Time-resolved velocity and pressure field quantification in a flow-focusing device for ultrafast microbubble production

Abstract

Flow-focusing devices have gained great interest in the past decade, due to their capability to produce monodisperse microbubbles for diagnostic and therapeutic medical ultrasound applications. However, up-scaling production to industrial scale requires a paradigm shift from single chip operation to highly parallelized systems. Parallelization gives rise to fluidic interactions between nozzles that, in turn, may lead to a decreased monodispersity. Here we study the velocity and pressure field fluctuations in a single flow-focusing nozzle during bubble production. We experimentally quantify the velocity field inside the nozzle at 100 ns time resolution, and a numerical model provides insight into both the oscillatory velocity and pressure fields. Our results demonstrate that, at the length scale of the flow-focusing channel, the velocity oscillations propagate at fluid dynamical timescale (order of μs) whereas the dominant pressure oscillations are linked to the bubble pinch-off and propagate at a much faster timescale (order of ns).

Microbubble formation by flow focusing: role of gas and liquid properties, and channel geometry

“…To support the experimental observations, we also conducted a series of numerical simulations. The numerical model used is explained in more detail in Cleve et al (2021). In short, both the liquid and gas dynamics were modelled using the incompressible Navier-Stokes equation using sharp-interface arbitrary Lagrangian-Eulerian finite element scheme.…”

“…The simulated domain is 'two-dimensional' assuming radial symmetry of the flow-focusing channel. This model was validated in terms of the bubble shape dynamics (formation and pinch-off), production rate, velocity of the bubbles and average and oscillatory velocity of the surrounding liquid (Cleve et al 2021). Note that, owing to the restriction of the axisymmetric geometry, the simulation cannot perfectly reproduce the non-axisymmetric experimental channel (see figure 1c), thereby displaying quantitative differences when compared with experimental results over a large parameter range.…”

“…Tween 20 Pluronic SDS Water pressure between bubble growth and detachment, Evangelio et al (2015) try to find an optimal description of the pressure gradient for each of their considered cases. From our previous paper (Cleve et al 2021), we know that the pressure variation in the inlet region scales as Δp ∼ ρ l Q 2 l /S 2 , with S being the cross-sectional surface area, and the viscous pressure loss along the channel as dp/dx ∼ μ l Q l /S 2 (see also more detailed explanation in the supplementary material). By plotting the bubble size and production rate as a function of Q l /S 2 (instead the previously used Q l ), we can obtain a perfect match between the data for the short and long channel from figure 2, see figure S3 in the supplementary material.…”

“…Experiments were conducted in flow-focusing devices, which were fabricated from two isotropically etched glass wafers, aligned and bonded together. The flow-focusing channel has a length l and its cross-section has a width w and a height h with rounded corners (radius of curvature h/2), see Cleve et al (2021) and Segers et al (2018a) for more detail. Two different channel sizes were used, as shown in figure 1(a,b).…”

“…The gas volume flow rate also varies along the channel axis: as bubbles flow downstream through the flow-focusing region, they experience a pressure drop of up to several bars (Cleve et al. 2021).…”