Master thesis : Bursting conditions of passive capillary valves and subsequent emptying dynamics in centrifugal microfluidic platforms

Abstract

Fully-automated sample-to-answer biochemical assays are needed in intensive care units, e.g., for the real-time monitoring of antibiotic concentration in blood samples from patients whose physiological state may quickly vary. The Lab-on-Disk developed in the Medicare project offers a promising integrated solution to this request. A Lab-on-Disk is a centrifugal microfluidic platform that consists in a rotating disk that contains microchambers and microchannels. The liquids flow therein thanks to centrifugal forces and can be mixed together to perform the assays. For the robustness of the assays, liquids have to be regulated by valves. In particular, the passive capillary valve is the simplest to implement as it only consists in an abrupt enlargement of the channel geometry.

This master's thesis aims at identifying the bursting conditions of a given valve and at characterizing the emptying dynamics of the reservoir upstream once the valve opens. Thanks to a first set of experiments, which consisted in performing a ramp on the angular frequency, we could identify the bursting pressure of a given valve. In a second set, we performed a pulse on the frequency to trigger the valve, followed by a steady frequency plateau during which the reservoir chamber emptied. The properties of the fluid interfaces were measured by computer vision. High-resolution videos of the experiments were processed by the Medicare HUB software developed as part of the master's thesis.

Considering the bursting conditions, the Laplace pressure of the interface in the reservoir chamber is shown to add a significant contribution to the pressure balance. The bursting pressures can be expressed as a Laplace law that involves a geometrical parameter independent of the angular acceleration in the range [0, 1000 RPM]. The main source of uncertainty is identified as the time discretization due to the video recording. Then, considering the emptying dynamics, the flow in the waste chamber is characterized by a Bond number greater than or close to unity. Five emptying scenarios are characterized on the basis of the outlet jet behaviour and a phase diagram is presented to discriminate between them. Lastly, an analytical model is presented to reproduce the experimental observations and its limitation are discussed thanks to the phase diagram.