Michael Baudoin

Low power sessile droplets actuation via modulated surface acoustic waves

Low power actuation of sessile droplets is of primary interest for portable or hybrid lab-on-a-chip and harmless manipulation of biofluids. In this paper, we show that the acoustic power required to move or deform droplets via surface acoustic waves can be substantially reduced through the forcing of the drops inertio-capillary modes of vibrations. Indeed, harmonic, superharmonic, and subharmonic (parametric) excitation of these modes are observed when the high frequency acoustic signal (19.5 MHz) is modulated around Rayleigh-Lamb inertio-capillary frequencies. This resonant behavior results in larger oscillations and quicker motion of the drops than in the non-modulated case.

The air-liquid flow in a microfluidic airway tree

Microfluidic techniques are employed to investigate air-liquid flows in the lung. A network of microchannels with five generations is made and used as a simplified model of a section of the pulmonary airway tree. Liquid plugs are injected into the network and pushed by a flow of air; they divide at every bifurcation until they reach the exits of the network. A resistance, associated with the presence of one plug in a given generation, is defined to establish a linear relation between the driving pressure and the total flow rate in the network. Based on this resistance, good predictions are obtained for the flow of two successive plugs in different generations. The total flow rate of a two-plug flow is found to depend not only on the driving pressure and lengths of the plugs, but also the initial distance between them. Furthermore, long range interactions between daughters of a dividing plug are observed and discussed, particularly when the plugs are flowing through the bifurcations. These interactions lead to different flow patterns for different forcing conditions: the flow develops symmetrically when subjected to constant pressure or high flow rate forcing, while a low flow rate driving yields an asymmetric flow.

Anisotropic Swirling Surface Acoustic Waves from Inverse Filtering for On-Chip Generation of Acoustic Vortices

Antoine Riaud, 2 Jean-Louis Thomas, Eric Charron, Adrien Bussonnière, Olivier Bou Matar, and Michael Baudoin ∗ Institut d’Electronique, de Microélectronique et Nanotechnologie (IEMN), LIA LICS, Université Lille 1 and EC Lille, UMR CNRS 8520, 59652 Villeneuve d’Ascq, France Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 7588, Institut des NanoSciences de Paris (INSP), F-75005, Paris, France (Dated: April 30, 2015)

An extended coupled phase theory for the sound propagation in polydisperse concentrated suspensions of rigid particles

An extension of the classical coupled phase theory is proposed to account for hydrodynamic interactions between neighboring rigid particles, which are essential to describe properly the sound propagation in concentrated suspensions. Rigorous ensemble-averaged equations are derived for each phase and simplified in the case of acoustical wave propagation. Then, closure is achieved by introducing a self-consistent scheme originally developed by Buyevich and Shchelchkova [Prog. Aerosp. Sci. 18, 121-151 (1978)] for incompressible flows, to model the transfer terms between the two phases. This provides an alternative to the effective medium self-consistent theory developed by Spelt et al. [J. Fluid Mech. 430, 51-86 (2001)] in which the suspension is considered as a whole. Here, a significantly simpler formulation is obtained in the long wavelength regime. Predictions of this self-consistent theory are compared with the classical coupled phase theory and with experimental data measuring the attenuation in concentrated suspensions of silica in water. Our calculation is shown to give a good description of the attenuation variation with volume fraction. This theory is also extended to the case of polydisperse suspensions. Finally, the link between the self-consistent theory and the different orders of the multiple scattering theory is clarified.

Dynamics of sessile and pendant drops excited by surface acoustic waves: Gravity effects and correlation between oscillatory and translational motions.

When sessile droplets are excited by ultrasonic traveling surface acoustic waves (SAWs), they undergo complex dynamics with both oscillations and translational motion. While the nature of the Rayleigh-Lamb quadrupolar drop oscillations has been identified, their origin and their influence on the drop mobility remains unexplained. Indeed, the physics behind this peculiar dynamics is complex with nonlinearities involved both at the excitation level (acoustic streaming and radiation pressure) and in the droplet response (nonlinear oscillations and contact line dynamics). In this paper, we investigate the dynamics of sessile and pendant drops excited by SAWs. For pendant drops, so-far unreported dynamics are observed close to the drop detachment threshold with the suppression of the translational motion. Away from this threshold, the comparison between pendant and sessile drop dynamics allows us to identify the role played by gravity or, more generally, by an initial or dynamically induced stretching of the drop. In turn, we elucidate the origin of the resonance frequency shift, as well as the origin of the strong correlation between oscillatory and translational motion. We show that for sessile drops, the velocity is mainly determined by the amplitude of oscillation and that the saturation observed is due to the nonlinear dependence of the drop response frequency on the dynamically induced stretching.

SAW Synthesis With IDTs Array and the Inverse Filter: Toward a Versatile SAW Toolbox for Microfluidics and Biological Applications

Surface acoustic waves (SAWs) are versatile tools to manipulate fluids at small scales for microfluidics and biological applications. A nonexhaustive list of operations that can be performed with SAW includes sessile droplet displacement, atomization, division, and merging but also the actuation of fluids embedded in microchannels or the manipulation of suspended particles. However, each of these operations requires a specific design of the wave generation system, the so-called interdigitated transducers (IDTs). Depending on the application, it might indeed be necessary to generate focused or plane, propagating or standing, and aligned or shifted waves. Furthermore, the possibilities offered by more complex wave fields such as acoustical vortices for particle tweezing and liquid twisting cannot be explored with classical IDTs. In this paper, we show that the inverse filter technique coupled with an IDTs array enables us to synthesize all classical wave fields used in microfluidics and biological applications with a single multifunctional platform. It also enables us to generate swirling SAWs, whose potential for the on-chip synthesis of tailored acoustical vortices has been demonstrated lately. The possibilities offered by this platform are illustrated by performing many operations successively on sessile droplets with the same system.

Sound, infrasound, and sonic boom absorption by atmospheric clouds.

This study quantifies the influence of atmospheric clouds on propagation of sound and infrasound, based on an existing model [Gubaidulin and Nigmatulin, Int. J. Multiphase Flow 26, 207-228 (2000)]. Clouds are considered as a dilute and polydisperse suspension of liquid water droplets within a mixture of dry air and water vapor, both considered as perfect gases. The model is limited to low and medium altitude clouds, with a small ice content. Four physical mechanisms are taken into account: viscoinertial effects, heat transfer, water phase changes (evaporation and condensation), and vapor diffusion. Physical properties of atmospheric clouds (altitude, thickness, water content and droplet size distribution) are collected, along with values of the thermodynamical coefficients. Different types of clouds have been selected. Quantitative evaluation shows that, for low audible and infrasound frequencies, absorption within clouds is several orders of magnitude larger than classical absorption. The importance of phase changes and vapor diffusion is outlined. Finally, numerical simulations for nonlinear propagation of sonic booms indicate that, for thick clouds, attenuation can lead to a very large decay of the boom at the ground level.

Dynamics of liquid plugs in prewetted capillary tubes: from acceleration and rupture to deceleration and airway obstruction.

The dynamics of individual liquid plugs pushed at a constant pressure head inside prewetted cylindrical capillary tubes is investigated experimentally and theoretically. It is shown that, depending on the thickness of the prewetting film and the magnitude of the pressure head, the plugs can either experience a continuous acceleration leading to a dramatic decrease of their size and eventually their rupture or conversely, a progressive deceleration associated with their growth and an exacerbation of the airway obstruction. These behaviors are quantitatively reproduced using a simple nonlinear model [Baudoin et al., Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 859] adapted here for cylindrical channels. Furthermore, an analytical criterion for the transition between these two regimes is derived and successfully compared with extensive experimental data. The potential implications of this work for pulmonary obstructive diseases are discussed.

Removal of living cells from biosensing surfaces in droplet-based microfluidics using surface acoustic waves

Removal of living biological cells from surfaces is a critical process for many applications in the area of biosensing and lab-on-a-chip. Trypsin is one of the most effective biochemical tools used to cleave the cells proteins that are responsible for bonding cells to surfaces [1]. We propose a method using Rayleigh-type (20MHz) surface acoustic wave (SAW)-based mixing [2] as an accelerator for trypsin mediated removal of living cells from surfaces. In the experiments, a 10 µL droplet of Hanks Balanced Salt Solution (HBSS)-trypsin is placed on a piezoelectric substrate covered with human embryonic kidney cells (HEK293). Using phase contrast microscopy, cells removal time for different acoustic power levels and trypsin concentrations is measured. Results from validation experiments show that a minimum of 180 seconds is necessary to completely release surface-bonded cells covered by the 10µL droplet without the use of SAW (negative control). By using microstreaming flow in the droplets generated by the SAW...

Inverse Saffman-Taylor Experiments with Particles Lead to Capillarity Driven Fingering Instabilities.

Using air to displace a viscous fluid contained in a Hele-Shaw cell can create a fingering pattern at the interface between the fluids if the capillary number exceeds a critical value. This Saffman-Taylor instability is revisited for the inverse case of a viscous fluid displacing air when partially wettable hydrophilic particles are lying on the walls. Though the inverse case is otherwise stable, the presence of the particles results in a fingering instability at low capillary number. This capillary-driven instability is driven by the integration of particles into the interface which results from the minimization of the interfacial energy. Both axisymmetric and rectangular geometries are considered in order to quantify this phenomenon.