The effect of contact angles and capillary dimensions on the burst frequency of super hydrophilic and hydrophilic centrifugal microfluidic platforms, a CFD study.

This paper employs the volume of fluid (VOF) method to numerically investigate the effect of the width, height, and contact angles on burst frequencies of super hydrophilic and hydrophilic capillary valves in centrifugal microfluidic systems. Existing experimental results in the literature have been used to validate the implementation of the numerical method. The performance of capillary valves in the rectangular and the circular microfluidic structures on super hydrophilic centrifugal microfluidic platforms is studied. The numerical results are also compared with the existing theoretical models and the differences are discussed. Our experimental and computed results show a minimum burst frequency occurring at square capillaries and this result is useful for designing and developing more sophisticated networks of capillary valves. It also predicts that in super hydrophilic microfluidics, the fluid leaks consistently from the capillary valve at low pressures which can disrupt the biomedical procedures in centrifugal microfluidic platforms

Motion of long bubbles in gravity- And pressure-driven flow through cylindrical capillaries up to moderate capillary numbers

The motion of bubbles and drops through tubes in gravity- and pressure-driven flows is intensively studied numerically and experimentally. The Bretherton asymptotic expressions predict axisymmetric bubbles movement at low velocities. They describe the dependence of capillary (Ca) and Bond (Bo) numbers on the system parameters but are valid only in the ranges 0 < Ca < 0.005 and 0.84 < Bo < 1.04. This paper investigates the gravity-induced motion of bubbles with free or tangentially immobile interfaces in pressure-driven flows. We derive the exact solution of the hydrodynamic problem using the lubrication approximation in the zero- and first-order approximations for pressure and fluid velocity. The respective boundary value problem for the bubble shape is solved numerically to obtain the wetting film thickness, h, between the bubble and the capillary and the dependence of the capillary numbers on the flow parameters and magnitude of gravity. The proposed model expands the applicable solution ranges by 400 and 38 times, respectively (0 < Ca < 2 and 0 < Bo < 7.5), validated with available experimental data. The model's simplicity and transparency open the possibility to generalize this approach including determining new physicochemical properties of liquids and interfaces

Gradient flow perspective on thin-film bilayer flows

We study gradient flow formulations of thin-film bilayer flows with triple-junctions between liquid/liquid/air phase. First we highlight the gradient structure in the Stokes free-boundary flow and identify its solutions with the well-known PDE with boundary conditions. Next we propose a similar gradient formulation for the corresponding reduced thin-film model and formally identify solutions with those of a PDE problem. A robust numerical algorithm for the thin-film gradient flow structure is then provided. Using this algorithm we compare the sharp triple-junction model with precursor models. For their stationary solutions a rigorous connection is established using Gamma -convergence. For time-dependent solutions the comparison of numerical solutions shows a good agreement for small and moderate times. Finally we study spreading in the zero-contact angle case, where we compare numerical solutions with asymptotically exact source-type solutions