New regime of droplet generation in a T-shape microfluidic junction

International audienceWe present an experimental study of a new regime of monodisperse micro-droplet generation that we named the balloon regime. A dispersion of oil in water in a T-junction microfluidic system was studied. Several microfluidic devices having different cross-sections of the continuous and the dispersed phases micro-channels were tested. This new regime appears only for low- dispersed phase velocity. The micro-droplet size is mainly related to the geometry of the T-junction micro-channels especially its width and depth, and independent of the continuous and dispersed phases velocities. In our experiments, the velocities of the continuous and the dispersed phases vc and vd respectively, have been varied in a wide range: vc from 0.5 to 500 mm/s, and vd from 0.01 to 30 mm/s. We show that the continuous phase only controls the micro-droplet density, while the dispersed phase linearly changes the frequency of the micro-droplet generation. Another particularity of the present regime, which differentiates it from all other known regimes, is that the micro-droplet retains its circular shape throughout its formation at the T junction, and undergoes no deformation due to the drag forces. We propose a mechanism to explain the formation of microdroplets in this new regime

Interaction between liquid layer and vibrating plates. Application to displacement of liquid droplets

Various experiments have been performed to study the interaction of a liquid layer and vibrating plates. A liquid layer deposited on a vibrating plate exhibits a deformation of the surface with a high amplitude of vibration (larger than 1 mum at 30kHz). Furthermore, a water droplet placed on the vibrating plate moves towards an antinode of vibration. These non-linear phenomena are explained by the action of acoustic radiation pressure. An application to the displacement of droplets is presented

Parallel active mixing microdroplet array

In the micro- and nano-fluidics field, the mixing of droplets is a difficult challenge. This function is usually achieved by mixing two or more continuous flows via their injection in the same micro-channel. This strategy may be further improved by the active mixing introduced by an additional energy source plugged into the system to create flow instabilities. For example, ultrasonic mixers using stationary wave patterns or surface acoustic waves (SAW) were developed in order to decrease the mixing time and to improve the homogeneity of continuous-flow mixtures. Some micro-devices using microchanels even permit to generate micro-drops of reagents and to coalesce them in a carrier continuous phase. An alternative approach to continuous microfluidic systems is the manipulation of discrete droplets. The electrowetting-based linear-array droplet mixer, for example, proves that microdroplets can be transported, merged and actively mixed using an electrostatic field. Acoustic field can also be used for that purpose, several examples have been presented using, for the most part, high frequency vibrations such as SAW devices. In this paper, we introduce a parallel microdroplet mixing strategy based on local acoustic field generation using low frequency vibrations. An active acoustic mixer array was designed and fabricated. This microfluidic device permits the creation in parallel of an active mixture in a matrix of 25 microdroplets that are localized on a surface. It was designed to avoid contamination between droplets while they are excited. Experiments showing the independent and active mixture of droplets will be presented

Performing microdroplets mixing using an acoustic transducer with low vibration frequencies

In this paper, we introduce a parallel microdroplet mixing strategy based on local acoustic field generation using low frequency vibrations. An active acoustic mixer array was designed and fabricated. This microfluidic device permits the creation in parallel of an active mixture in a matrix of 25 microdroplets that are localized on a surface. It was designed to avoid contamination between droplets while they are excited. Experiments showing the active mixture of droplets will be presented

FEM modelling of Piezo-actuated Microswitches

International audienceThis article presents a study of microswitches with piezoelectric actuation. With the help of analysis modelling and FEM commercial software (ANSYS), we investigated the potentiality of AlN as piezoelectric actuator. Firstly, we compared AlN with PZT to actuate simple structures by bimorph effect as cantilever or clamped-clamped membrane. After this investigation, we focused on means to improve the deflection and the contact force of structures, by mechanical considerations, in case of an AlN actuation. This resulted in several designs of microswitches, with two different actuation mechanisms: bending or buckling. To go further, we evaluated some technological aspects as the influence of residual stresses and the shape of membrane on an AlN piezo-actuated structure

Temperature compensation of lamb wave sensor by combined antisymmetric mode and symmetric mode

International audienceBoth thermal sensitivity and mass sensitivity in liquid of the first antisymmetric (A0) mode and the first symmetric (S0) mode of Lamb wave biosensor were investigated. A0 and S0 modes are sensitive to the mass change on the surface of the sensor but A0 mode is also sensitive to the liquid in the region of evanescent wave associated with Lamb wave. By combining A0 mode and S0 mode, the measurement error due to the environmental temperature drift decreased by a large factor, therefore, the environmental temperature was efficiently compensated without changing the structure of Lamb wave sensor

Measurements of evanescent wave in a sandwich Lamb wave sensor

International audienceOne method for evanescent wave measurement of the Lamb wave biosensor is proposed by putting another Lamb wave device above the first with the distance less than the evanescent field penetration depth in the liquid. The liquid layer is sandwiched with the two Lamb wave devices. The devices are interacted by an evanescent field; thus the evanescent wave can be studied. The mode is split by the interaction of the evanescent wave. The investigation of the evanescent field gives insight into acoustic biosensors and provides precise and multiparameter measurements. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3009562