Microdrop generation and deposition of ionic liquids
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lia Perera-Nuñezd) and Antonio Mendez-Vilase) Department of Applied Physics, University of Extremadura, Badajoz 06071, Spain
Luis Labajos-Broncano and Maria-Luisa González-Martín Department of Applied Physics, University of Extremadura, Badajoz 06071, Spain; and Networking Research Center of Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Badajoz 06071, Spain
Jürgen Bruggerb) Microsystem Laboratory, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland (Received 21 March 2014; accepted 10 June 2014)
This work describes the use of a piezo-actuated inkjet print head with a nozzle aperture of 50 lm to obtain picoliter drops of different model ionic liquids (ILs). A theoretical analysis of the microdrop generation of three model ILs is confirmed by experiments. The inkjet print process was optimized to enable a stable and reproducible drop ejection in both continuous and drop-on-demand modes by controlling the temperature of the nozzle, as well as the electrical signal sent to the piezo actuator used to generate the drops. Controlled volumes ranging from 43 6 3 pL to 319 6 1 pL have been achieved, with a volume control down to 3 pL. The null volatility of ILs yields an extremely high stability of the inkjet process, obtaining drops with very constant volumes during the entire print process. It also avoids the coffee staining effect observed in the deposition of conventional liquid drops. The possibility to deposit controlled volumes in a reproducible way is demonstrated here and applied to a proof-of-concept application with the aim to create dense concave optical lens arrays by replicating the deposited ionic liquid microdrops in poly (dimethylsiloxane) (PDMS). I. INTRODUCTION
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: juergen.brugger@epfl.ch c) Present address: Laboratory of Micro and Nanotechnology, Paul Scherrer Institute (PSI), Villigen PSI 5232, Switzerland. d) J. Perera-Nuñez and V.J. Cadarso have contributed equally to the work presented in this manuscript. e) Present address: Department of Didactics of Mathematics, University of Sevilla, Sevilla 41013, Spain DOI: 10.1557/jmr.2014.162
thermal stability, and intrinsically smooth surface topography.7 For instance, ILs have been applied in solar cells,8,9 in super-smooth mirrors in vacuum and low pressure environments,7 in high-density data storage,10 as lubricants,11 in batteries,12 in nanocomposites for immunosensors,13 in light sources,14 and for chemical propulsion.15 In some applications, it is needed to work with small volumes of ILs, e.g., as imaging fluids for submicrometerscale monolayer patterns,16 in microreactors,17,18 as contact angle probe fluids,19 in micro- and nano-electromechanical systems (MEMS/NEMS),20,21 in micro-optical applications,18 or in a variety of sensors.22–25 However, generally the use of ILs in such applications is limited by the difficulty to obtain reproducible volumes below 0.4–0.2 lL.17,24 This issue is mainly due to the exceptionally high viscosity of
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