Lecture 17-19 Electrowetting PDF

Electrowetting and Digital Microfluidics 1 Different driving forces become pertinent in microscale systems in contrast to their macroscale counterparts to drive fluids. Surface tension force Important in microscale domain owing to the fact that...

Electrowetting and Digital Microfluidics 1 Different driving forces become pertinent in microscale systems in contrast to their macroscale counterparts to drive fluids. Surface tension force Important in microscale domain owing to the fact that the surface to volume ratio increases significantly in microscale. The control of surfaces and surface energies is one of the most important challenges both in microtechnology in general as well as in microfluidics. For liquid droplets of sub-millimetre dimensions, capillary forces dominate. 2 The control of interfacial energies (both liquid–vapour and solid–liquid interfaces) has therefore become an important strategy for manipulating droplets at surfaces. Precise manipulation of droplets in the micrometer scale. Four fundamental fluidic operations (creating, transporting, cutting, and merging) with droplets are utilized to digitize droplet- based fluidic system which has evolved into a new microfluidics paradigm Digital Microfluidics 3  Ways to influence the interfaces Temperature gradients Interface motion induced by a thermal gradient between two regions of the surface. The interface motion propagates into the bulk due to viscous forces. 4 Gradients in the concentration of surfactants across droplets Gives rise to gradients in interfacial energies, mainly at the liquid–vapour interface, and thus produce forces that can propel droplets making use of the thermocapillary and Marangoni effects. 5 Marangoni convection occurs if the variation of the surface tension force dominates the viscosity forces. A dimensionless number—the Marangoni number (thermal)—determines the strength of the convective motion where R is the radius of the spherical cap, ρ the density of the liquid, ν the kinematic viscosity, α the thermal diffusivity, and Δγ the variation of surface tension on the interface. The Marangoni number represents the ratio between the tangential stress and the viscosity. 6  Drops Moving by Capillarity At the microscopic scale there are other forces to move fluids that are not efficient at the macroscopic scale. These forces are electro-osmosis and capillarity. In particular capillarity is widely used for actuating droplets. 1. Drop Moving Over a Transition of Wettability Resulting force directed towards the hydrophilic Droplet at equilibrium region If L1 and L2 are the contact lines in the hydrophilic and hydrophobic planes, and θ1 and θ2 the contact angles, the force acting on the drop is 7 2. Drop Moving Uphill Capillary forces may be sufficient to make micro-drops move upwards on an inclined plane. M.K. Chaudhury and G.M. Whitesides, “How to make water run uphill,” Science, Vol. 256, pp. 1539–1541, 1992. The required gradient in surface free energy was generated on a polished silicon wafer by exposing it to the diffusing front of a vapor of decyltrichlorosilane, The average velocity is approximately 1 to 2 mm/s A drop moves uphill towards the more hydrophilic region. 8  3. Drop Moving up a Step A micro-drop of water is initially located on a step at the boundary of a hydrophilic region (on top of the step) and a hydrophobic region (at the base of the step). The drop progressively moves towards the hydrophilic region, even if this region is located at a higher level (simulation result) Motion of a drop up a step towards the hydrophilic plane (simulation) 9  4. Drop Moving Over a Gradient of Surface Concentration of Surfactant Chemical reactions between the liquid of the droplet and the substrate can create droplet motion. A droplet of n-alkanes containing silane molecules is placed on a hydrophilic surface. Silane molecules form dense grafted monolayers on silicon or glass, rendering the surface hydrophobic. If such a droplet is deposited on a glass surface and pushed with a pipette, then the droplet continues to move on the substrate. It moves in nearly linear segments and changes its direction each time it encounters a hydrophobic barrier. The droplet cannot cross its own tracks. The advancing contact line has a hydrophilic Young contact angle. Molecules of silane concentrate at the vicinity of the receding contact line and form a hydrophobic layer. 10 Chemical and topographical structuring of surfaces - local wettability The main disadvantage of chemical and topographical patterns is their static nature, which prevents active control of the liquids.  Examples of the role of the interface/contact line Evaporation of sessile droplets It has been observed experimentally that wetting and non-wetting droplets do not evaporate in the same way. Water Droplet on hydrophobic (top) and hydrophilic (bottom) surfaces In the case of a non-wetting droplet, it is the contact angle that remains constant (CCA), the contact radius decreases gradually. In the case of a wetting droplet the contact radius remains constant (CCR) during the evaporation process, the contact angle decreases gradually. It is as if the contact line was pinned on the initial contact line. 12 Electrocapillarity, the basis of modern electrowetting, was first described in detail in 1875 by Gabriel Lippmann. the capillary depression of mercury in contact with electrolyte solutions could be varied by applying a voltage between the mercury and electrolyte Generic electrowetting set-up. Partially wetting liquid droplet at zero voltage (dashed) and at high voltage (solid). Electrowetting (EW) has proven to be very successful: Contact angle variations of several tens of degrees are routinely achieved. 13  Electrowetting (EW) In EW an electrical double layer (EDL) is formed between the electrode and aqueous solution that is between 1 nm and 10 nm thick. Applying a voltage difference may cause a hydrophobic surface to behave like a hydrophilic one. The electric energy counterbalances the free surface energy and lowers the surface tension γsl. 14 Switching speeds are limited (typically to several milliseconds) by the hydrodynamic response of the droplet rather than the actual switching of the equilibrium value of the contact angle. Excellent stability without noticeable degradation Nowadays, droplets can be moved along freely programmable paths on surfaces; they can be split, merged, and mixed with a high degree of flexibility. However, electrolysis start within a few milli-volts to make EW difficult to use for practical applications. 15  Electrowetting-on-dielectric (EWOD) Berge in the early 1990 introduced the idea of using a thin insulating layer to separate the conductive liquid from the metallic electrode in order to eliminate the problem of electrolysis - Electrowetting on dielectric (EWOD). In EWOD there is no electric double layer, but the change in the energy balance takes place in the hydrophobic dielectric layer; A Teflon layer, 0.8 µm thick was used as the dielecric. Lee, J., Moon, H., Fowler, J., Schoellhammer, T., and Kim, C.-J. (2002). Electrowetting and electrowetting-on-dielectric for microscale liquid handling. Sens. Actuators, A, 95:259–268. 16 EWOD 17  Electrowetting: basics to applications Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from ‘lab-on-a-chip’ devices to adjustable lenses and new kinds of electronic displays. Issues Fundamental and applied aspects. Basic electrowetting equation, Origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. 18 Issues – contd. Limitations of the electrowetting equation Failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, The dynamics of electrowetting Overview of commercial applications 19  Theoretical background Basic aspects of wetting In electrowetting, one is generically dealing with droplets (typical size of the order of 1 mm or less) of partially wetting liquids (aqueous salt solutions) on planar solid substrates. Bond number Bo = g ∆ ρ R 2 /σ lv measures the strength of gravity with respect to surface tension. If Bo < 1 , the strength of gravity is neglected and the behavior of the droplets is determined by surface tension alone. The free energy F of a droplet is a function of the droplet shape. 20  The free energy F of a droplet is the sum of the areas Ai of the interfaces between the three phases, weighted by the respective interfacial energies, i.e. σsv, σsl , and σlv : Force balance at the contact line F = Fif = ∑ Ai σ i − λV (1) i λ is equal to the pressure drop across the liquid–vapour interface. Minimization of Eq. (l) leads to the following condition that any equilibrium liquid morphology has to fulfill -  1 1  ∆ P = σ lv  +  = σ lv K (2) Laplace Equation  r1 r 2  relates Young’s equilibrium σ sv − σ sl Young Equation cos θ Y = (3) contact angle to the σ lv interfacial energies Both equations are approximations intended for mesoscopic scales. 21  Electrowetting theory for homogeneous substrates The thermodynamic and electrochemical approach - Lippmann’s derivation - direct metal – electrolyte interfaces Upon applying a voltage dU, an electric double layer builds up spontaneously at the solid–liquid interface consisting of charges on the metal surface on one hand and of a cloud of oppositely charged counter-ions on the liquid side of the interface. Since the accumulation is a spontaneous process, it leads to a reduction of the (effective) interfacial tension, σeff d σ sleff = − ρ sl d U (4) ρ sl = ρ sl (U )is the surface charge density of the counter-ions, U = applied voltage The voltage dependence of σ eff is calculated by integrating this equation sl 22  Simplifying assumption - the counter-ions are all located at a fixed distance dH (of the order of a few nanometres) from the surface (Helmholtz model). In this case, the double layer has a fixed capacitance per unit area, ε oε1 CH= ρ / U cH = dH Where ε1 is the dielectric constant of the liquid. γeffSL denotes the effective surface tension at the liquid–solid interface U ε ε γ SL eff γ SL − ∫ C H U d U = (U ) = γ SL − 0 1 (U ) 2 (5) U pzc 2d H Upzc is the potential (difference) of zero charge and approximated to zero. Mercury surfaces—like those of most other materials—acquire a spontaneous charge when immersed into electrolyte solutions at zero voltage. The voltage required to compensate for this spontaneous charging is Upzc 23  γLG cos θ = γSG − γSL Young’s law applied successively at zero potential and at potential V can be written γSG − γSL = γLG cos θ0 γSG − γeffSL(V ) = γLG cos θ where θ, θ0 are respectively the actuated and non-actuated contact angles. 24  γLG cos θ = γSG − γSL This equation for γ eff sl is inserted into Young’s equation. For an electrolyte droplet placed directly on an electrode surface one can find (Upzc approximated to zero) ε 0 ε1 cosθ cosθY + (U ) 2 = (6) 2d H σ lv θY - equilibrium contact angle at zero applied voltage, ε0 - permittivity of free space, ε1 - dielectric constant of the insulating layer, σlv -surface tension between the liquid and the vapor, and U the applied voltage For typical values of dH (2 nm), εl (81), and σlv (0.072 mJ m−2) the ratio on the rhs of equation is on the order of 1 V−2. The contact angle thus decreases rapidly upon the application of a voltage. This equation is only applicable within a voltage range below the onset of electrolytic processes, i.e. typically up to a few hundred millivolts. 25 Modern applications of electrowetting usually circumvent this problem by introducing a thin dielectric film, which insulates the droplet from the electrode. In EWOD, the electric double layer builds up at the insulator–droplet interface. Since the insulator thickness d is usually much larger than dH, the total capacitance of the system is reduced tremendously. The system may be described as two capacitors in series, namely the solid–insulator interface (capacitance cH ) and the dielectric layer with ε 0ε d cd = d εd is the dielectric constant of the insulator. Since c d