5.4 Micro-Sonoreactors: Miscellaneous Applications
81
or impurities in liquids suffice to create gas bubbles at pressures much lower than
those required to overcome cohesion forces in liquids (the Blake threshold pressure).
In addition, acoustically driven capillary waves travelling on the liquid surface can
induce cavitation in narrow channels as well (Fernandez Rivas and Kuhn 2016).
As mentioned before, standing waves represent a convenient tool for the fabrication of microactuators and microsensors, which are suitable for analytical and bioanalytical applications. The acoustic radiation pressure along with other secondary
forces arising from such high ultrasonic frequencies causes the levitation of suspended matter that can further be concentrated and separated. This acoustophoresis
has been employed in the fractionation and separation of particles, including blood
cells in continuous flow (Lenshof et al. 2012), and dynamic photobioreactor systems
that enhance the medium replacement process in concentrated microalgae cultivation
(Lee and Li 2017). In routine proteomics protocols, a microscale in-gel processing
and tryptic digestion of proteins takes no more than 30 min (overall) using SAWs
(at ~20 MHz). The method competes favourably with the standard experiment that
involves overnight digestion (Kulkarni et al. 2010).
Ultrasonic equipment, such as bath or probes (also known as horns), can be considerably inefficient to transmit the acoustic power as the system becomes smaller if
done without a proper understanding of the phenomena behind acoustic cavitation.
The miniaturization of these devices is also a technical challenge often overlooked.
In contrast, integrating a piezoelectric actuator directly into the microfluidic system
has been proven to be more energetically efficient, but design considerations need
to be followed (Navarro-Brull et al. 2014). The diagram shown in Fig. 5.9 shows
a home-made design, where a piezo-element (transducer) converts electricity into
mechanical vibrations that are transferred to the liquid contained in the microreactor. Typical phenomena resulting from acoustic cavitation are summarized on the
top, namely, formation of radical species, shock waves, streaming or sonoluminescence (Fernandez Rivas et al. 2012a, b, c; Fernandez Rivas and Kuhn 2016). The
presence of artificially created crevices ensures the formation of bubbles at desired
spots, which resulted in lowering the energy threshold for cavitation, and a significant
increase in the energy efficiency measured in terms of radical production (Fernandez
Rivas et al. 2010). Such microreactors are ideal for studying fundamental aspects of
sonochemistry under spatial control, and other mechanisms or effects of cavitation,
such as bubble nucleation, erosion, surface cleaning and sonoluminescence (Fernandez Rivas et al. 2012b, c, 2013a, b). Additionally, bubble structures generated from
micromachined crevices on surfaces show more homogeneous size and distribution,
as inferred from luminol luminescence, than the multibubble configuration arising
from bath or horns at a given power and frequency (Fernandez Rivas et al. 2010).
The same microreactor principle based on artificial crevices has been used for
improving mixing when acoustic pressures lower than required for bubble nucleation
are employed (Bolaños-Jiménez et al. 2017). As shown in Fig. 5.9b), this reactor has
also been scaled-up and numbered up, though still as a batch type. The authors have
proposed further studies to attain continuous flow conditions.
In a different multilayered design, shown in Fig. 5.10, a piezoelectric actuator is
integrated with a Teflon microreactor that was employed for handling solid-forming
81
or impurities in liquids suffice to create gas bubbles at pressures much lower than
those required to overcome cohesion forces in liquids (the Blake threshold pressure).
In addition, acoustically driven capillary waves travelling on the liquid surface can
induce cavitation in narrow channels as well (Fernandez Rivas and Kuhn 2016).
As mentioned before, standing waves represent a convenient tool for the fabrication of microactuators and microsensors, which are suitable for analytical and bioanalytical applications. The acoustic radiation pressure along with other secondary
forces arising from such high ultrasonic frequencies causes the levitation of suspended matter that can further be concentrated and separated. This acoustophoresis
has been employed in the fractionation and separation of particles, including blood
cells in continuous flow (Lenshof et al. 2012), and dynamic photobioreactor systems
that enhance the medium replacement process in concentrated microalgae cultivation
(Lee and Li 2017). In routine proteomics protocols, a microscale in-gel processing
and tryptic digestion of proteins takes no more than 30 min (overall) using SAWs
(at ~20 MHz). The method competes favourably with the standard experiment that
involves overnight digestion (Kulkarni et al. 2010).
Ultrasonic equipment, such as bath or probes (also known as horns), can be considerably inefficient to transmit the acoustic power as the system becomes smaller if
done without a proper understanding of the phenomena behind acoustic cavitation.
The miniaturization of these devices is also a technical challenge often overlooked.
In contrast, integrating a piezoelectric actuator directly into the microfluidic system
has been proven to be more energetically efficient, but design considerations need
to be followed (Navarro-Brull et al. 2014). The diagram shown in Fig. 5.9 shows
a home-made design, where a piezo-element (transducer) converts electricity into
mechanical vibrations that are transferred to the liquid contained in the microreactor. Typical phenomena resulting from acoustic cavitation are summarized on the
top, namely, formation of radical species, shock waves, streaming or sonoluminescence (Fernandez Rivas et al. 2012a, b, c; Fernandez Rivas and Kuhn 2016). The
presence of artificially created crevices ensures the formation of bubbles at desired
spots, which resulted in lowering the energy threshold for cavitation, and a significant
increase in the energy efficiency measured in terms of radical production (Fernandez
Rivas et al. 2010). Such microreactors are ideal for studying fundamental aspects of
sonochemistry under spatial control, and other mechanisms or effects of cavitation,
such as bubble nucleation, erosion, surface cleaning and sonoluminescence (Fernandez Rivas et al. 2012b, c, 2013a, b). Additionally, bubble structures generated from
micromachined crevices on surfaces show more homogeneous size and distribution,
as inferred from luminol luminescence, than the multibubble configuration arising
from bath or horns at a given power and frequency (Fernandez Rivas et al. 2010).
The same microreactor principle based on artificial crevices has been used for
improving mixing when acoustic pressures lower than required for bubble nucleation
are employed (Bolaños-Jiménez et al. 2017). As shown in Fig. 5.9b), this reactor has
also been scaled-up and numbered up, though still as a batch type. The authors have
proposed further studies to attain continuous flow conditions.
In a different multilayered design, shown in Fig. 5.10, a piezoelectric actuator is
integrated with a Teflon microreactor that was employed for handling solid-forming