Sensors and Actuators A 211 (2014) 19–26 Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l h o mepage: www.elsevier.com/locate/sna Acoustic mixer using low frequency vibration for biological and chemical applications Faten Kardous a,b,∗ , Réda Yahiaoui c , Boujemâa Aoubiza d , Jean-Franc ¸ ois Manceau c a Nanotechnology Group, INSAT, Bp 676, Centre Urbain Nord, 1080 Charguia Cedex, Tunisia b Nanomedicine Lab, Imagery and Therapeutics, Université de Franche-Comté Besanc ¸ on, France c Institut FEMTO-ST, Université de Franche-Comté, CNRS, ENSMM, UTBM, F-25044 Besanc ¸ on, France d Laboratoire de Mathématiques Université de Franche-Comté, CNRS, Besanc ¸ on, France a r t i c l e i n f o Article history: Received 20 September 2013 Received in revised form 26 February 2014 Accepted 1 March 2014 Available online 12 March 2014 Keywords: Microdroplet mixing Acoustic mixing Thermal acoustic effect a b s t r a c t Liquid mixing at micro-scale is considered a challenge which is even tougher to overcome in the case of discrete microfluidic. Many researchers have developed strategies and tried to be pioneer in mixing solutions for lab on chip. In this paper, we present a parallel microdroplet mixer based on acoustic field generation using a low frequency vibration (up to few hundreds of kilohertz). This device can be used for lab on chip applications, since the liquid characteristics are not disturbed by the plugged energy and involve relatively simple microfabrication techniques. We designed, fabricated, evaluated, presented experiments showing the microdroplet active mixing, and investigated the thermal effect of the created acoustic energy. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Liquid mixing is usually achieved in continuous flows via liquid injection in the same micro-channel [1]. Nevertheless, at micro- and nano-scale, liquid mixing is a difficult challenge. In fact, in these cases Reynolds number R e is doubly reduced by characteristic length and speed decrease: R e =  0 · v · L  (1) where  0 is density,  is liquid speed, L is characteristic length and  is liquid viscosity.  0 and  are supposed to be independent from scale changing. The low noted Reynolds number reflects an absence of turbu- lence in the channel, thus a low mixing efficiency. To overcome this difficulty, researchers added an additional energy source to introduce an active mixing by creating flow instabilities. For example, in order to decrease mixing time and improve the continuous-flow mixture homogeneity, they developed ultrasonic mixers using stationary wave patterns or Surface Acoustic Waves (SAW) [2–5]. They equally used other energies with the same aim ∗ Corresponding author at: Nanotechnology Group, INSAT, Bp 676, Centre Urbain Nord, 1080 Charguia Cedex, Tunisia. Tel.: +216 55280785. E-mail addresses: faten1 kardous@yahoo.com, fk.professionnel@gmail.com (F. Kardous). such as pressure field [6,7], electric field [8,9], or magnetic field variation [10]. However, continuous flow systems are limited in term of maximum number of micro-channels which are realizable despite the significant progress in micro-fabrication technologies. This limitation is pricey in biological and chemical domains, since the number of simultaneously treatable samples is low. To exploit a larger number of samples, they proposed an alternative approach to continuous microfluidic systems by manipulating discrete droplets. To create flow instabilities in droplet, many techniques can be used like the methodology for introducing thorough chaotic mixing in microdroplets by moving it along a two-dimensional path, which are presented by some studies [11]. Paik et al. developed an electrowetting-based linear-array droplet mixer. In this device, the droplets act as virtual mixing chambers, and mixing occurs by transporting the droplet across an electrode array thanks to an electrostatic field [12]. Droplet mixing was equally performed using electrically tunable superhydrophobic nanostructured surfaces. By applying electrical voltage and current, droplets can be reversibly switched from a wetting to a non-wetting state, which induces fluid motion within the droplet [13]. A different approach can be the use of Magneto Hydrodynamic (MHD) principle. In fact, a MHD driven microfluidic system was developed to transport and mix two or more microdroplets [14]. We can also imagine introducing superparamagnetic micro- particles inside the droplet so they can be afar controlled by an http://dx.doi.org/10.1016/j.sna.2014.03.003 0924-4247/© 2014 Elsevier B.V. All rights reserved.