developed a new experimental platform made up of an inclined flume coupled to an imaging system to measure velocity in granular suspension. The inclined flume was 3.5 m long and 10 cm wide and could be inclined from 0 to 35°. A reservoir with the capacity for 10 l of fluid was located in the upper part of the flume and closed with a pneumatic controlled gate. Velocity profiles were obtained using Particle Image Velocimetry (PIV) and index-of-refraction matching of the solid and liquid phases. We used transparent PMMA beads with mean diameters of 200 µm and the interstitial fluid was composed of a mixture of three fluids. The interstitial fluid was adapted in order to match the refraction index and the density of the solid phase. Using pulsed laser and a high speed camera we were able to measure velocity profiles at frequencies up to 1000 Hz with very good precision. Two additional cameras tracked the front position along the flume with a frequency of 30 Hz and a spatial resolution of 1 mm. Prior to acquiring data on the granular suspension, we tested our system on Newtonian fluids. Eight flow configurations were selected with different fluids (glycerol and triton X100), different slopes and different released masses. Velocity profiles were found to be parabolic far from the front as well as very close to the contact line. However, near the front, quantitative theoretical predictions given by lubrication theory diverged from experimental results. Velocities were significantly overestimated (∼ 400%) by the theory at low Reynolds numbers (Re < 2) and slightly underestimated (∼ 10%) at high Reynolds numbers (Re > 8). Very good agreement with theory far from the front indicated that the accuracy of the setup was good (reliable calibration procedure and image processing methods). Experiments on granular suspensions revealed a variety of behaviors depending on the particle concentration, the slope and the mass released. At solid fractions up to 45%, suspensions behaved as homogeneous viscous fluids. For the duration of the experiment, it was not possible to detect any inhomogeneity due to migration or sedimentation. In the range of shear rate tested and with the precision allowed by the setup no shear thickening or shear thinning was observed since velocity profiles remained perfectly Newtonian. For slightly more concentrated suspensions (up to 55%), we found that the flow dynamics at the bulk scale could still be described using a viscous theory. However, at the local scale, migration gave rise to concentration inhomogeneities producing a blunted velocity profile. The shape of the blunted profile was well described by the Mills and Snabre migration model coupled to a Krieger-Dougherty effective viscosity. However, magnitudes of the velocities were largely overestimated, most probably because we fitted the effective viscosity at higher shear rates. Above 55%, small released masses with high solid fractions stopped after a finite time and separation between fluid and solid phases occurred. The solid frame stayed at rest while the fluid seeped through the granular media eroding the front. For larger released masses, we observed successions of different regimes: After an inertial regime and a pseudo-viscous regime, the flow slowed down, corresponding to a new regime in which the shearing was localized in a thin layer at bottom and there was no shearing of the front. At the same time, we observed that the free surface deformed and became wavy. Fractures developed on the top of the flow and, if they grew sufficiently, modified the local velocity field substantially. Finally, at longer time (≥ 4 min) an intermittent motion (stick-slip) was observed with phases during which the suspension was flowing in a quasi-steady regime and phases during which the suspension was at a halt.

EPFL2012

SMOOTHED PARTICLE HYDRODYNAMICS APPLIED TO THE SIMULATION OF LOW REYNOLDS NUMBER FLOWS

Pierre Maruzewski

In this work we use the Smoothed Particle Hydrodynamics methodology (SPH) to simulate incompressible viscous flows of Newtonian fluids. The SPH is a meshfree Lagrangian method which simulates the flow like a set of moving fluid particles that interact one with others by hydrodynamics forces. The forces acting on each particle are evaluated using a spatial filtering process, taken over a certain region in the neighborhood of the particle which is defined by a smoothing length, h. The filter kernel function and the ratio of the mean spatial displacement of the particles, and the smoothing length, h, are both of central importance for the accuracy of this methodology. A study of the relative importance of this parameters and also of the boundary condition scheme and pressure constitutive model are performed in a context of low Reynolds numbers flow. The model is validated by comparison with analytical solution of the Stokes equations. Adicionalmente, simula-se o caso da cavidade para Re = 1 e Re = 100. Good agreement of the results of the simulations and the reference theoretical data was observed.