Farzam Zoueshtiagh

Droplet displacements and oscillations induced by ultrasonic surface acoustic waves: A quantitative study

We present an experimental study of a droplet interacting with an ultrasonic surface acoustic wave. Depending on the amplitude of the wave, the drop can either experience an internal flow with its contact line pinned, or (at higher amplitude) move along the direction of the wave also with internal flow. Both situations come with oscillations of the drop free surface. The physical origins of the internal mixing flow as well as the drop displacement and surface waves are still not well understood. In order to give insights of the underlying physics involved in these phenomena, we carried out an experimental and numerical study. The results suggest that the surface deformation of the drop can be related to a combination between acoustic streaming effect and radiation pressure inside the drop.

Laminar-turbulent boundary-layer transition over a rough rotating disk

Boundary-layer transition over a disk spinning under water is investigated. Transitional Reynolds numbers, Rec, and associated boundary-layer velocity profiles are determined from flow-visualizations and hot-film measurements, respectively. The value of Rec and the velocity profiles are studied as a function of the disk’s surface roughness. It is found that transition over rough disks occurs in a similar fashion to that over smooth disks, i.e., abruptly and axisymmetrically at well-defined radii. Wall roughness has little effect on Rec until a threshold relative roughness is reached. Above the threshold Rec decreases sharply. The decrease is consistent with the drop one expects for our flow for the absolute instability discovered by Lingwood [J. Fluid Mech. 299, 17 (1995); 314, 373 (1996); 331, 405 (1997)]. This indicates that the Lingwood absolute instability may continue to play a major role in the transition process even for large relative roughness.

To grate a liquid into tiny droplets by its impact on a hydrophobic microgrid

We report on experiments of drop impacting a hydrophobic microgrid of typical spacing a few tens of micrometers. Above a threshold in impact speed, liquid emerges to the other side, forming microdroplets of size about that of the grid holes. We propose a method to produce either a monodisperse spray or a single tiny droplet of volume as small as a few picolitres corresponding to a volume division of the liquid drop by a factor of up to 105. We also discuss the discrepancy of the measured thresholds with that predicted by a balance between inertia and capillarity.

Experimental and numerical study of miscible Faraday instability

A study of the Faraday instability of diffuse interfaces between pairs of miscible liquids of different densities, by means of experiments and by a nonlinear numerical model, is presented. The experimental set-up consisted of a rectangular cell in which the lighter liquid was placed above the denser one. The cell in this initially stable configuration was then subjected to vertical vibrations. The subsequent behaviour of the ‘interface’ between the two liquids was observed with a high-speed camera. This study shows that above a certain acceleration threshold an instability developed at the interface. The amplitude of the instability grew during the experiments which then led to the mixing of the liquids. The instability finally disappeared once the two liquids were fully mixed over a volume, considerably larger than the initial diffuse region. The results of a companion two-dimensional nonlinear numerical model that employs a finite volume method show very good agreement with the experiments. A physical explanation of the instability and the observations are advanced.

Bubble splitting in oscillatory flows on ground and in reduced gravity

The stability of centimeter scale air bubbles is studied in quiescent suspending liquid under an imposed oscillatory acceleration field. Experiments were performed in reduced- and normal-gravity environments. A strong acceleration resulted in an instability leading to the breakups of the bubbles in both gravity environments. The breakup onset was investigated and found to be characterized by a critical acceleration acr . The influence of the liquid viscosity and the gravitational environment was studied. Empirical correlations for the onset are presented and discussed with the intention to reveal splitting mechanism. The inertial mechanism often deemed to cause the breakup of drops subjected to a rapid gas stream is shown to give explanations consistent with the experiments. A breakup criterion for both gravitational environments is proposed through discussions from an energetic point of view.

Enhancement of biosensing performance in a droplet-based bioreactor by in situ microstreaming

A droplet-based micro-total-analysis system involving biosensor performance enhancement by integrated surface-acoustic-wave (SAW) microstreaming is shown. The bioreactor consists of an encapsulated droplet with a biosensor on its periphery, with in situ streaming induced by SAW. This paper highlights the characterization by particle image tracking of the speed distribution inside the droplet. The analyte-biosensor interaction is then evaluated by finite element simulation with different streaming conditions. Calculation of the biosensing enhancement shows an optimum in the biosensor response. These results confirm that the evaluation of the Damköhler and Peclet numbers is of primary importance when designing biosensors enhanced by streaming.

Experimental verification of Type-II-eigenmode destabilization in the boundary layer over a compliant rotating disk

Destabilization of the Type-II eigenmode in boundary layers over compliant rotating disks was predicted theoretically by Cooper and Carpenter [J. Fluid Mech. 350, 231 (1997)]. Their results showed that for relatively low levels of compliance the Type-II eigenmode was destabilized, to be stabilized and ultimately eliminated for higher levels of compliance. The goal of the present study was to obtain the first experimental verification of the prediction that the Type-II mode can be destabilized at low levels of compliance. To this end a new type of rotating-disk apparatus was designed and a new type of material was used to produce suitable compliant walls for the experiments. Background noise in the new facility is substantially reduced in comparison with that in facilities used in related previous studies. This enabled the detection of substantially cleaner hot-film signals. Although the mean base flow remained unchanged, noise characteristics have been improved and turbulence intensities are significantly reduced. The measurements reveal not only the comparatively strong signals from the Type-I (cross-flow vortices) instability mode but also clear evidence of the Type-II eigenmode. In agreement with the theory of Cooper and Carpenter the data analysis shows that relatively low levels of wall compliance destabilize the Type-II mode.

The Faraday instability in miscible fluid systems

The Faraday instability arising in distinct miscible fluid layers, when the parametric forcing is parallel to the gravity vector, is analysed. A time-dependent density gradient is established from the moment the fluid layers are placed in contact with one another. The operating parameters in a miscible Faraday system are the frequency of parametric forcing and the wait time between the initial contact of fluids and the commencement of oscillations. Using a linearized theory that invokes a quasi-steady approximation, the vibrational threshold required for the onset of Faraday instability is evaluated for these parameters and several observations are made. First, the criticality is observed to occur at a sub-harmonic frequency. Second, the large magnitude of the concentration gradient at early wait times is found to make the thin layers highly unstable. Third, the stability increases with forcing frequency, owing to the increased dissipation of the resulting choppy waves. All these observations qualitatively agree with experiments. Finally, a calculation reveals that an increase in gravity increases the critical wavelength of flow onset and results in the reduction of critical input acceleration.

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.

Bubble rupture in a vibrated liquid under microgravity

The response of an air bubble surrounded by a liquid in a sealed cell submitted to vibrations was investigated experimentally under microgravity conditions and compared to experiments under normal gravity conditions. As in normal gravity [1], it was observed that the bubble split into smaller parts when the acceleration of the vibrations reached a threshold. This threshold in microgravity is substantially smaller than that in normal gravity. Experimental results are presented in terms of an acceleration based Bond number which has been found to characterize the bubble behaviour in the laboratory experiments [1].