Taux de cisaillement | EPFL Graph Search

vignette|redresse=1.7|Un fluide s'écoule de gauche à droite le long d'une limite solide. Ce faisant, il subit un frottement avec elle, ce qui entraîne l'application d'une contrainte de cisaillement. Selon la condition de non-glissement, la vitesse de l'écoulement au contact de cette limite est égale à zéro, mais la vitesse augmente de 0 à v (v = la vitesse du fluide) à une certaine hauteur. La région située entre la frontière et cette hauteur est appelée couche limite et c'est dans cette couche que l'on peut observer la formation de tourbillons contrarotatifs dans l'écoulement. Agrandissement ×100 La vitesse de cisaillement ou « gradient de vitesse » ou « taux de cisaillement », symboles \dot \gamma (gamma point) ou D, mesure le cisaillement appliqué au sein d’un fluide. Elle dépend de la contrainte de cisaillement et de la nature du fluide. Le gradient de vitesse décrit la variation spatiale de la vitesse d’écoulement. La viscosité est la propriété physique intrinsèque

CO2-Driven diffusiophoresis and water cleaning: similarity solutions for predicting the exclusion zone in a channel flow

Mrudhula Baskaran

We investigate experimentally and theoretically diffusiophoretic separation of negatively charged particles in a rectangular channel flow, driven by CO2 dissolution from one side-wall. Since the negatively charged particles create an exclusion zone near the boundary where CO2 is introduced, we model the problem by applying a shear flow approximation in a two-dimensional configuration. From the form of the equations we define a similarity variable to transform the reaction-diffusion equations for CO2 and ions and the advection-diffusion equation for the particle distribution to ordinary differential equations. The definition of the similarity variable suggests a characteristic length scale for the particle exclusion zone. We consider height-averaged flow behaviors in rectangular channels to rationalize and connect our experimental observations with the model, by calculating the wall shear rate as functions of channel dimensions. Our observations and the theoretical model provide the design parameters such as flow speed, channel dimensions and CO2 pressure for the in-flow water cleaning systems.

ROYAL SOC CHEMISTRY2021

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

Swimming by reciprocal motion at low Reynolds number

Peer Fischer, Tian Qiu

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.

Nature Publishing Group2014

Microdevices for Locomotion in Complex Biological Fluids

Tian Qiu

EPFL2016