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Questions and Answers
What are the two main types of digital microfluidics systems?
What are the two main types of digital microfluidics systems?
Open and Closed systems
What are the four fundamental fluidic operations in digital microfluidics?
What are the four fundamental fluidic operations in digital microfluidics?
Creating, Transporting, Cutting, and Merging
What is the driving force for electrowetting?
What is the driving force for electrowetting?
Electrostatic forces
What is the main disadvantage of using chemical and topographical patterns for controlling liquids?
What is the main disadvantage of using chemical and topographical patterns for controlling liquids?
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What is the name of the dimensionless number that measures the strength of gravity with respect to surface tension?
What is the name of the dimensionless number that measures the strength of gravity with respect to surface tension?
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What is the name of the dimensionless number that measures the strength of the electrostatic energy compared to surface tension in electrowetting?
What is the name of the dimensionless number that measures the strength of the electrostatic energy compared to surface tension in electrowetting?
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The contact angle in electrowetting always decreases as the voltage increases.
The contact angle in electrowetting always decreases as the voltage increases.
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The electrowetting number is typically larger in EWOD compared to direct electrowetting.
The electrowetting number is typically larger in EWOD compared to direct electrowetting.
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What is the key advantage of using an insulating layer in EWOD?
What is the key advantage of using an insulating layer in EWOD?
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What is the primary function of the hydrophobic coating in DMF devices?
What is the primary function of the hydrophobic coating in DMF devices?
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What is the main benefit of using droplets suspended in oil for DMF devices?
What is the main benefit of using droplets suspended in oil for DMF devices?
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Oil-based DMF systems have the disadvantage of requiring a separate container for the oil bath.
Oil-based DMF systems have the disadvantage of requiring a separate container for the oil bath.
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What is the main benefit of using DMF for biological applications?
What is the main benefit of using DMF for biological applications?
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What is the main principle behind DMF actuation?
What is the main principle behind DMF actuation?
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What is the typical range of droplet volumes used in DMF devices?
What is the typical range of droplet volumes used in DMF devices?
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Contact line friction can be ignored when analyzing droplet movement in DMF.
Contact line friction can be ignored when analyzing droplet movement in DMF.
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The contact angle in electrowetting is independent of the droplet speed.
The contact angle in electrowetting is independent of the droplet speed.
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The contact angle saturation phenomenon in electrowetting is fully understood.
The contact angle saturation phenomenon in electrowetting is fully understood.
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What is the primary driving force in DMF systems?
What is the primary driving force in DMF systems?
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Study Notes
Electrowetting and Digital Microfluidics
- Electrowetting and digital microfluidics are used to manipulate fluids at the microscale.
- Surface tension significantly impacts microscale fluids due to the high surface-to-volume ratio.
- Controlling surface energies is crucial for microtechnology and microfluidics, especially for sub-millimeter liquid droplets where capillary forces dominate.
- Interfacial energies (liquid-vapor and solid-liquid) are essential for droplet manipulation at the microscale.
- Fundamental fluidic operations (creating, transporting, cutting, and merging) are used to digitize droplet-based fluidic systems.
Ways to Influence Interfaces
- Temperature gradients influence interfacial tension. Warmer interfaces have lower surface tension.
- Temperature gradients induce interface motion, which propagates into the bulk due to viscous forces.
- Gradients in surfactant concentration also influence interfacial energies.
- Surfactant gradients generate forces, such as thermocapillary and Marangoni effects, to propel droplets.
Marangoni Convection
- Marangoni convection occurs when surface tension variations dominate viscosity forces for the movement of fluids.
- The Marangoni number (thermal) is a dimensionless number that determines the strength of the convective motion.
- The Marangoni number relates tangential stress to viscosity.
- The Marangoni number is determined by the parameters such as radius of a spherical cap, liquid density, kinematic viscosity, thermal diffusivity, and variation of surface tension.
Drops Moving by Capillarity
- Capillarity is a crucial force for moving fluids at the microscale.
- Electro-osmosis and capillarity are other forces at the microscopic scales that are not efficient at the macroscopic level, and are used for actuating droplets.
- Drop movement is affected by the transition of wettability (hydrophilic and hydrophobic regions).
- The force acting on a drop moving between hydrophilic and hydrophobic regions depends on contact lines and contact angles.
Drop Moving Uphill
- Capillary forces can move micro-droplets uphill against gravity.
- The required gradient in surface free energy is generated by a polished silicon wafer exposed to a diffusing vapor like decyltrichlorosilane.
- Micro-droplets move uphill towards the more hydrophilic regions.
- The average velocity of the drop is approximately 1 to 2 mm/s.
Drop Moving up a Step
- Microdroplets can move upwards from a lower to a higher level between hydrophilic and hydrophobic regions, even if both regions are at different heights (simulation result). The droplet progressively moves up towards the higher hydrophilic region.
Drop Moving over Gradient of Surface Concentration
- Chemical reactions between the droplet and the substrate can create droplet motion.
- Silane molecules form dense monolayers on silicon or glass, resulting in a hydrophobic surface.
- Droplets move along a substrate in a more or less linear fashion, altering direction when encountering a hydrophobic barrier and cannot cross their own paths. -The advancing contact line is hydrophilic and the receding contact line is hydrophobic.
Chemical and Topographical Structuring of Surfaces (Local Wettability)
- The disadvantage of patterns created by chemical and topographical structures for surface modification is their static nature. Consequently, active control of liquids is not possible.
- Patterns are created on surfaces which have a static nature which makes active control of liquids not possible
Evaporation of Sessile Droplets
- Wetting and non-wetting droplets do not evaporate in the same manner.
- For non-wetting droplets, the contact angle remains constant, but the contact radius decreases during evaporation.
- For wetting droplets, the contact radius remains constant, and the contact angle decreases during evaporation
Electrocapillarity
- Electrocapillarity is the basis of modern electrowetting.
- It was first described in 1875 by Gabriel Lippmann.
- Capillary depression of mercury in contact with electrolytic solutions can be varied by applying voltage between the mercury and electrolyte.
- Partially wetting liquid droplet behavior can be controlled.
Electrowetting (EW)
- In electrowetting, an electrical double layer forms between the electrode and aqueous solution.
- The double layer is about 1 to 10 nm thick and Applying a voltage on the electrode can cause a hydrophobic to act like it is hydrophilic by modifying the surface tension.
- The electric energy counterbalances the free surface energy, lowering the surface tension.
Switching Speeds and Stability
- Switching speeds in electrowetting are limited by the hydrodynamic response of the droplet, typically to a few milliseconds.
- Electrowetting exhibits good stability without noticeable degradation.
- Droplets can be moved, split, merged, mixed on surfaces with high flexibility.
Electrowetting-on-Dielectric (EWOD)
- To avoid electrolysis problems, a thin insulating layer is placed between the electrode and the conductive liquid.
- In EWOD there is no electric double layer.
- The change in the energy balance takes place within a hydrophobic dielectric layer (e.g., Teflon layer).
- The insulator is typically much thicker than the double -layer, thus reducing the total capacitance dramatically.
Electrowetting: Basics to Applications
- Electrowetting is a tool to manipulate tiny amounts of liquid on surfaces.
- Applications include "lab-on-a-chip" devices, adjustable lenses, and new display technologies.
Issues with Electrowetting
- Failure of electrowetting equation, namely the saturation of the contact angle at high voltage.
- Limitations of electrowetting in dynamic conditions.
- The dynamics of electrowetting.
- Overview of commercial electrowetting applications.
Theoretical Background
- The bond number is a crucial parameter to determine the relative importance of gravity against surface tension effects when dealing with droplets
- The behavior of droplets in many cases is determined by surface tension alone.
- The free energy of a droplet is largely a function of the droplet shape.
- Force balance at the contact line
- Minimization of the free energy leads to Laplace and Young equations, which are approximations for mesoscopic scales
Electrowetting Theory for Homogeneous Substrates
- The thermodynamic and electrochemical approach derived by Lippmann for describing the behavior of droplets on metal or electrodes.
- Applying voltage can build an electric double layer.
- Effective interfacial tension is reduced.
- The surface charge density depends on the voltage.
- The voltage dependence of the effective interfacial tension can be calculated.
Simplifying Assumptions
- Counter-ions are located at a fixed distance from the surface (Helmholtz model) leading to a fixed capacitance per unit area.
- The dielectric constant of the liquid and the effective surface tension at the liquid-solid interfacial are key factors.
- The voltage dependence for the effective surface tension is calculated integrating the equation.
- Mercury surfaces, like many materials, acquire spontaneous charges.
Young's Law
- Young's law can be successively applied at zero potential and at a certain potential V to determine variations of surface tension.
- The contact angle decreases rapidly upon application of a voltage.
- The equation for effective surface tension is applied to Young's Equation to estimate the contact angle
- The voltage range is limited by electrolysis.
Modern Applications of Electrowetting
- Electrowetting typically addresses this problem by introducing a thin dielectric film.
Digital Microfluidics (DMF)
- DMF refers to two different technologies involving an open and confined system to describe droplet motion.
- Droplet position in open systems is controlled by actuation electrodes arranged in the two-dimensional array, and droplets are manipulated in microchannels in a confined system.
- Droplets serve as microvessels for reactions without cross-talk with multiple advantages such as compartmentalization enabling rapid reactions, mixing of reagents, control of reaction timing, control of interfacial properties and ability to synthesize and transport solid reagents and products.
DMF Configurations
- Systems can be described as either a closed format or open format.
- Closed format devices involve droplets positioned between two substrates with patterned electrodes.
- For open format devices, droplets are positioned on a single substrate with actuation and ground electrodes.
Considerations of DMF Devices
- An insulator layer is typically deposited to decrease potential for electrolysis.
- Hydrophobic coating is applied to the insulator to minimize droplet sticking.
- Closed format devices are suitable for a wide range of droplet operations (e.g., dispensing, moving, splitting, merging).
- Open format devices are useful for rapid mixing, enabling better access to sensors and larger droplets.
- Evaporation rates are higher in open-format devices and should be considered depending on the specific application.
Liquid Motion Thresholds
- Liquid motion is achieved above a threshold voltage resulting from the contact angle hysteresis.
- Reynolds numbers of electrowetting-induced flows are low.
- Electrowetting-induced motion is analogous to droplet motion on chemically heterogeneous substrates.
Droplet Actuation
- Polarizable and conductive liquid droplets are sandwiched between two sets of planar electrodes with the upper plate consisting of a single continuous ground electrode and the lower plate comprised of independently addressable control electrodes.
- Droplet edges in the system overlap at least two adjacent control electrodes while touching the upper ground electrode and all electrodes are initially grounded.
- An equilibrium contact angle is maintained everywhere.
- Applying a potential to the control electrodes leads to a charge layer and a reduction in interfacial energy between the droplet and electrode.
- This process does not depend on the electrolyte, unlike uninsulated electrodes.
- The meniscus is also asymmetrically deformed, producing a pressure gradient forcing bulk flow towards the energized electrode.
Sequential Images of Successful Cutting and Merging of Droplets
One Substrate, Pattened Electrodes for Liquid Actuation
- One substrate is used for patterning electrodes for liquid actuation
- The other substrate consists of a homogeneous electrode to provide electrical contact with the droplet.
Dynamic Aspects of Electrowetting
- Contact angle dynamics are time and speed dependent and influenced by local dissipative processes at the contact line.
Contact Line Friction
- Dynamic wetting can be treated by examining the displacement of the three-phase contact line
- The advancement of the contact line results in dissipation of energy at the molecular level.
Contact Line Friction- Theoretical Model
-The instantaneous velocity of the droplet on a surface can be evaluated using the surface tension gradient inspired driving force and the resistive forces (hydrodynamic and three phase contact forces) and resulting in a force balance..
Driving Force Acting on the Droplet
- The driving force acting on the droplet is influenced by the interfacial tension and the contact angle
- A drop encapsulated in a filler experiences drag force which is often negligible.
Wedge Approximation
- A wedge approximation to the Stokes flow condition analysis to predict the hydrodynamic force exerted by the solid or electrode surface to the liquid drop
- Key parameters of the analysis are the velocity ( U), dynamic contact angle, slip length
Combining Governing Equation
- The governing equations are combined to achieve the general governing equation related to droplet movement
- The governing equation is integrated to predict the velocity
- For a specific solid-liquid system, the values of the contact-line friction coefficient should be calculated experimentally.
Digital Revolution in Microfluidics
- Discrete droplets are manipulated using electrical fields applied to an array of electrodes.
- Each sample and reagent are addressable leading to excellent control of chemical reactions
- This contrast to microchannels where each sample or reagent may not be addressable.
- Configurations for the DMF device include open and closed methods.
- The four principle processes carried out in a DMF device are dispensing, moving, splitting and merging.
Biological Applications of DMF
- DMF is suitable for handling, purifying, detecting and characterizing DNA samples in genome research
- Assays like proteomics and enzyme assays can benefit from DMF
- Cell-based assays benefit from cost effective reagent and material use
- Immunassays can benefit from selective detection of analytes in biological samples.
Optical Applications
- Microlenses are flexible and tunable, achieving a change in light focal length based on contact angle modification.
- Microlenses allow the design of optical systems with different focal lengths controlled electrically.
- Fiber optic displays are built using electrowetting.
Particle Synthesis
- DMF is used to synthesize particles such as conductive gold/SU-8, polypyrrole (semiconducting particles).
- "Eyeball" microbeads are formed by drying water droplets after encapsulating the latex.
Electronics Cooling
- Microchannels are used for cooling electronics, though DMF can be superior for cooling specific "hot spots" on integrated circuits.
- DMF can effectively cool "hot spots" by deploying droplets directly to these spots, ignoring surrounding areas.
Microbelt Conveyer System
- DMF is used for developing microbelt systems able to support objects like lady bugs on silicon wafers using droplets.
Complex Surfaces
- Morphological transitions on structured surfaces like e.g. hydrophobic surface with stripes of variable wettability can be achieved for certain wettability contrasts.
- Patterned electrodes are ideal for creating multilayer substrates with patterned electrodes separated by dielectric layers, enabling good control.
- Topographically patterned surfaces influence the behavior of droplets on the surface substantially like superhydrophobicity and hydrophilicity.
Additional Considerations
- DMF devices are fabricated using conventional clean room techniques
- Electrodes, insulator and hydrophobic coatings are fundamental components.
- The use of oil for the suspended droplets can further reduce evaporation rates and voltages needed for actuation.
- Oil-immersed systems have limitations on compatibility with some liquids (e.g., organic solvents) or experimental analysis requiring drying.
- Gaskets and other containment structures are also needed with this approach
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