Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266)
Under electrostatic actuation, mercury droplet can act as a contact and moving part in a microswi... more Under electrostatic actuation, mercury droplet can act as a contact and moving part in a microswitch system. In order to reduce the actuation voltage while keeping the electrical advantages of liquid-solid contact, the contact properties of mercury droplet on structured surfaces are investigated in this paper. Forces to actuate a mercury droplet on different structured surfaces are theoretically analyzed and experimentally tested. Both results confirm our claim that the adhesion forces of liquid metal droplets on a solid surface can be designed by physical modification of the surface. The criteria for detaching a mercury droplet from solid surface was predicted and verified by experimental results.
As far as plastron is sustained, superhydrophobic (SHPo) surfaces are expected to reduce skin-fri... more As far as plastron is sustained, superhydrophobic (SHPo) surfaces are expected to reduce skin-friction drag in any flow conditions including large-scale turbulent boundary-layer flows of marine vessels. However, despite many successful drag reductions reported using laboratory facilities, the plastron on SHPo surfaces was persistently lost in high-Reynolds-number flows on open water, and no reduction has been reported until a recent study using certain microtrench SHPo surfaces underneath a boat (Xu et al., Phys. Rev. Appl., vol. 13, no. 3, 2020, 034056). Since scientific studies with controlled flows are difficult with a boat on ocean water, in this paper we test similar SHPo surfaces in a high-speed towing tank, which provides well-controlled open-water flows, by developing a novel 0.7 m × 1.4 m towing plate, which subjects a 4 cm × 7 cm sample to the high-Reynolds-number flows of the plate. In addition to the 7 cm long microtrenches, trenches divided into two in length are also tested and reveal an improvement. The skin-friction drag ratio relative to a smooth surface is found to be decreasing with increasing Reynolds number, down to 73 % (i.e. 27 % drag reduction) at Re x ∼ 8 × 10 6 , before starting to increase at higher speeds. For a given gas fraction, the trench width non-dimensionalized to the viscous length scale is found to govern the drag reduction, in agreement with previous numerical results.
Journal of Micromechanics and Microengineering, 2020
Multilayer interconnections are needed for microdevices with a large number of independent electr... more Multilayer interconnections are needed for microdevices with a large number of independent electrodes. A multi-level photolithographic process is commonly employed to provide multilayer interconnections in integrated circuit (IC) devices, but it is often too expensive for large-area or disposable devices frequently needed for microfluidics. The printed circuit board (PCB) can provide multilayer interconnection at low cost, but its rough topography poses a challenge for small droplets to slide over. Here we report a low-cost fabrication of lowtopography multilayer interconnects by selective and controlled anodization of thin-film metal layers. The process utilizes anodization of metal (tantalum in this paper) or, more specifically, repetitions of a partial anodization to form insulation layers between conductive layers and a full anodization to form isolating regions between electrodes, replacing the usual process of depositing, planarizing, and etching insulation layers. After verifying the electric connections and insulations as intended, the developed method is applied to electrowetting-on-dielectric (EWOD), whose complex microfluidic products are currently built on PCB or thin-film transistor (TFT) substrates. To demonstrate the utility, we fabricated a 3 metal-layer EWOD device with steps (surface topography) less than 1 micrometer (vs. > 10 micrometers of PCB EWOD devices) and confirmed basic digital microfluidic operations.
Accurate measurement of shear stress on a solid surface is a crucial but challenging task in flui... more Accurate measurement of shear stress on a solid surface is a crucial but challenging task in fluid mechanics. Different sensors are usually used for different experimental settings: water channel, wind tunnel, towing tank, watercraft, aircraft, etc. This paper presents a direct shear sensor designed to work for varying test objects and flow conditions. Designed to compare two different sample surfaces, the shear sensor is comprised of two floating elements, whose displacement is proportional to the shear stress they experience, and two optical encoders, which measure the displacements precisely, right under the floating elements. The main plate includes two identical sets of floating elements and flexure beams machined monolithically from a thick piece of metal, allowing displacements in only one in-plane direction. The sideby-side arrangement allows the two floating elements to experience essentially the same flow conditions, regardless of test condition, enabling the comparative sensing. The method of machining these folded-beam flexures, whose width is on the scale of micrometers, while thickness and length are in millimeters and centimeters, respectively, is presented. The main plate is designed with the help of finite element analysis to ensure dynamic response of the floating elements is appropriate for target flow conditions. The utility of the shear sensor is verified in three different flow settings, i.e., water tunnel, boat in open water, and wind tunnel. A miniature underwater camera system is also developed to observe the sample surfaces during testing on a moving object, such as a boat.
The Sixteenth Annual International Conference on Micro Electro Mechanical Systems, 2003. MEMS-03 Kyoto. IEEE
This paper introduces a distributed gas breather to remove the gas byproduct (CO 2) from the micr... more This paper introduces a distributed gas breather to remove the gas byproduct (CO 2) from the micro direct methanol fuel cell (µDMFC). The concept and device configuration of " distributed breathing" aim to improve the performance of the µDMFC by (1) decreasing flow resistance, (2) increasing the active electrode area, and (3) reducing or possibly eliminating the discrete gas separator. Experimental verification is provided by a prototype breather with microscale hydrophobic breathing holes, which successfully removed CO 2 bubbles from a mixture of weak sulfuric acid and sodium bicarbonate aqueous solution. The techniques to fabricate sub-micron hydrophobic breathing holes with good stability, which would enable integration of the breather into the eventual µDMFC system, are discussed.
Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266)
Enhancement of mixing using electrowetting-on-dielectric (EWOD) actuation is demonstrated. Expone... more Enhancement of mixing using electrowetting-on-dielectric (EWOD) actuation is demonstrated. Exponential improvement over simple diffusion is theorized, based on convolutions generated by discrete droplet flow. The relationship between particle size, distance diffused, and diffusion time is studied. An improvement of 50 times over simple diffusion is experimentally shown, using a simple dye-mixing experiment.
Different types of Cassie-to-Wenzel transitions on superhydrophobic surfaces with the impact velo... more Different types of Cassie-to-Wenzel transitions on superhydrophobic surfaces with the impact velocity of water drop.
Superhydrophobic and superoleophobic surfaces have so far been made by roughening a hydrophobic m... more Superhydrophobic and superoleophobic surfaces have so far been made by roughening a hydrophobic material. However, no surfaces were able to repel extremely-low-energy liquids such as fluorinated solvents, which completely wet even the most hydrophobic material. We show how roughness alone, if made of a specific doubly reentrant structure that enables very low liquid-solid contact fraction, can render the surface of any material superrepellent. Starting from a completely wettable material (silica), we micro- and nanostructure its surface to make it superomniphobic and bounce off all available liquids, including perfluorohexane. The same superomniphobicity is further confirmed with identical surfaces of a metal and a polymer. Free of any hydrophobic coating, the superomniphobic silica surface also withstands temperatures over 1000°C and resists biofouling.
Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266)
This paper reports a breakthrough in our quest for digital microfluidic circuits-full completion ... more This paper reports a breakthrough in our quest for digital microfluidic circuits-full completion of all four fundamental microfluidic operations: (1) creating, (2) transporting, (3) cutting, and (4) merging of liquid droplets, based on electrowetting-on-dielectric (EWOD) actuation. All the operations were achieved with 25 V DC , lower than EWOD actuation voltages previously reported. We also report conditions to reduce the driving voltage even further and conditions to drive a droplet as fast as 250 mm/s.
2007 IEEE 20th International Conference on Micro Electro Mechanical Systems (MEMS), 2007
A novel liquid-metal (LM) droplet switch is presented, which addresses the switching speed limita... more A novel liquid-metal (LM) droplet switch is presented, which addresses the switching speed limitation of previously reported LM switches. An actuation technique based on electrowetting-ondielectric (EWOD) gives contact line speeds of 50 cm/s. High accuracy positioning by placing the droplet inside a micro-frame allows us to fabricate devices with switching gaps as small as ~10 µm. We report switches with switch-on latency of 60 µs and switch-off latency of 150 µs. The rise and fall time for the switched signal was measured to be below 5 µs. In preliminary tests, 1 W of power was hotswitched for 10's of cycles without any visual degradation. Combining with the RF performance results and the inherent contact reliability of LM, we are developing microswitches for RF applications.
While many recent studies have confirmed the existence of liquid slip over certain solid surfaces... more While many recent studies have confirmed the existence of liquid slip over certain solid surfaces, there has not been a deliberate effort to design and fabricate a surface that would maximize the slip under practical conditions. Here, we have engineered a nanostructured superhydrophobic surface that minimizes the liquid-solid contact area so that the liquid flows predominantly over a layer of air. Measured through a cone-and-plate rheometer system, the surface has demonstrated dramatic slip effects: a slip length of 20 m for water flow and 50 m for 30 wt % glycerin. The essential geometrical characteristics lie with the nanoposts populated on the surface: tall and slender (i.e., needlelike) profile and submicron periodicity (i.e., pitch).
A new model predicts the receding contact angle of a liquid suspended on microstructures for a wi... more A new model predicts the receding contact angle of a liquid suspended on microstructures for a wide range of data in the literature regardless of their distinct patterns and receding modes.
This paper reports the design, fabrication and test of a micromechanical memory cell (essentially... more This paper reports the design, fabrication and test of a micromechanical memory cell (essentially a switch) using a microscale liquid-metal droplet as the moving and contact part. The droplet is driven by electrostatic force between a grounded liquid metal and imbedded driving electrodes, all placed inside of an anisotropically etched silicon cavity. The electrodes inside the silicon cavity and {111} side walls are patterned by a new shadow masking technique using thin wafers.
Superhydrophobic (SHPo) surfaces have shown promise for passive drag reduction because their surf... more Superhydrophobic (SHPo) surfaces have shown promise for passive drag reduction because their surface structures can hold a lubricating gas film between the solid surface and the liquid in contact with it. However, the types of SHPo surfaces that would produce any meaningful amount of reduction get wet under liquid pressure or at surface defects, both of which are unavoidable in real world. In this report, we solve the above problem by (1) discovering surface structures that allow the restoration of a gas blanket from a wetted state while fully immersed underwater and (2) devising a self-controlled gas generation mechanism that maintains the SHPo condition under high liquid pressures (tested up to 7 atm) as well as in the presence of surface defects, thus removing a fundamental barrier against the implementation of SHPo surfaces for drag reduction.
Digital microfluidic chips provide a new platform for manipulating chemicals for multi-step chemi... more Digital microfluidic chips provide a new platform for manipulating chemicals for multi-step chemical synthesis or assays at the microscale. The organic solvents and reagents needed for these applications are often volatile, sensitive to contamination, and wetting, i.e. have contact angles of < 90° even on the highly hydrophobic surfaces (e.g., Teflon ® or Cytop ®) typically used on digital microfluidic chips. Furthermore, often the applications dictate that the processes are performed in a gas environment, not allowing the use of a filler liquid (e.g., oil). These properties pose challenges for delivering controlled volumes of liquid to the chip. An automated, simple, accurate and reliable method of delivering reagents from sealed, off-chip reservoirs is presented here. This platform overcomes the issues of evaporative losses of volatile solvents, cross-contamination, and flooding of the chip by combining a syringe pump, a simple on-chip liquid detector and a robust interface design. The impedance-based liquid detection requires only minimal added hardware to provide a feedback signal to ensure accurate volumes of volatile solvents are introduced to the chip, independent of time delays between dispensing operations. On-demand dispensing of multiple droplets of acetonitrile, a frequently used but difficult to handle solvent due to its wetting properties and volatility, was demonstrated and used to synthesize the positron emission tomography (PET) probe [ 18 F]FDG reliably. 2 INTRODUCTION Digital microfluidic devices based on electrowetting-on-dielectric (EWOD) and dielectrophoresis (DEP) electronically manipulate droplets on chip to perform a variety of applications in biology [1][2], biochemistry [3][4][5][6][7] and chemistry [8][9][10]. To automate these applications, reagents and samples can be stored on chip as "reservoir droplets" [11], above the chip in wells [12], or in off-chip locations [13] and dispensed ondemand when they are needed.
A micromachined chip capable of generating liquid microfilaments has been developed for a miniatu... more A micromachined chip capable of generating liquid microfilaments has been developed for a miniature version of the Capillary Breakup Extensional Rheometer (CaBERÒ). The proposed system is exceptionally simple and compact because liquid samples are actuated by voltages administered onchip, which therefore requires only electrical connections (rather than a linear motor, an integral part of the CaBERÒ). Since chip features are photolithographically defined, the miniature rheometer can handle sub-microlitre samples. Following the CaBERÒ, we show that a commercial LED micrometer effectively measures diameters of filaments generated by the electrowetting-on-dielectric (EWOD) forces. Since negligible electric fields are sustained within the liquid far away from the measurement region, the applied EWOD voltage does not influence tested material properties. Through breakup experiments using a wide range of Newtonian and complex fluids (e.g., glycerol, xanthan gum, dilute polystyrene, and dilute solutions of various molecular weight polyethylene oxide) we demonstrate a versatile testing platform for scarce and precious samples such as biochemical fluids and novel materials. Measured Newtonian and complex dynamics agree well with published theories and experiments.
This paper describes a concept of concentration and binary separation of particles and its experi... more This paper describes a concept of concentration and binary separation of particles and its experimental confirmations for digital microfluidics where droplets are driven by the mechanism of electrowetting-on-dielectric (EWOD). As a fundamental separation unit, a binary separation scheme is developed, separating two different types of particles in one droplet into two droplets, one type each. The separation scheme consists of three distinctive steps, each with their own challenges: (1) isolate two different types of particles by electrophoresis into two regions inside a mother droplet, (2) physically split the mother droplet into two daughter droplets by EWOD actuation so that each type of particle is concentrated in each daughter droplet, and (3) free the daughter droplets from the separation site by EWOD to ready them for follow-up microfluidic operations. By applying a similar procedure to a droplet containing only one type of particle, two daughter droplets of different particle concentrations can be created. Using negatively charged carboxylate modified latex (CML) particles, 83% of the total particles are concentrated in a daughter droplet. Successful binary separation is also demonstrated using negatively charged CML particles and no-chargetreated polystyrene particles. Despite the undesired vortex developed inside the mother droplet, about 70% of the total CML particles are concentrated in one daughter droplet while about 70% of the total polystyrene particles are concentrated in the other daughter droplet.
Due to the lack of continuous flows that would wash unwanted specifies and impurities off from a ... more Due to the lack of continuous flows that would wash unwanted specifies and impurities off from a target location, droplet microfluidics commonly employs a long serial dilution process to purify target species. In this work, we achieve high-purity separation for the case of electrowetting-on-dielectric (EWOD) based droplet microfluidics by introducing a ''fluidic conduit'' between a sample droplet and a buffer droplet. The long and slender fluidic path minimizes the diffusion and fluidic mixing between the two droplets (thus eliminating non-specific transport) but provides a conduit between them for actively transported particles (thus allowing the specific transport). The conduit is purely fluidic, stabilized chemically (e.g. using surfactants) and controlled by EWOD. The effectiveness of the technique is demonstrated by eliminating $97% non-magnetic beads in just one purification step, while maintaining high collection efficiency (>99%) of magnetic beads.
We report the integration of two technologies: droplet microfluidics using electrowetting-on-diel... more We report the integration of two technologies: droplet microfluidics using electrowetting-on-dielectric (EWOD) and individual particle manipulation using optoelectronic tweezers (OET)-in one microfluidic device. The integrated device successfully demonstrates a sequence involving both EWOD and OET operations. We encountered various challenges during integration of the two different technologies and present how they are addressed. To show the applicability of the device in cellular biology, live HeLa cells are used in the experiments. The unique advantages of EWOD and OET make their integration a significant step towards a powerful tool for many applications, such as single cell studies involving multiplexed environmental stimuli.
Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266)
Under electrostatic actuation, mercury droplet can act as a contact and moving part in a microswi... more Under electrostatic actuation, mercury droplet can act as a contact and moving part in a microswitch system. In order to reduce the actuation voltage while keeping the electrical advantages of liquid-solid contact, the contact properties of mercury droplet on structured surfaces are investigated in this paper. Forces to actuate a mercury droplet on different structured surfaces are theoretically analyzed and experimentally tested. Both results confirm our claim that the adhesion forces of liquid metal droplets on a solid surface can be designed by physical modification of the surface. The criteria for detaching a mercury droplet from solid surface was predicted and verified by experimental results.
As far as plastron is sustained, superhydrophobic (SHPo) surfaces are expected to reduce skin-fri... more As far as plastron is sustained, superhydrophobic (SHPo) surfaces are expected to reduce skin-friction drag in any flow conditions including large-scale turbulent boundary-layer flows of marine vessels. However, despite many successful drag reductions reported using laboratory facilities, the plastron on SHPo surfaces was persistently lost in high-Reynolds-number flows on open water, and no reduction has been reported until a recent study using certain microtrench SHPo surfaces underneath a boat (Xu et al., Phys. Rev. Appl., vol. 13, no. 3, 2020, 034056). Since scientific studies with controlled flows are difficult with a boat on ocean water, in this paper we test similar SHPo surfaces in a high-speed towing tank, which provides well-controlled open-water flows, by developing a novel 0.7 m × 1.4 m towing plate, which subjects a 4 cm × 7 cm sample to the high-Reynolds-number flows of the plate. In addition to the 7 cm long microtrenches, trenches divided into two in length are also tested and reveal an improvement. The skin-friction drag ratio relative to a smooth surface is found to be decreasing with increasing Reynolds number, down to 73 % (i.e. 27 % drag reduction) at Re x ∼ 8 × 10 6 , before starting to increase at higher speeds. For a given gas fraction, the trench width non-dimensionalized to the viscous length scale is found to govern the drag reduction, in agreement with previous numerical results.
Journal of Micromechanics and Microengineering, 2020
Multilayer interconnections are needed for microdevices with a large number of independent electr... more Multilayer interconnections are needed for microdevices with a large number of independent electrodes. A multi-level photolithographic process is commonly employed to provide multilayer interconnections in integrated circuit (IC) devices, but it is often too expensive for large-area or disposable devices frequently needed for microfluidics. The printed circuit board (PCB) can provide multilayer interconnection at low cost, but its rough topography poses a challenge for small droplets to slide over. Here we report a low-cost fabrication of lowtopography multilayer interconnects by selective and controlled anodization of thin-film metal layers. The process utilizes anodization of metal (tantalum in this paper) or, more specifically, repetitions of a partial anodization to form insulation layers between conductive layers and a full anodization to form isolating regions between electrodes, replacing the usual process of depositing, planarizing, and etching insulation layers. After verifying the electric connections and insulations as intended, the developed method is applied to electrowetting-on-dielectric (EWOD), whose complex microfluidic products are currently built on PCB or thin-film transistor (TFT) substrates. To demonstrate the utility, we fabricated a 3 metal-layer EWOD device with steps (surface topography) less than 1 micrometer (vs. > 10 micrometers of PCB EWOD devices) and confirmed basic digital microfluidic operations.
Accurate measurement of shear stress on a solid surface is a crucial but challenging task in flui... more Accurate measurement of shear stress on a solid surface is a crucial but challenging task in fluid mechanics. Different sensors are usually used for different experimental settings: water channel, wind tunnel, towing tank, watercraft, aircraft, etc. This paper presents a direct shear sensor designed to work for varying test objects and flow conditions. Designed to compare two different sample surfaces, the shear sensor is comprised of two floating elements, whose displacement is proportional to the shear stress they experience, and two optical encoders, which measure the displacements precisely, right under the floating elements. The main plate includes two identical sets of floating elements and flexure beams machined monolithically from a thick piece of metal, allowing displacements in only one in-plane direction. The sideby-side arrangement allows the two floating elements to experience essentially the same flow conditions, regardless of test condition, enabling the comparative sensing. The method of machining these folded-beam flexures, whose width is on the scale of micrometers, while thickness and length are in millimeters and centimeters, respectively, is presented. The main plate is designed with the help of finite element analysis to ensure dynamic response of the floating elements is appropriate for target flow conditions. The utility of the shear sensor is verified in three different flow settings, i.e., water tunnel, boat in open water, and wind tunnel. A miniature underwater camera system is also developed to observe the sample surfaces during testing on a moving object, such as a boat.
The Sixteenth Annual International Conference on Micro Electro Mechanical Systems, 2003. MEMS-03 Kyoto. IEEE
This paper introduces a distributed gas breather to remove the gas byproduct (CO 2) from the micr... more This paper introduces a distributed gas breather to remove the gas byproduct (CO 2) from the micro direct methanol fuel cell (µDMFC). The concept and device configuration of " distributed breathing" aim to improve the performance of the µDMFC by (1) decreasing flow resistance, (2) increasing the active electrode area, and (3) reducing or possibly eliminating the discrete gas separator. Experimental verification is provided by a prototype breather with microscale hydrophobic breathing holes, which successfully removed CO 2 bubbles from a mixture of weak sulfuric acid and sodium bicarbonate aqueous solution. The techniques to fabricate sub-micron hydrophobic breathing holes with good stability, which would enable integration of the breather into the eventual µDMFC system, are discussed.
Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266)
Enhancement of mixing using electrowetting-on-dielectric (EWOD) actuation is demonstrated. Expone... more Enhancement of mixing using electrowetting-on-dielectric (EWOD) actuation is demonstrated. Exponential improvement over simple diffusion is theorized, based on convolutions generated by discrete droplet flow. The relationship between particle size, distance diffused, and diffusion time is studied. An improvement of 50 times over simple diffusion is experimentally shown, using a simple dye-mixing experiment.
Different types of Cassie-to-Wenzel transitions on superhydrophobic surfaces with the impact velo... more Different types of Cassie-to-Wenzel transitions on superhydrophobic surfaces with the impact velocity of water drop.
Superhydrophobic and superoleophobic surfaces have so far been made by roughening a hydrophobic m... more Superhydrophobic and superoleophobic surfaces have so far been made by roughening a hydrophobic material. However, no surfaces were able to repel extremely-low-energy liquids such as fluorinated solvents, which completely wet even the most hydrophobic material. We show how roughness alone, if made of a specific doubly reentrant structure that enables very low liquid-solid contact fraction, can render the surface of any material superrepellent. Starting from a completely wettable material (silica), we micro- and nanostructure its surface to make it superomniphobic and bounce off all available liquids, including perfluorohexane. The same superomniphobicity is further confirmed with identical surfaces of a metal and a polymer. Free of any hydrophobic coating, the superomniphobic silica surface also withstands temperatures over 1000°C and resists biofouling.
Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266)
This paper reports a breakthrough in our quest for digital microfluidic circuits-full completion ... more This paper reports a breakthrough in our quest for digital microfluidic circuits-full completion of all four fundamental microfluidic operations: (1) creating, (2) transporting, (3) cutting, and (4) merging of liquid droplets, based on electrowetting-on-dielectric (EWOD) actuation. All the operations were achieved with 25 V DC , lower than EWOD actuation voltages previously reported. We also report conditions to reduce the driving voltage even further and conditions to drive a droplet as fast as 250 mm/s.
2007 IEEE 20th International Conference on Micro Electro Mechanical Systems (MEMS), 2007
A novel liquid-metal (LM) droplet switch is presented, which addresses the switching speed limita... more A novel liquid-metal (LM) droplet switch is presented, which addresses the switching speed limitation of previously reported LM switches. An actuation technique based on electrowetting-ondielectric (EWOD) gives contact line speeds of 50 cm/s. High accuracy positioning by placing the droplet inside a micro-frame allows us to fabricate devices with switching gaps as small as ~10 µm. We report switches with switch-on latency of 60 µs and switch-off latency of 150 µs. The rise and fall time for the switched signal was measured to be below 5 µs. In preliminary tests, 1 W of power was hotswitched for 10's of cycles without any visual degradation. Combining with the RF performance results and the inherent contact reliability of LM, we are developing microswitches for RF applications.
While many recent studies have confirmed the existence of liquid slip over certain solid surfaces... more While many recent studies have confirmed the existence of liquid slip over certain solid surfaces, there has not been a deliberate effort to design and fabricate a surface that would maximize the slip under practical conditions. Here, we have engineered a nanostructured superhydrophobic surface that minimizes the liquid-solid contact area so that the liquid flows predominantly over a layer of air. Measured through a cone-and-plate rheometer system, the surface has demonstrated dramatic slip effects: a slip length of 20 m for water flow and 50 m for 30 wt % glycerin. The essential geometrical characteristics lie with the nanoposts populated on the surface: tall and slender (i.e., needlelike) profile and submicron periodicity (i.e., pitch).
A new model predicts the receding contact angle of a liquid suspended on microstructures for a wi... more A new model predicts the receding contact angle of a liquid suspended on microstructures for a wide range of data in the literature regardless of their distinct patterns and receding modes.
This paper reports the design, fabrication and test of a micromechanical memory cell (essentially... more This paper reports the design, fabrication and test of a micromechanical memory cell (essentially a switch) using a microscale liquid-metal droplet as the moving and contact part. The droplet is driven by electrostatic force between a grounded liquid metal and imbedded driving electrodes, all placed inside of an anisotropically etched silicon cavity. The electrodes inside the silicon cavity and {111} side walls are patterned by a new shadow masking technique using thin wafers.
Superhydrophobic (SHPo) surfaces have shown promise for passive drag reduction because their surf... more Superhydrophobic (SHPo) surfaces have shown promise for passive drag reduction because their surface structures can hold a lubricating gas film between the solid surface and the liquid in contact with it. However, the types of SHPo surfaces that would produce any meaningful amount of reduction get wet under liquid pressure or at surface defects, both of which are unavoidable in real world. In this report, we solve the above problem by (1) discovering surface structures that allow the restoration of a gas blanket from a wetted state while fully immersed underwater and (2) devising a self-controlled gas generation mechanism that maintains the SHPo condition under high liquid pressures (tested up to 7 atm) as well as in the presence of surface defects, thus removing a fundamental barrier against the implementation of SHPo surfaces for drag reduction.
Digital microfluidic chips provide a new platform for manipulating chemicals for multi-step chemi... more Digital microfluidic chips provide a new platform for manipulating chemicals for multi-step chemical synthesis or assays at the microscale. The organic solvents and reagents needed for these applications are often volatile, sensitive to contamination, and wetting, i.e. have contact angles of < 90° even on the highly hydrophobic surfaces (e.g., Teflon ® or Cytop ®) typically used on digital microfluidic chips. Furthermore, often the applications dictate that the processes are performed in a gas environment, not allowing the use of a filler liquid (e.g., oil). These properties pose challenges for delivering controlled volumes of liquid to the chip. An automated, simple, accurate and reliable method of delivering reagents from sealed, off-chip reservoirs is presented here. This platform overcomes the issues of evaporative losses of volatile solvents, cross-contamination, and flooding of the chip by combining a syringe pump, a simple on-chip liquid detector and a robust interface design. The impedance-based liquid detection requires only minimal added hardware to provide a feedback signal to ensure accurate volumes of volatile solvents are introduced to the chip, independent of time delays between dispensing operations. On-demand dispensing of multiple droplets of acetonitrile, a frequently used but difficult to handle solvent due to its wetting properties and volatility, was demonstrated and used to synthesize the positron emission tomography (PET) probe [ 18 F]FDG reliably. 2 INTRODUCTION Digital microfluidic devices based on electrowetting-on-dielectric (EWOD) and dielectrophoresis (DEP) electronically manipulate droplets on chip to perform a variety of applications in biology [1][2], biochemistry [3][4][5][6][7] and chemistry [8][9][10]. To automate these applications, reagents and samples can be stored on chip as "reservoir droplets" [11], above the chip in wells [12], or in off-chip locations [13] and dispensed ondemand when they are needed.
A micromachined chip capable of generating liquid microfilaments has been developed for a miniatu... more A micromachined chip capable of generating liquid microfilaments has been developed for a miniature version of the Capillary Breakup Extensional Rheometer (CaBERÒ). The proposed system is exceptionally simple and compact because liquid samples are actuated by voltages administered onchip, which therefore requires only electrical connections (rather than a linear motor, an integral part of the CaBERÒ). Since chip features are photolithographically defined, the miniature rheometer can handle sub-microlitre samples. Following the CaBERÒ, we show that a commercial LED micrometer effectively measures diameters of filaments generated by the electrowetting-on-dielectric (EWOD) forces. Since negligible electric fields are sustained within the liquid far away from the measurement region, the applied EWOD voltage does not influence tested material properties. Through breakup experiments using a wide range of Newtonian and complex fluids (e.g., glycerol, xanthan gum, dilute polystyrene, and dilute solutions of various molecular weight polyethylene oxide) we demonstrate a versatile testing platform for scarce and precious samples such as biochemical fluids and novel materials. Measured Newtonian and complex dynamics agree well with published theories and experiments.
This paper describes a concept of concentration and binary separation of particles and its experi... more This paper describes a concept of concentration and binary separation of particles and its experimental confirmations for digital microfluidics where droplets are driven by the mechanism of electrowetting-on-dielectric (EWOD). As a fundamental separation unit, a binary separation scheme is developed, separating two different types of particles in one droplet into two droplets, one type each. The separation scheme consists of three distinctive steps, each with their own challenges: (1) isolate two different types of particles by electrophoresis into two regions inside a mother droplet, (2) physically split the mother droplet into two daughter droplets by EWOD actuation so that each type of particle is concentrated in each daughter droplet, and (3) free the daughter droplets from the separation site by EWOD to ready them for follow-up microfluidic operations. By applying a similar procedure to a droplet containing only one type of particle, two daughter droplets of different particle concentrations can be created. Using negatively charged carboxylate modified latex (CML) particles, 83% of the total particles are concentrated in a daughter droplet. Successful binary separation is also demonstrated using negatively charged CML particles and no-chargetreated polystyrene particles. Despite the undesired vortex developed inside the mother droplet, about 70% of the total CML particles are concentrated in one daughter droplet while about 70% of the total polystyrene particles are concentrated in the other daughter droplet.
Due to the lack of continuous flows that would wash unwanted specifies and impurities off from a ... more Due to the lack of continuous flows that would wash unwanted specifies and impurities off from a target location, droplet microfluidics commonly employs a long serial dilution process to purify target species. In this work, we achieve high-purity separation for the case of electrowetting-on-dielectric (EWOD) based droplet microfluidics by introducing a ''fluidic conduit'' between a sample droplet and a buffer droplet. The long and slender fluidic path minimizes the diffusion and fluidic mixing between the two droplets (thus eliminating non-specific transport) but provides a conduit between them for actively transported particles (thus allowing the specific transport). The conduit is purely fluidic, stabilized chemically (e.g. using surfactants) and controlled by EWOD. The effectiveness of the technique is demonstrated by eliminating $97% non-magnetic beads in just one purification step, while maintaining high collection efficiency (>99%) of magnetic beads.
We report the integration of two technologies: droplet microfluidics using electrowetting-on-diel... more We report the integration of two technologies: droplet microfluidics using electrowetting-on-dielectric (EWOD) and individual particle manipulation using optoelectronic tweezers (OET)-in one microfluidic device. The integrated device successfully demonstrates a sequence involving both EWOD and OET operations. We encountered various challenges during integration of the two different technologies and present how they are addressed. To show the applicability of the device in cellular biology, live HeLa cells are used in the experiments. The unique advantages of EWOD and OET make their integration a significant step towards a powerful tool for many applications, such as single cell studies involving multiplexed environmental stimuli.
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Papers by Chang-Jin Kim