A Study of Nanosuspension Droplets Free Evaporation and Electrowetting.

1. A Study of Nanosuspension Droplets Free Evaporation and Electrowetting. Daniel Orejon 13th September 2013 *d.orejon@ed.ac.uk

2. Introduction Wetting Droplet Evaporation Evaporation of Colloidal Suspensions Electrowetting

3. Aims Understand further the mechanisms involved during droplet evaporation for nanofluids and pure fluids. Uncover the mechanisms present during particle deposition for fluids laden with nanoparticles. Systematic study at sub-atmospheric pressures. Electrowetting for the manipulation of nanofluid droplets. Electrowetting as a method for the control of the internal particle mechanisms and the final nanoparticle deposits.

4. Materials & Methods Fluids Pure fluids Nanoparticles Nanofluid preparation Substrates Hydrophilic Hydrophobic EWOD • Apparatous • DSA100 • EW • Pendant Drop • AFM • EPM

5. Pure Fluids & Substrate Hydrophobicity Substrates varying in hydrophobicity Evident differences in evaporative behaviour regarding the different hydrophobicity. Unbalance Young Force Hydrophoilicity

6. Nanofluid Concentration and Substrate Hydrophobicity Different % TiO2-water on Hydrophilic and Hydrophobic Substrates Jump of the CL and change in CA. Two different analogous expressions already derived in literature having used to calculate the free energy before the jump: Silicon TEFLON CYTOP

7. Building-up at the Contact Line TiO2-ethanol on Teflon

8. Deposition Theory There is local an increase in concentration of nanoparticles over the region ‘epsilon’ • Radial flow follows the features predicted by Deegan et al Deposition Theory • Nanoparticles build-up

9. Evaporation for pinned CL at sub-atmospheric pressures Comparison between evaporative behaviour for pure fluid and Al2O3-water nanofluid Al2O3-water Water

10. Electrowetting of Nanosuspension Droplets Effect of dielectric thickness Looking at the Electrowetting Equation % % %

11. Evaporative Behaviour under DC Electrowetting Electric Field Different TiO2-water nanofluid concentration left to dry out under DC EW conditions 0.025% TiO2-water 0.1% TiO2-water 0.05% TiO2-water 0.01% TiO2-water

12. Electrophoretic Mobility vs. Advective Flow Advective Motion: Electrophoretic Motion: Comparisson

13. Conclusion Demonstrated the effect of hydrophilicity in the forces involved during pinning of the contact line. The dependence of nanoparticle concentration on the evaporative behaviour is evident. Deposition theory developed. No apparent differences between the evaporative behaviour of nanofluids and pure fluids on rough Aluminium and at different sub-atmospheric pressure.

14. Conclusions Nanofluids can be manipulated in a similar way as the pure fluid. An enhancement in spreading was observed and explained in terms of adsorption of nanoparticles triggered by voltage. Receding movement of the contact line follows the exponential decay observed when discharging capacitors More homogeneous deposits and the quasi-absence of “stick-slip” was found under Electrowetting conditions.

15. Further Work The complexity of this non-equilibrium phenomena that makes each system particle-solid-liquid-gas unique, offers the potential to uncover new interactions. Electrowetting can be used to create new nanostructured materials. The use of electrostatic charges to maintain particles in suspension in microchannels.

16. Thank you very much for your time

17. Effect of the aFP thickness EWOD dictated by the thickness of the dielectric layer.

18. Effect of Nanofluid Concentration Different nanofluid concentrations on the three different substrates • Modifying EWOD equation it is possible to represent cosθ(V)-cos θvs. V2.

19. Discussion Ki,j vs. thickness of the aFP • Ki,j vs. concentration

20. Mechanisms EWOD experiments are based on Young-Lippmann equation. • Where the capacitance is • It was found that nanoparticles modify the final electrowetting contact angle, therefore the total capacitance changes

21. Conclusions Simple circuit principles can be applied to the dielectric layers to predict the electrowetting behaviour • Results presented for TiO2-water nanofluids agree fairly well with Young-Lippmann equation, linear trend when representing cosθ(V)-cosθ0 vs. V2. • Enhancement of droplet wettability when adding nanoparticles to the base fluid and with increasing concentration. • Able to predict the electrowetting contact angle when changing the aFP thickness.

22. Dziękuję / Thank you

23. Displace oil pixels becoming transparent at low voltages [Liquavista]. • Manipulate small volumes of liquid altering its interfacial tension at low power consumption and no mechanical components [Liquavista]. • Optical micromirrors using EW as a tilting method [Kang et al. 2006]. • Mixing within sessile drops can be enhanced generating internal flow patterns bye electrothermal effect [Garcia et al., 2010]. • Microfluidic, biochip, micro-optics [Lin et al. 2005]. • Patterned areas increase gradually with the applied voltage. EW is reversible and large patterns can be obtained with a minimum of volume. [Olivier et al. 2007]. • EW-assisted drop deposition for controlled spotting using minimum volume of reagents (at high voltages area decreases) [Leïchlé et al. 2007]. Growing interest as driving mechanisms for a wide range of fluidic and electro-optics applications. • Fast control of wettabiltiy, suitable for studying dynamics of wetting. • Oil in water EW due to its weak hysteresis • Microactuation for mix in, in high density microcell devices and programmable filter. • Electrical Engineering, insulators depends on atmospheric humidity and wetting props of the insulator surface. • Optics Gradients of wettability, as focusing lens, whose optical power can be electrically controlled [Quillet et al. 2001]

24. Proceed • Different concentrations of TiO2-water nanofluids are prepared with the aim of observing “stick-slip” behaviour during the evaporation. • A Copper wire is immersed inside of the drop standing on an hydrophobic surface and the evolution of the CA and Contact Radius are recorded  “Stick-slip” behaviour is checked prior Electrowetting experiments. • The drop is placed on the dielectric substrate (positive pole), the wire is immersed (acting as the negative pole) and a voltage of 10 volts is applied to the drop. • The evolution of the droplet profile is recorded and analyzed.

25. Different behaviours observed • There is an instantly spreading of the droplet due to charges change interfacial tension and there is a subsequent contraction of the base line during the first instants due to charges trapped on the substrate. • Suppression of the “stick-slip” behaviour is observed.

26. Other authors work [1] conducting liquid V Example: Water droplet on Cytop® surface conductive electrode d dielectric film  [º] Equilibrium [J.L. Lin 2011, Intro to micro/nano-fludic]

27. Other authors work [2] [Bahadur et al. 2008] There is an energy barrier for the reverese transition and the roughness dependent dissipative forces are the major factors that inhibit droplet reversibility. Disipative forces need to be minimized to enable the reverse transition.

28. 0.1% TiO2-water on CYTOP Evaporation with a 0.09 mm wire Evaporation under EW (10v) effect

29. 0.05% TiO2-water on CYTOP Evaporation under EW (10v) effect Evaporation with a 0.09 mm wire

30. 0.025% TiO2-water on CYTOP Evaporation with a 0.09 mm wire Evaporation under EW effect

31. 0.01% TiO2-water on CYTOP Evaporation with a 0.09 mm wire Evaporation under EW effect

32. Problems encountered • Reliability • Controllability • Response • Compatibility • Use of AC can avoid electrolysis effects • Residual surface charge for DC • Breakdown of the dielectric layer • Dielectric polarization • Current Flow

33. Comparison with actual findings Coffee stain effect is suppressed.

34. Other findings • The net negative charges on the NPs induce repulsive electrostatic forces between NPs and inhibit agglomeration [Raj et al. 2007]. • NFs show enhanced droplet stability compared with water solutions and absence of the CAS [Raj et al. 2007]. • Electric field in the liquid bulk becomes important, leading to energy dissipation due to Joule heating. Fluid flow generated by electrothermal effect . (natural convection + thermocapillary effect) [Garcia et al. 2010]. • Although electrothermal effect is absent for both low and high frequencies for particles ca. 10 microns [Mugele et al. 2011]*. • Fluid mixing at 1kHz and at 128kHz. IN the middle range 18kHz flow gradually dies out, as liquid cannot follow the applied forcing [Garcia et al. 2010]. • Thousands of actuations cycles without degradation [Mugele 2009]. • Internal flow fields generated by AC counteract the evaporation driven flux, preventing the accumulation of solutes at the CL. Unique way to exert forces directly to the CL of sessile drops [Eral et al. 2011]. • With a DC field, the CA decreases with an increase in voltage and there is no CA hysteresis for DC field [Bhushan et al. 2011]. • Hydrodynamic flows generated inside a droplet in EW when an AC voltage is applied depend on the frequency and on the electrolyte concentration (at high freq. and position of the electrode). Oscillatory motion observed driven by the electrical force acting at the TCL [Ko et al. 2008]**. • Patterned areas increase with the applied voltage demonstrating that the spreading of the drop cover a larger surface. At very high voltages, however, the drop volume and area seems to decrease [Leïchlé et al. 2007].