Electromouillage réversible sur nanofils de silicium superhydrophobes

Transcription

1 Electromouillage réversible sur nanofils de silicium superhydrophobes EWOD: Electro Wetting On Dielectrics Rabah Boukherroub 1,2 N. Verplanck, 2 G.Piret 1,2 Y. Coffinier, 1,2 E. Galopin, 2 V. Thomy, 2 I. Fournier, 3 M. Salzet, 3 J.-C. Camart 2 1 Institut de Recherche Interdisciplinaire (IRI) 2 Institut d Électronique, de Microélectronique et de Nanotechnologie (IEMN) 3 Laboratoire de Neuroimmunologie des Annélides Cité Scientifique Villeneuve D Ascq, France

2 Why use droplet based Microfluidics? La biologie n échappe pas à la tendance générale à la miniaturisation, notamment dans le domaine analytique de la biochimie. En quelques années, les concepts de laboratoires sur puces, labo puces ou Lab-on-a-chip sont devenus une réalité. Les avantages de la miniaturisation sont multiples: - amélioration de la sensibilité d analyse - rapidité d exécution - parallélisation des essais - réduction des volumes d échantillons - diminution des coûts des dispositifs - automatisation - standartisation des procédés, etc. La réalisation de microsystèmes pour l analyse biologique est un domaine de recherche interdisciplinaire faisant appel à des connaissances aussi diverses que: - la microfluidique, - la chimie des surfaces - la microélectronique, la mécanique, l optique

3 Why use droplet based Microfluidics (EWOD)? basic operations all on one chip - e.g. pumping, mixing achieved by simply programming signals fabrication is simple no moving parts like pumps, etc. flexible - changing of signals instead of adding new physical structures small amounts of fluids can be handled operations become digital Integration

4 Voltage Dependence of Contact Angle - for higher voltages saturation of the contact angle - contact angle saturation still largely not well understood -hydrophobic surface needed because of contact angle saturation Lippmann equation Ewod_120.wmv The main criteria required to ensure effective reversible electrowetting is that the hydrophobic layer hysteresis is as low as possible and the contact angle at zero voltage is as high as possible

5 Setups in EWOD (I) - Open Setup - insulator needed: as dielectric medium (for capacity) in order to prevent electrolysis (SiO 2, Si 3 N 4, organic polymers) - droplet sitting on hydrophobic surface (hydrophobicity induced through chemical modification or capping with a hydrophobic layer such as fluoropolymers) Droplet deformation and moving - ground electrode is sitting in the droplet - bottom electrode is structured in order to control droplet actuation (motion)

6 Setups in EWOD (II) - Closed Setup - structured electrodes on the bottom side - unstructured, electrically isolated electrodes on the top side -hydrophobizationon top and bottom side - electrolysis prevented by electrical insulator on the bottom/top layer

7 EWOD setup used in the laboratory Si counter electrode Liquid droplet Hydrophobic surfaces (Cytop, Teflon ) SiO 2 insulator Glass substrate +V Control electrodes Distance between the plans: 300 µm, Applied voltage: 80 V Speed: environ 10 mm/s (5 plots/s)

8

9 It works and so what is next?!!! The main criteria required to ensure effective reversible electrowetting is that the Teflon is relatively an inert material hydrophobic layer hysteresis is as low as possible and the contact angle at zero voltage is as high as possible Si counter electrode Liquid droplet superhydrophobic surfaces (SiNWs) SiO 2 insulator Glass substrate +V Control electrodes The main advantages associated with such a substrate are: (i) flow resistance of droplets is dramatically reduced (low voltage), (ii) simple realization of hydrophilic and functionalized pads in the superhydrophobic surface allowing analytes trapping and enhancement of the liquid / surface interaction, (iii) subsequent analysis by matrixfree desorption/ionization MS-DIOS on these pads.

10 Previous work on reversible electrowetting on superhydrophobic surfaces

11 - Herbertson et al.sens. Actuators, A 2006, , Electrowetting on patterned layers of SU-8 photoresist with an amorphous Teflon-AF coating was examined by Newton et al.16 After a cycle from 0 to 130 V and back to 0 V, a decrease of the contact angle from its original value ( 152 ) to 114 was observed. The contact angle continued to fall down even when the voltage was reduced, which is a good indication of the non-reversibility of the system.

12 - Heikenfled et al. Langmuir 2006, 22, Krupenkin et al. Langmuir 2004, 20, The nanostructured material exhibited a water contact angle of 160 in air for saline solution with an irreversible behavior. A method for dynamic electrical control over the wetting behavior of liquid droplets on superhydrophobic nanostructured surfaces prepared by etching microscopic array of cylindrical nanoposts into the surface of a silicon wafer was first demonstrated by Krupenkin et al. They found that the wetting properties of the surface can be tuned from superhydrophobic behavior to nearly complete wetting as a function of applied voltage and liquid surface tension, but with no reversible effect.

13 SiNWs Synthesis VLS growth technique

14 1. Preparation of Superhydrophobic Surfaces a) SEM image of silicon nanowires grown on Si/SiO 2 using the vapor-liquid-solid (VLS) mechanism (PSiH 4 = 0.4 T, T = 500 C, t = 60 min). diameter in the range of nm and 30 µm in length b) SEM image of the same silicon nanowires coated with C4F8. C4F8 was deposited using a plasma technique to yield a conformal hydrophobic layer (60 nm thick) N. Verplanck et al. Nano Lett. 7 (2007) 813

15 1. Preparation of Superhydrophobic Surfaces CF 3 (CF 2 ) 7 (CH 2 ) 2 Si O O O SiONWs Y. Coffinier et al. Langmuir 23 (2007) 1608

16 It is well-established that the air trapped in the solid surface plays an important role on hydrophobicity. Cassie and Baxter have proposed the following equation to express the contact angle on a composite surface (θr): cos θ r = f 1 cosθ f 2 θ r and θ are the water contact angles of the chemically-modified a-sionws and Smooth Si/SiO 2 surfaces, respectively, and f 1 and f 2 are the fractions of solid surfaces and air in contact with water, respectively. It is assumed that f 1 + f 2 = 1. θ = 109 reported for Si/SiO 2 modified with perfluorodecyl trichlorosilane was used for the calculation. f 1 = f 2 = Y. Coffinier et al. Langmuir 23 (2007) 1608

17 EWOD actuation on SiNWs

18 2. Reversible EWOD on SiNWs Electrowetting on silicon nanowires coated with hydrophobic fluoropolymer C 4 F 8 displaying reversible electrowetting of a saline solution (100 mm KCl) in oil: (a) contact angle ) 164 at 0 V, (b) contact angle ) 106 at 150 VTRMS. Contact angle vs time for a saline solution droplet (100mM KCl) in air at 150 VTRMS proving the repeatable reversibility of the electrowetting. A zoom of this curve shows the speed of the droplet relaxation: each point is a contact angle of the droplet taken from a frame of a video (25 frames/s). Reversible electrowetting is demonstrated on modified SiNWs. Even though the decrease of the contact angle is limited to ~ 25, it is expected that such surfaces will allow liquid displacement according to the quasi null hysteresis due to a recessed contact angle near the advanced contact angle. In order to move a droplet, the contact angle under applied voltage has to be lower than the recessed contact angle.

19 2. Reversible EWOD on SiNWs Capot_SH1.wmv

20 Conclusions - Comprendre le phénomène électromouillage sur nanofils de silicium (modélisation + expérience) - Préparer des surfaces avec deux niveaux de rugosité (feuilles de Lotus) - Réaliser les fonctions principales d électromouillage sur surfaces superhydrophobes (scission, fusion, ) -Déplacer des liquides contenant des molécules biologiques - Optimiser le système pour réaliser une actuation à faibles tensions Control electrodes SiNWS for EWOD actuation Modified SiNWs for 2D chromatography