Microfluidos: ¿cuánto hay de nuevo?

Microfluidic groups many branches of Physics ranging from Fluid Dynamics to Electronics and closely linked with Biology Sciences. Its interdisciplinary character is a distinctive feature, as the times we are living today. In the last ten years, along with the development of biotechnology, microelectronics, materials science and many others, the use of microfluidic devices has increased remarkably. Few scientists in Cuba have worked in topics related with Microfluidics; nevertheless, there is no reference in which this activity has been pointed out explicitly. This article aims to introduce some terms and divulge basic aspects of Microfluidics; besides this, another article will be published with most applications of Microfluidics that could be of interest for Cuba

Cavitation induced by pulsed and continuous-wave fiber lasers in confinement

Bubbles generated with lasers under confinement have been investigated for their potential use as the driving mechanism for liquid micro-jets in various microfluidic devices, such as needle free jet injectors. Here, we report on the study of bubble formation by a continuous-wave (CW) and a pulsed laser inside an open-ended microfluidic capillary. This results in a direct comparison between bubbles generated by laser sources emitting light in different time scales (ms and ns). The bubble kinetics represents an important parameter because it determines the available kinetic energy for a subsequent liquid jet. We show that the bubble growth rate increases linearly with the delivered energy for both the CW and the pulsed laser. Experiments show that at equal absorption coefficient, the bubble growth for both lasers is similar, which indicates that they can be used interchangeably for a jet generation. However, bubbles generated by a CW laser require more optical energy, which is due to heat dissipation. Furthermore, the bubbles generated by the CW laser show a slightly larger variation in size and growth rate for identical initial conditions, which we attribute to the stochastic nature of thermocavitation

Gas density influences the transition from capillary collapse to surface seal in microfluidic jet impacts on deep pools

Studies of liquid jet impacts onto a deep liquid pool are of great significance for a multitude of engineering and environmental applications. During jet impact, the free surface of the pool deforms and a cavity is generated. Simultaneously, the free surface of the cavity extends radially outward and forms a rim. Eventually the cavity collapses by means of gas inertia and surface tension. In this work we study numerically such cavity collapse, under different impact velocities and ambient gas density conditions. An axisymmetric numerical model, based on the volume of fluid method is constructed in Basilisk C. This model is validated by qualitative and quantitative comparison with theory and experiments, in a parameter range that has not been previously explored. Our results show two distinct regimes in the cavity collapse mechanism. By considering forces pulling along the interface, we derive scaling arguments for the time of closure and maximum radius of the cavity, based on the Weber number. For jets with uniform constant velocity from tip to tail and $We \leq 150$ the cavity closure is capillary dominated and happens below the surface (deep seal). In contrast, for $We \geq 200$ the cavity closure happens above the surface (surface seal) and is dominated by the gas entrainment and the pressure gradient that it causes. Our results provide information for understanding pollutant transport during droplet impacts on large bodies of water, and other engineering application, like additive manufacturing, lithography and needle-free injections

Laser beam properties and microfluidic confinement control thermocavitation

Thermocavitation, the creation of a vapor bubble by heating a liquid with a continuous-wave laser, has been studied for a wide range of applications. Examples include the development of an actuator for needle-free jet injectors, as the pumping mechanism in microfluidic channels and crystallization or nanoparticle synthesis. Optimal use in these applications require control over the dynamics of the laser-generated bubble through the laser power and beam radius. In contrast to pulsed lasers, for continuous-wave lasers the influence of the laser beam radius on the bubble characteristics is not fully understood. Here, we present a novel way to control the size of the beam from an optical fiber by changing the distance from the glass-liquid interface. We show that the increase in beam size results in a longer nucleation time. Numerical simulations of the experiment show that the maximum temperature at the moment of nucleation is 237$\pm$5{\deg}C and independent of laser parameters. Due to delayed nucleation for larger beam sizes, more energy is absorbed by the liquid at the nucleation instant. Consequently, a larger beam size results in a faster growing bubble, producing the same effect as reducing the laser power. We conclude that the total bubble energy only depends on the amount of absorbed optical energy and it is independent of the beam radius and laser power for any amount of absorbed energy. This effect contrasts with pulsed lasers, where an increase in beam radius results in a reduction of bubble energy. Our results are of relevance for the use of continuous-wave laser-actuated cavitation in needle-free jet injectors as well as other applications of thermocavitation in microfluidic confinement

Controllable production of Janus ligaments by AC fields in a flow-focusing junction

We report the production of bicomponent Janus filaments of miscible aqueous fluids in a micro- fluidic electro-flow focusing device under the action of an AC electric field. The production of liquid filaments can lead to the generation of microfibers by adding a subsequent process of polymerization. Janus microfi- bers are of paramount importance in biomedical appli- cations such as tissue production on crimped scaffolds. We show that the filament length is a function of the frequency signal, voltage amplitude and of the visco- sity and conductivity of the dispersed phase. In par- ticular, Janus filaments with diameters ∼ 10μm and longer than 1 mm are produced by AC voltages with frequencies below 150 kHz and a viscosity of the dis- persed phase ∼ 10 cP

A novel ultrasonic cavitation enhancer

We introduce a Cavitation Intensifying Bag as a versatile tool for acoustic cavitation control. The cavitation activity is spatially controlled by the modification of the inner surface of the bag with patterned pits of microscopic dimensions. We report on different measurements such as the transmission of ultrasound, temperature increase inside the bag during sonication. Several applications of interest to other scientific activities are also demonstrated