Surface-Driven Fluid Manipulation

ESS5855 LectureFall 2011

Contents

• Microfluidics • Electroosmosis• Electrowetting• Optoelectrowetting• Thermocapillarity

Microfluidics

• Basic functions:– Pumping– Purification– Separation– Metering– Mixing– Dispensing– Reaction– Detection– …

Surface-Directed Liquid FlowInside Microchannels (28)

D.J. Beebe’s Group, U. WisconsinScience 2001

AbstractSelf-assembled monolayer chemistry was used incombination with either multistream laminar flow orphotolithography to pattern surface free energies in-side micro-channel networks. Aqueous liquids intro-duced into these patterned channels are confined tothe hydrophilic pathways, provided the pressure ismaintained below a critical value. The maximumpressure is determined by the surface free energy ofthe liquid, the advancing contact angle of the liquidon the hydrophobic regions, and the channel depth.Surface-directed liquid flow was used to createpressure-sensitive switches inside channel networks.The ability to confine liquid flow inside microchan-nels with only two physical walls is expected to beuseful in applications where a large gas-liquid inter-face is critical, as demonstrated here by a gas-liquidreaction.

Multi-stream Laminar Flows

hydrophilic glass substrate

Pressure-Sensitive Valves

UV Photopatterning

Microfluidic Devices Fabricated in PDMS for Biological Studies (10)

G.M. Whitesides’s Group, Harvard U.Electrophoresis, 2003

Abstract

This review describes microfluidic systems inPDMS for biological studies. Properties of PDMSthat make it a suitable platform for miniaturizedbiological studies, techniques for fabricatingPDMS microstructures, and methods for controll-ing fluid flow in microchannels are discussed.Biological procedures that have been miniaturizedinto PDMS-based microdevices include immuno-assays, separation of proteins and DNA, sortingand manipulation of cells, studies of cells inmicrochannels exposed to laminar flows of fluids,and large-scale, combinatorial screening. Thereview emphasizes the advantages of miniaturi-zation for biological analysis, such as efficiency ofthe device and special insights into cell biology.

Surface Properties• PDMS consists of repeating -OSi(CH3)2- units; the

CH3 groups make its surface hydrophobic• This hydrophobicity results in poor wettability with

aqueous solvents, renders micro-channels susceptible to the trapping of air bubbles, and makes the surface prone to nonspecific adsorptionto proteins and cells

• The surface can be made hydrophilic by exposure to an air plasma; the plasma oxidizes the surface to silanol (Si-OH)

• The plasma-oxidized surface remains hydrophilic if it stays in contact with water

• In air, rearrangements occur within 30 min, which bring hydrophobic groups to the surface to lower the surface free energy

• The surface of oxidized PDMS can be modified further by treatment with functionalized silanes

Surface Modification of PDMSMicrochannels (9)

H. Makamba, et al., Pohang U,Electrophoresis, 2003

Approaches

• by exposure to energy• Ultraviolet and Ultraviolet/Ozone

treatment• Dynamic modification using

charged surfactants• Polymer grafting• Chemical vapor deposition• Phospholipid bilayer modification• Protein modification

PDMS-Based Microfluidic Devices in the Private Sector

A Review of Micropumps (1)

J.G. Santiago’s Group, Stanford U.J. Micromech. Microeng., 2004

Zeta Potential of Microfluidic Substrates: 1. Theory,

Experimental Techniques, and Effects on Separations (2)

B.J. Kirby, et al., SNLElectrophoresis, 2004

Measuring Techniques

• by measuring electroosmotic mobility• by measuring streaming current or

streaming potential generated by pressure-driven flow through a conduit

• by measuring response of a small spherical particle in an applied E-field

Electroosmotic Mobility

• Straightforward

• The transit time of an electrically neutral, optically active or conductive tracer through a microchannel or capillary is measured as a function of electrical field, and the electroosmotic mobility is directly related to the zeta potential, assuming thin Debye layers

• Electroosmotic mobility

Streaming Current/Potential

Equates the shear forces at the wall with the pressure difference

Electroosmotic Flow

Key Parameters

• the magnitude of the applied electric fieldand applied voltage

• the cross-sectional dimensions of the structure in which flow is generated

• the surface charge density of the solid surface that is in contact with the working liquid

• ion density and pH of the working fluid

Performance Comparison

Micromachined Porous Polymer for Bubble Free Electro-Osmosis

Pump (18)

C.H. Mastrangelo’s Group, U. Michigan MEMS 2002

Abstract

A novel porous polymer was microfabricated to serve as a porous plug for a new device, the porous plug electro-osmotic pump (pp-EOP).

The plug eliminates any back pressure effects while enhances electro-osmotic flow in a

channel. The pp-EOP was batch fabricated by surface micromachining on top of a silicon

wafer. The pp-EOP device is driven by a periodic, zero-average injected current signal at low frequencies producing bubble-free electro-

osmotic flow with reversible net movement. Testing of the device produced an average

water-air interface velocity of 1.8 μm/s at 0.8 Hz. The velocity was increased to 4.8 and to 13.9

μm/s by necking the channel size.

Problems of Conventional EOP

• In aqueous solutions, electrode potentials larger than 1.1 V cause electrolysis and bubble generation which leads to blockage of the EOP channel

• EOPs are driven by the motion of charges in the double layer along the surface of the channel hence susceptible to counter flows driven by hydrostatic pressure

New Techniques

• A porous plug is placed between the electrodes creating a high flow resistancethat reduces the pressure driven counter flow substantially– Permits the use of short channels and drive

voltages as low as 20-30 V• Bubble generation is minimized by applying a

zero net current (and zero net charge) pulsed current signal across electrodes at very low frequencies– Allows the use of voltages much larger

than 1.1 V without causing significant bubble generation

Bubble Free• When two electrodes in a cell are in contact with

water they have a linear current-voltage relation-ship under low voltages or high frequencies

• A zero average current results in zero average voltage, and no net fluid movement

• At low frequencies, the cell shows a non-linearcurrent-voltage characteristic due to the activation control of the electrochemical cell; hence a zero-averaged current signal yields a non-zero averaged voltage and net motion

Fabrication

Characteristics

Field-Effect Flow Control for Microfabricated Fluidic

Networks (31)

R. Schasfoort, et al., MESA+, U. TwenteScience, 1999

Abstract

The magnitude and direction of the electro-osmotic flow (EOF) inside a microfabricatedfluid channel can be controlled by a per-pendicular electric field of 1.5 megavolts percentimeter generated by a voltage of only 50volts. A microdevice called a flowFET, withfunctionality comparable to that of a field-effecttransistor (FET) in microelectronics, has beenrealized. Two flowFETs integrated with achannel junction have been used to generateopposite flows inside a single EOF-pumpedchannel, thus illustrating the potential of theflowFET as a controlling and switching elementin microfluidic networks.

Principles

Creating, Transporting, Cutting, and Merging Liquid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits (16)

C.J. Kim’s Group, UCLAJ. MEMS, 2003

AbstractThis paper reports the completion of four fundamental fluidic operations considered essential to build digital microfluidic circuits, which can be

used for lab-on-a-chip or micro total analysis system (μTAS): 1) creating, 2) transporting, 3) cutting, and 4) merging liquid droplets, all by electro-

wetting, i.e., controlling the wetting property of the surface through electric potential. The surface used in this report is, more specifically, an electrode covered with dielectrics, hence, called electrowetting-on-

dielectric (EWOD). All the fluidic movement is confined between two plates, which we call parallel-plate channel, rather than through closed channels or on open surfaces. While transporting and merging droplets are easily verified, we discover that there exists a design criterion for a given set of materials beyond which the droplet simply cannot be cut by EWOD mechanism. The condition for successful cutting is theoretically analyzed by examining the channel gap, the droplet size and the degree

of contact angle change by electrowetting on dielectric (EWOD). A series of experiments is run and verifies the criterion. A smaller channel

gap, a larger droplet size and a larger change in the contact angleenhance the necking of the droplet, helping the completion of the

cutting process. Creating droplets from a pool of liquid is highly related to cutting, but much more challenging. Although droplets may be

created by simply pulling liquid out of a reservoir, the location of cutting is sensitive to initial conditions and turns out unpredictable. This

problem of an inconsistent cutting location is overcome by introducing side electrodes, which pull the liquid perpendicularly to the main fluid

path before activating the cutting. All four operations are carried out in air environment at 25 Vdc applied voltage.

Principle

Lippmann’s equation: the solid–liquid interfacial tensioncan be controlled by the electric potential across the interface

Cutting

Design Criterion

Design Criterion

Configuration

Electrowetting-Based Actuation of Droplets for Integrated

Microfluidics (4)

R.B. Fair’s Group, Duke U.Lab on a Chip, 2002

An Integrated Digital Microfluidic Lab-on-a-Chip for Clinical Diagnostics

on Human Physiological Fluids (5)

R.B. Fair’s Group, Duke U.Lab on a Chip, 2004

AbstractClinical diagnostics is one of the most promising applications formicrofluidic lab-on-a-chip systems, especially in a point-of-care setting.Conventional microfluidic devices are usually based on continuous-flow inmicrochannels, and offer little flexibility in terms of reconfigurability andscalability. Handling of real physiological samples has also been a majorchallenge in these devices. We present an alternative paradigm - a fullyintegrated and reconfigurable droplet-based “digital” microfluidic lab-on-a-chip for clinical diagnostics on human physiological fluids. The micro-droplets, which act as solution-phase reaction chambers, are manipu-lated using the electrowetting effect. Reliable and repeatable high-speedtransport of microdroplets of human whole blood, serum, plasma, urine,saliva, sweat and tear, is demonstrated to establish the basic compati-bility of these physiological fluids with the electrowetting plat-form. Wefurther performed a colorimetric enzymatic glucose assay on serum,plasma, urine, and saliva, to show the feasibility of performing bioassayson real samples in our system. The concentrations obtained compare wellwith those obtained using a reference method, except for urine, wherethere is a significant difference due to interference by uric acid. A lab-on-a-chip architecture, integrating previously developed digital microfluidiccomponents, is proposed for integrated and auto-mated analysis ofmultiple analytes on a monolithic device. The lab-on-a-chip integratessample injection, on-chip reservoirs, droplet formation structures, fluidicpathways, mixing areas and optical detection sites, on the same substrate.The pipelined operation of two glucose assays is shown on a prototypedigital microfluidic lab-on-chip, as a proof-of-concept.

Configuration

An Electrohydrodynamic Polarization Micropump for

Electronic Cooling (19)

D. DeVoe’s Group, U. MarylandJ. MEMS, 2001

Principles

f: net force on each dipole

p: dipole moment

P: Polarization density

F: Polarization force density

E: Electrical field

Light-Driven Motion of Liquidson a Photoresponsive Surface (21)

K. Ichimura, et al., TITScience, 2000

AbstractThe macroscopic motion of liquids on a flat solid

surface was manipulated reversibly by photoirradi-ation of a photoisomerizable monolayer covering thesurface. When a liquid droplet several millimeters indiameter was placed on a substrate surfacemodified with a calix[4]resorcinarene derivativehaving photochromic azobenzene units, asym-metrical photoirradiation caused a gradient insurface free energy due to the photoisomerization ofsurface azobenzenes, leading to the directionalmotion of the droplet. The direction and velocity ofthe motion were tunable by varying the directionand steepness of the gradient in light intensity. Thelight-driven motion of a fluid substance in a surface-modified glass tube suggests potential applicabilityto microscale chemical process systems.

Light Actuation of Liquid by Optoelectrowetting (20)

M.C. Wu’s Group, UCLASensors and Actuators, 2003

AbstractOptical actuation of liquid droplets has been

experimentally demonstrated for the first time using a novel optoelectrowetting (OEW) principle. The optoelectrowetting surface is realized by inte-grating a photoconductive material underneath a

two-dimensional array of electrowetting electrodes. Contact angle change as large as 30° has been

achieved when illuminated by a light beam with an intensity of 65 mW/cm2. A micro-liter droplet of DI

water has been successfully transported by a 4 mW laser beam across a 1 cm x 1 cm OEW surface. The

droplet speed is measured to be 7 mm/s. Light actuation enables complex microfluidic functions to be performed on a single chip without encountering the wiring bottleneck of two-dimensional array of

electrowetting electrodes.

Principles

Configuration

Optical TweezersTweezing is due to the dipole-, or gradient-, force of light incident on a dielectric object

Principles

• Tweezing is due to the dipole-, or gradient-, force of light incident on a dielectric object

• Dipole force is due to the interaction between the dipole induced by the electric field from the laser and the electric field itself

V = -d×E– E: electric field d: induced dipole moment

• Gradient force is proportional to the gradient of the intensity of the laser field

F = ½(a× E²)– a: polarizability of the particle

• Using a focused beam with a power of 1 W striking a particle of radius of ~1 wavelength we get, by conservation of momentum, a force F of ~10-3 dynes, assuming the particle acts as a perfect mirror reflecting all of the incident light momentum back on itself

• In absolute terms, this is small• However, the particle acceleration F/m,

because of the small mass m, is ~105 g, where g is the acceleration of gravity

• This is quite large and should give rise to significant dynamical effects

Order of Magnitude Estimation

Optical Tweezers

CELL ADDRESSING AND TRAPPING USING NOVEL OPTOELECTRONIC TWEEZERS

MICROVISON-ACTIVATED AUTOMATIC OPTICAL MANIPULATOR FOR MICROSCOPIC PARTICLES

Massively Parallel Manipulation of Single Cells and Microparticles

Using Optical Images (32)

M.C. Wu’s Group, UC BerkeleyNature, 2005

Device Structure

The optical images shown on the digital micro-mirror display (DMD) are focused onto the photo-sensitive surface and create the non-uniform electric field for DEP manipulation

Microfabricated Structures for Integrated DNA Analysis (29)

M.A. Burns’s, et al., U. MichiganPNAS, 1996

AbstractPhotolithographic micromachining of silicon is a candidatetechnology for the construction of highthroughput DNA analysisdevices. However, the development of complex silicon micro-fabricated systems has been hindered in part by the lack of asimple, versatile pumping method for integrating individualcomponents. Here we describe a surface-tension-based pumpable to move discrete nanoliter drops through enclosedchannels using only local heating. This thermocapillary pumpcan accurately mix, measure, and divide drops by simpleelectronic control. In addition, we have constructed thermal-cycling chambers, gel electrophoresis channels, and radio-labeled DNA detectors that are compatible with the fabricationof thermocapillary pump channels. Since all of the componentsare made by conventional photolithographic techniques, theycan be assembled into more complex integrated systems. Thecombination of pump and components into self-containedminiaturized devices may provide significant improvements inDNA analysis speed, portability, and cost. The potential ofmicrofabricated systems lies in the low unit cost of silicon-based construction and in the efficient sample handlingafforded by component integration.

Principle

Thermocapillary Actuation of Liquids Using Patterned Microheater Arrays

• A liquid film which is heated locally at some position x reduces the surface tension (γ) at that point and gives rise to a gradient in surface tension across the liquid

• A thermocapillary shear stress, (τ ), is induced which pulls the liquid away from the heated region,

where T is temperature• For a thin flat liquid film, the flow speed is given by

where h is film thickness, μ is local viscosity, and t is time

On-Chip Thermopneumatic Pressure for Discrete Drop

Pumping (15)

C.H. Mastrangelo’s Group, U. MichiganAnalytical Chemistry, 2001

AbstractA class of “lab-on-a-chip” devices use external air pressure for pumping discrete drops in a microchannel network. External

air connectors can be cumbersome and are real-estate intensive. We have developed an on-chip technique to

generate pressures required for metering and pumping of nanoliter-volume discrete drops. This is achieved by heating of

trapped air in a pressure-generating chamber. The pressure-generating chamber is connected to the point of pressure

application in the liquid-conveying microchannel through an air-delivery channel. The trapped air volume on the order of

100 nL is heated by resistive metal heaters by tens of degrees celcius to generate air pressures on the order of 7.5 kN/m2.

The rate of discrete drop pumping is electronically controlled in the microchannel device by controlling the rate of air

heating. Flow rates on the order of 20 nL/s are obtained in the microchannel (300μm x 30μm) by heating the air chamber at

the rate of 6°C/s. In this paper, we describe the design, fabrication, and operation of this new technique of generating on-chip air pressure, used for metering and pumping nanoliter

discrete drops in microchannels.

Principle