Journal of the Korean Industrial and Engineering Chemistry, 2007
Bimodally porous γ-alumina granules, including mesopores (2∼50 nm) and macropores (>50 nm), we... more Bimodally porous γ-alumina granules, including mesopores (2∼50 nm) and macropores (>50 nm), were prepared by sol-gel and oil-drop method. Mesopores are made from the voids among the alumina crystallites, while macropores are from the space of the decomposed PS particles used as physical templates during the granulation process. The product γ-alumina granules with the average diameter of 2 mm were characterized by FE-SEM, XRD, FT-IR, N2 porosimetry, and universal mechanical testing system.
A consistent and reproducible degree of stretching of DNA is important for single DNA molecule an... more A consistent and reproducible degree of stretching of DNA is important for single DNA molecule analysis such as DNA mapping and fast sequencing. Micro/nanopost or posts arrays are known to be able to uncoil genomic scale DNAs upon collision on the posts. Here, we built periodical hexagon nanoposts array embedded in a polymeric micro/nanofluidic device at low costs by a novel NanoImprinting process. The electrophoresis of λ-DNA and T4 DNA were studied in the posts array. A periodical stretching/recoiling motion was observed for both DNAs. Experiments showed a stretching to over 90% of its contour length was highly reproducible for both λ-DNA and T4 DNA. This non-gel electrophoresis of DNA also can be useful for the study of fundamental of DNA separation.
This study presents low cost fabrication of free-standing membranes in polymer with perforated po... more This study presents low cost fabrication of free-standing membranes in polymer with perforated pores down to subµm diameter, which provides platforms for fundamental studies of many biosystems. For the fabrication, a combination of imprint lithography and a sacrificial layer technique was employed in order to obtain a clean, fully released, and mechanically stable membrane with perforated pores. Lift-off resist (LOR) was used as a sacrificial layer first while SU-8 resist spin-coated on the LOR layer was used as the active membrane layer in which micro- and nanopores patterns are formed via a combined thermal- and UV-imprint process. With this method, we could achieve a large area, free-standing SU-8 membrane with micropores up to 4 inch diameter. As a demonstration of the use of the membrane in the study of a biosystem, the membrane was exposed to a solution with lipid vesicles. Lipid vesicles preferentially adsorb at the pore sites in the membrane, the extent of which depends on a...
Hypothesis: Compared to vertical micro-pillars, re-entrant micro-structures exhibited superior om... more Hypothesis: Compared to vertical micro-pillars, re-entrant micro-structures exhibited superior omniphobicity for suspending liquids to Cassie-Baxter state. However, the existing re-entrant structures rely on complex multi-step deposition and etching procedures. The conventional, rigid templated imprinting would instead damage the re-entrant structures. This leads to the question: is it possible to preserve the re-entrant curvatures by a flexible-templated imprinting? Experiments: We facilely imprinted the re-entrant structures on a plastic substrate using a flexible nylon-mesh template. The effect of imprinting time (15-35 min), temperature (110-120 °C) and pressure (15-50 Bar) was investigated. To further improve the liquid-repellency and abrasion resistance, the silica nanoparticles (30-650 nm) along with epoxy resin binder (10 mg/mL) were pre-coated. Findings: A one-step imprinting is sufficient to fabricate the re-entrant structures by utilizing flexible nylon-mesh template, without damaging the imprinted structures after the demolding process. The pre-coated silica nanoparticles and epoxy resin (1) improved liquid repellency by introducing hierarchical surface structures (e.g. contact angle hysteresis of olive oil reduced > 10°), and (2) acted as a protective layer against mechanical abrasion (omniphobicity maintained after 25 cycles, ~1.6 kPa sand paper abrasion). Additionally, the fluorine-free post-treatment was sufficient for the omniphobicity on the obtained plastic structures.
Nanoscale electrophoresis allows for unique separations of single molecules, such as DNA/RNA nucl... more Nanoscale electrophoresis allows for unique separations of single molecules, such as DNA/RNA nucleobases, and thus has the potential to be used as single molecular sensors for exonuclease sequencing. For this to be envisioned, label-free detection of the nucleotides to determine their electrophoretic mobility (i.e., time-of-flight, TOF) for highly accurate identification must be realized. Here, we for the first time show a novel nanosensor that allows discriminating four 2 deoxyribonucleoside 5'-monophosphates, dNMPs, molecules in a label free manner by nanoscale electrophoresis. This was made possible by positioning two sub-10 nm in-plane pores at both ends of a nanochannel column used for nanoscale electrophoresis and measuring the longitudinal transient current during translocation of the molecules. The dual nanopore TOF sensor with 0.5, 1, and 5 μm long nanochannel column lengths discriminated different dNMPs with a mean accuracy
With advances in the design and fabrication of nanofluidic devices during the last decade, there ... more With advances in the design and fabrication of nanofluidic devices during the last decade, there have been a few reports on nucleic acid analysis using nanoscale electrophoresis. The attractive nature of nanofluidics is the unique phenomena associated with this length scale that are not observed using microchip electrophoresis. Many of these effects are surface-related and include electrostatics, surface roughness, van de Waals interactions, hydrogen bonding, and the electric double layer. The majority of reports related to nanoscale electrophoresis have utilized glass-based devices, which are not suitable for broad dissemination into the separation community because of the sophisticated, time consuming, and high-cost fabrication methods required to produce the relevant devices. In this study, we report the use of thermoplastic nanochannels (110 nm x 110 nm, depth x width) for the free solution electrokinetic analysis of ribonucleotide monophosphates (rNMPs). Thermoplastic devices with micro-and nanofluidic networks were fabricated using nanoimprint lithography (NIL) with the structures enclosed via thermal fusion bonding of a cover
Hypothesis-The superhydrophobic lotus leaf has dual-scale surface structures, that is, nanobumps ... more Hypothesis-The superhydrophobic lotus leaf has dual-scale surface structures, that is, nanobumps on micro-mountains. Large hydrophilic particles, due to its high surface energy and weight, have high affility to substrates and tend to precipitate at the bottom of coating films. Small hydrophobic particles, due to its low surface energy and weight, tends to sit on the top of coating films and form porous structures. To mimic the lotus leaf surface, it may be possible to develop dual-sized particle films, in which small particles are decorated on large particles. Experiments-A one-step spin coating of a mixture of dual-sized silica particles (55/200 nm) was used. Epoxy resin was added to improve the adhesion of particle films. The single-sized and dual-sized particle films were compared. The mechanical robustness of particle films was tested by tape peeling and droplet impact.
A combination of electromagnetic alignment and topological pattern assisted alignment to position... more A combination of electromagnetic alignment and topological pattern assisted alignment to position magnetic nanowires, which is referred to as the Patterned Electromagnetic Alignment (PEA), is developed and examined. Electrodeposited, FeNiCo nanowires with different lengths were used as the test nanomaterial, and the microscale grooved surface was formed by UV nanoimprint lithography. The accuracy of the PEA with FeNiCo nanowires was evaluated by measuring the deviation angle from the direction of the magnetic field line for different magnetic field strengths and nanowire lengths, and a statistical alignment distribution was reported for different nanowire length groups. The results were compared with those of the electromagnetic alignment on flat surfaces and in grooved-patterned substrates without electromagnetic alignment. Overall, the deviation angle for the PEA was lower than that for the electromagnetic alignment when all other experimental conditions were identical, indicating that the alignment accuracy along the direction of the magnetic field lines was enhanced in the presence of surface micro grooves. This can be attributed to the fact that, upon attachment of nanowires to the substrate surface, the surface micro grooves in the PEA add additional deterministic characteristics to the otherwise stochastic nature of the nanowire deposition and solvent evaporation processes compared to the sole electromagnetic alignment.
Nanoimprint molds made of soft polymeric materials have advantages of low demolding force and low... more Nanoimprint molds made of soft polymeric materials have advantages of low demolding force and low fabrication cost over Si or metal-based hard molds. However, such advantages are often sacrificed by their reduced replication fidelity associated with the low mechanical strength. In this paper, we studied replication fidelity of different UV-resin molds copied from a Si master mold via UV nanoimprint lithography (NIL) and their thermal imprinting performance into a thermoplastic polymer. Four different UV resins were studied: two were high surface energy UV resins based on tripropyleneglycol diacrylate (TPGDA resin) and polypropyleneglycol diacrylate (PPGDA resin), and the other two were commercially available, low surface energy poly-urethane acrylate (PUA resin) and fluorine-containing (MD 700) UV resins. The replication fidelity among the four UVresins during UV nanoimprint lithograph from a Si master with sharp nanostructures was in the increasing order of (poorest) PUA resin < MD 700 < PPGDA resin < TPGDA resin (best). The results show that the high surface energy and small monomer size are keys to achieving good UV resin filling into sharp nanostructures over the viscosity of the resin solution. When the four UVresin molds were used for thermal-NIL into a thermoplastic polymer, the replicationfidelity was in the increasing order of (poorest) MD 700 < TPGDA resin < PUA resin (best), which follows the same order of their Young's moduli. Our results indicate that the selection of an appropriate UV resin for NIL molds requires consideration of the replication fidelities in the mold fabrication and the subsequent thermal-NIL into thermoplastic polymers.
We report on a simple and effective process that allows fabricating polymeric dual-scale nanoimpr... more We report on a simple and effective process that allows fabricating polymeric dual-scale nanoimprinting molds. The key for the process is the use of a thin flexible SU-8 stencil membrane, which was fabricated by either photolithography or thermal nanoimprint lithography (NIL). The stencil membrane with microscale pores was assembled into a nanopatterned substrate, producing a dual-scale structure. The assembled structure was used as a template to produce polymeric imprinting molds via UV-NIL. With this method, we demonstrated dual-scale nanoimprint molds having nano-pillars of 251 nm diameter and 146 nm high on top of microscale square protrusions of 5 μm wide and 3.6 μm high. The resin mold with the dual-scale structure was successfully used to produce a freestanding membrane with dual-scale perforated pores via UV-NIL. After metal coating and integrated into microfluidic devices, this freestanding membrane can potentially be used as a substrate for surface plasmon resonance sensors.
Hydrophobic silane coatings have been successfully applied to the surface of Si stamps to improve... more Hydrophobic silane coatings have been successfully applied to the surface of Si stamps to improve demolding in nanoimprint lithography (NIL). However, the role of the silane coating has only been studied either indirectly, by measuring adhesion or friction coefficients for Si and substrate surfaces without patterns, or collectively, by measuring the overall demolding force that does not differentiate contributions of friction dissipation, stored elastic energy, and adhesion. Here, for the first time, we present experimental evidence on the role of the silane coating in improving demolding in UV-NIL by using different silane coatings. The silane coatings were characterized by x-ray photoelectron spectroscopy, water contact angle, and friction force measurements. Then, the work of demolding was systematically measured for different silane coatings using stamps with the same micropattern but different pattern depths. Comparison of the results to the theoretical model developed for fiber-matrix debonding energy by Sutcu and Hillig [Acta Metall. Mater. 38(12), 2653-2662] indicated that with a hydrophobic silane coating, the main parameter contributing to overall demolding work shifts from adhesion to stored elastic energy and frictional dissipation as surface adhesion keeps decreasing. The results confirm that the main role of the silane coating in reducing the demolding is to reduce surface adhesion rather than friction at the stamp/substrate interface.
Leidenfrost droplets, liquid droplets placed on a hot flat surface above the Leidenfrost temperat... more Leidenfrost droplets, liquid droplets placed on a hot flat surface above the Leidenfrost temperature of the liquid, are an interesting model system to understand and achieve frictionless motion of droplets on a surface. Controlled unidirectional motion of otherwise random Leidenfrost droplets can be achieved by replacing the flat surface by a surface with topological ratchets. In this study, we show how an increase in the vapor layer thickness below the Leidenfrost droplet influences the droplet motion for underlying ratchets with various periods ranging from 1.5mm down to 800nm. This was exploited by systematically studying the Leidenfrost droplet motion of various liquids with low boiling points including acetone, isopropanol, and R134a on the aforementioned various ratchets. For all liquids with boiling points lower than water, no unidirectional motion was observed for 800 nm. This indicates that the asymmetric vapor flow beneath the Leidenfrost droplet becomes negligible due to the large vapor layer thickness relative to the ratchet depth. However, unidirectional droplet motion was still observed for the micron and millimeter scale ratchets even when the ratchet surface temperature was increased up to 360°C and 230°C for acetone and isopropanol, respectively. This can be attributed to the insulating property of the thick vapor layer which prevent the droplet from producing more vapor with increasing temperature. We also report the effect of the ratchet period on the droplet motion at room temperature using R134a droplets.
Volume 7B: Fluids Engineering Systems and Technologies, 2015
Template synthesis of silver nanowires is studied in this paper. Unlike previous studies, highly ... more Template synthesis of silver nanowires is studied in this paper. Unlike previous studies, highly ordered polymer-based template fabricated by nanoimprinting lithography was used instead of porous polycarbonate (PC) and anodic aluminum oxide (AAO) templates. This new template combines the advantages of the traditional templates and overcomes their drawbacks at the same time. Electrodeposition was carried out under different overpotential and time periods. Long single silver nanowires were found in the templates. The average length of the nanowires was much shorter than the length of the template, which can be explained by difficulty of plating solution diffusion. Wettability of the template and how to increase channel filling rate were also discussed in this paper.
Volume 10: Micro- and Nano-Systems Engineering and Packaging, 2015
Nanowires are widely used as sensing components for lab-on-a-chip devices. One major problem in u... more Nanowires are widely used as sensing components for lab-on-a-chip devices. One major problem in utilizing pre-grown nanowires in lab-on-a-chip applications is the agglomeration of nanowires during their preparation process. The common methods to reduce the agglomeration of nanowires include stirring, sonication and using of surfactants. However, these methods break the long nanowires and are not efficient to produce enough single nanowires. This paper shows a new method to improve the deposition process of individual nanowires. An intermediate membrane was used for the deposition of the nanowires after their preparation process. The membrane helps to filter the nanowire agglomerates and to deposit separated individual nanowires over a silicon surface underneath. The study also shows that the number of single nanowires is increased by increasing the tilt angle of the membrane. The method also helps achieving single long nanowires.
Volume 11: Micro and Nano Systems, Parts A and B, 2007
This study presents fabrication of free-standing, perforated membranes in polymer with the pore d... more This study presents fabrication of free-standing, perforated membranes in polymer with the pore diameter down to 500 nm via novel imprint lithography and use of the membrane to selectively immobilize lipid vesicles at the micropores in the membrane. For the fabrication, a combination of imprint lithography and a sacrificial layer technique was employed in order to get a clean, fully released, and mechanically stable membrane with perforated pores. Si molds with microscale pillar structures fabricated via photolithography were used to define patterns in SU-8 resist layer which was spin-coated on lift-off resist (LOR) used as a sacrificial layer. This was followed by a UV curing process to achieve enough mechanical strength in the SU-8 layer to be fee-standing. Release of the SU-8 layer from the LOR sacrificial layer by lift-off results in the free-standing, perforated membranes with pore diameter down to sub-micrometer range. Prior to the application of lipid vesicles, the SU-8 membr...
Volume 12: Micro and Nano Systems, Parts A and B, 2009
This study presents a stable and flexible method for fabricating a free-standing polymer membrane... more This study presents a stable and flexible method for fabricating a free-standing polymer membrane with perforated micro- and nanopores using an imprint lithography combined with a pressed self-perfection method and a sacrificial layer technique. For the fabrication, micropores were initially patterned on a double resist layer: the upper SU-8 resist layer as an active membrane layer and the lower life-off resist used as a sacrificial layer. The membrane with micropores was then pressed with a flat quartz wafer to reduce pore size down to sub-micrometer. Finally, a free-standing SU-8 membrane with perforated micro- and nanopores was successfully lifted-off from the substrate by dissolving the sacrificial layer.
Volume 9: Micro- and Nano-Systems Engineering and Packaging, Parts A and B, 2012
ABSTRACT Nanopores have proven to be an important sensing element in biosensors to detect and ana... more ABSTRACT Nanopores have proven to be an important sensing element in biosensors to detect and analyze single biomolecules such as DNAs, RNAs, or proteins. The charged biomolecules are driven by an electric field and detected as transient current blocks associated with their translocation through the pores. While protein nanopores, such as alpha-hemolysin and MspA protein nanopores embedded within a lipid bilayer membrane [1], promise to be a rapid, sensitive and label-free sensing paradigm, their duration of usage is too short to perform repetitive experiments due to the mechanical instability of the lipid bilayer. A variety of methods have been developed to prepare synthetic nanopores, which can substrate for protein nanopores, including a direct milling with a focused high-energy electron or ion beam in insulating substrates, an ion track etching in polymer substrates, and an anodizing in aluminum substrates. However, those methods do not allow for control over both the size and location of pores and the high yield of production.
Journal of the Korean Industrial and Engineering Chemistry, 2007
Bimodally porous γ-alumina granules, including mesopores (2∼50 nm) and macropores (>50 nm), we... more Bimodally porous γ-alumina granules, including mesopores (2∼50 nm) and macropores (>50 nm), were prepared by sol-gel and oil-drop method. Mesopores are made from the voids among the alumina crystallites, while macropores are from the space of the decomposed PS particles used as physical templates during the granulation process. The product γ-alumina granules with the average diameter of 2 mm were characterized by FE-SEM, XRD, FT-IR, N2 porosimetry, and universal mechanical testing system.
A consistent and reproducible degree of stretching of DNA is important for single DNA molecule an... more A consistent and reproducible degree of stretching of DNA is important for single DNA molecule analysis such as DNA mapping and fast sequencing. Micro/nanopost or posts arrays are known to be able to uncoil genomic scale DNAs upon collision on the posts. Here, we built periodical hexagon nanoposts array embedded in a polymeric micro/nanofluidic device at low costs by a novel NanoImprinting process. The electrophoresis of λ-DNA and T4 DNA were studied in the posts array. A periodical stretching/recoiling motion was observed for both DNAs. Experiments showed a stretching to over 90% of its contour length was highly reproducible for both λ-DNA and T4 DNA. This non-gel electrophoresis of DNA also can be useful for the study of fundamental of DNA separation.
This study presents low cost fabrication of free-standing membranes in polymer with perforated po... more This study presents low cost fabrication of free-standing membranes in polymer with perforated pores down to subµm diameter, which provides platforms for fundamental studies of many biosystems. For the fabrication, a combination of imprint lithography and a sacrificial layer technique was employed in order to obtain a clean, fully released, and mechanically stable membrane with perforated pores. Lift-off resist (LOR) was used as a sacrificial layer first while SU-8 resist spin-coated on the LOR layer was used as the active membrane layer in which micro- and nanopores patterns are formed via a combined thermal- and UV-imprint process. With this method, we could achieve a large area, free-standing SU-8 membrane with micropores up to 4 inch diameter. As a demonstration of the use of the membrane in the study of a biosystem, the membrane was exposed to a solution with lipid vesicles. Lipid vesicles preferentially adsorb at the pore sites in the membrane, the extent of which depends on a...
Hypothesis: Compared to vertical micro-pillars, re-entrant micro-structures exhibited superior om... more Hypothesis: Compared to vertical micro-pillars, re-entrant micro-structures exhibited superior omniphobicity for suspending liquids to Cassie-Baxter state. However, the existing re-entrant structures rely on complex multi-step deposition and etching procedures. The conventional, rigid templated imprinting would instead damage the re-entrant structures. This leads to the question: is it possible to preserve the re-entrant curvatures by a flexible-templated imprinting? Experiments: We facilely imprinted the re-entrant structures on a plastic substrate using a flexible nylon-mesh template. The effect of imprinting time (15-35 min), temperature (110-120 °C) and pressure (15-50 Bar) was investigated. To further improve the liquid-repellency and abrasion resistance, the silica nanoparticles (30-650 nm) along with epoxy resin binder (10 mg/mL) were pre-coated. Findings: A one-step imprinting is sufficient to fabricate the re-entrant structures by utilizing flexible nylon-mesh template, without damaging the imprinted structures after the demolding process. The pre-coated silica nanoparticles and epoxy resin (1) improved liquid repellency by introducing hierarchical surface structures (e.g. contact angle hysteresis of olive oil reduced > 10°), and (2) acted as a protective layer against mechanical abrasion (omniphobicity maintained after 25 cycles, ~1.6 kPa sand paper abrasion). Additionally, the fluorine-free post-treatment was sufficient for the omniphobicity on the obtained plastic structures.
Nanoscale electrophoresis allows for unique separations of single molecules, such as DNA/RNA nucl... more Nanoscale electrophoresis allows for unique separations of single molecules, such as DNA/RNA nucleobases, and thus has the potential to be used as single molecular sensors for exonuclease sequencing. For this to be envisioned, label-free detection of the nucleotides to determine their electrophoretic mobility (i.e., time-of-flight, TOF) for highly accurate identification must be realized. Here, we for the first time show a novel nanosensor that allows discriminating four 2 deoxyribonucleoside 5'-monophosphates, dNMPs, molecules in a label free manner by nanoscale electrophoresis. This was made possible by positioning two sub-10 nm in-plane pores at both ends of a nanochannel column used for nanoscale electrophoresis and measuring the longitudinal transient current during translocation of the molecules. The dual nanopore TOF sensor with 0.5, 1, and 5 μm long nanochannel column lengths discriminated different dNMPs with a mean accuracy
With advances in the design and fabrication of nanofluidic devices during the last decade, there ... more With advances in the design and fabrication of nanofluidic devices during the last decade, there have been a few reports on nucleic acid analysis using nanoscale electrophoresis. The attractive nature of nanofluidics is the unique phenomena associated with this length scale that are not observed using microchip electrophoresis. Many of these effects are surface-related and include electrostatics, surface roughness, van de Waals interactions, hydrogen bonding, and the electric double layer. The majority of reports related to nanoscale electrophoresis have utilized glass-based devices, which are not suitable for broad dissemination into the separation community because of the sophisticated, time consuming, and high-cost fabrication methods required to produce the relevant devices. In this study, we report the use of thermoplastic nanochannels (110 nm x 110 nm, depth x width) for the free solution electrokinetic analysis of ribonucleotide monophosphates (rNMPs). Thermoplastic devices with micro-and nanofluidic networks were fabricated using nanoimprint lithography (NIL) with the structures enclosed via thermal fusion bonding of a cover
Hypothesis-The superhydrophobic lotus leaf has dual-scale surface structures, that is, nanobumps ... more Hypothesis-The superhydrophobic lotus leaf has dual-scale surface structures, that is, nanobumps on micro-mountains. Large hydrophilic particles, due to its high surface energy and weight, have high affility to substrates and tend to precipitate at the bottom of coating films. Small hydrophobic particles, due to its low surface energy and weight, tends to sit on the top of coating films and form porous structures. To mimic the lotus leaf surface, it may be possible to develop dual-sized particle films, in which small particles are decorated on large particles. Experiments-A one-step spin coating of a mixture of dual-sized silica particles (55/200 nm) was used. Epoxy resin was added to improve the adhesion of particle films. The single-sized and dual-sized particle films were compared. The mechanical robustness of particle films was tested by tape peeling and droplet impact.
A combination of electromagnetic alignment and topological pattern assisted alignment to position... more A combination of electromagnetic alignment and topological pattern assisted alignment to position magnetic nanowires, which is referred to as the Patterned Electromagnetic Alignment (PEA), is developed and examined. Electrodeposited, FeNiCo nanowires with different lengths were used as the test nanomaterial, and the microscale grooved surface was formed by UV nanoimprint lithography. The accuracy of the PEA with FeNiCo nanowires was evaluated by measuring the deviation angle from the direction of the magnetic field line for different magnetic field strengths and nanowire lengths, and a statistical alignment distribution was reported for different nanowire length groups. The results were compared with those of the electromagnetic alignment on flat surfaces and in grooved-patterned substrates without electromagnetic alignment. Overall, the deviation angle for the PEA was lower than that for the electromagnetic alignment when all other experimental conditions were identical, indicating that the alignment accuracy along the direction of the magnetic field lines was enhanced in the presence of surface micro grooves. This can be attributed to the fact that, upon attachment of nanowires to the substrate surface, the surface micro grooves in the PEA add additional deterministic characteristics to the otherwise stochastic nature of the nanowire deposition and solvent evaporation processes compared to the sole electromagnetic alignment.
Nanoimprint molds made of soft polymeric materials have advantages of low demolding force and low... more Nanoimprint molds made of soft polymeric materials have advantages of low demolding force and low fabrication cost over Si or metal-based hard molds. However, such advantages are often sacrificed by their reduced replication fidelity associated with the low mechanical strength. In this paper, we studied replication fidelity of different UV-resin molds copied from a Si master mold via UV nanoimprint lithography (NIL) and their thermal imprinting performance into a thermoplastic polymer. Four different UV resins were studied: two were high surface energy UV resins based on tripropyleneglycol diacrylate (TPGDA resin) and polypropyleneglycol diacrylate (PPGDA resin), and the other two were commercially available, low surface energy poly-urethane acrylate (PUA resin) and fluorine-containing (MD 700) UV resins. The replication fidelity among the four UVresins during UV nanoimprint lithograph from a Si master with sharp nanostructures was in the increasing order of (poorest) PUA resin < MD 700 < PPGDA resin < TPGDA resin (best). The results show that the high surface energy and small monomer size are keys to achieving good UV resin filling into sharp nanostructures over the viscosity of the resin solution. When the four UVresin molds were used for thermal-NIL into a thermoplastic polymer, the replicationfidelity was in the increasing order of (poorest) MD 700 < TPGDA resin < PUA resin (best), which follows the same order of their Young's moduli. Our results indicate that the selection of an appropriate UV resin for NIL molds requires consideration of the replication fidelities in the mold fabrication and the subsequent thermal-NIL into thermoplastic polymers.
We report on a simple and effective process that allows fabricating polymeric dual-scale nanoimpr... more We report on a simple and effective process that allows fabricating polymeric dual-scale nanoimprinting molds. The key for the process is the use of a thin flexible SU-8 stencil membrane, which was fabricated by either photolithography or thermal nanoimprint lithography (NIL). The stencil membrane with microscale pores was assembled into a nanopatterned substrate, producing a dual-scale structure. The assembled structure was used as a template to produce polymeric imprinting molds via UV-NIL. With this method, we demonstrated dual-scale nanoimprint molds having nano-pillars of 251 nm diameter and 146 nm high on top of microscale square protrusions of 5 μm wide and 3.6 μm high. The resin mold with the dual-scale structure was successfully used to produce a freestanding membrane with dual-scale perforated pores via UV-NIL. After metal coating and integrated into microfluidic devices, this freestanding membrane can potentially be used as a substrate for surface plasmon resonance sensors.
Hydrophobic silane coatings have been successfully applied to the surface of Si stamps to improve... more Hydrophobic silane coatings have been successfully applied to the surface of Si stamps to improve demolding in nanoimprint lithography (NIL). However, the role of the silane coating has only been studied either indirectly, by measuring adhesion or friction coefficients for Si and substrate surfaces without patterns, or collectively, by measuring the overall demolding force that does not differentiate contributions of friction dissipation, stored elastic energy, and adhesion. Here, for the first time, we present experimental evidence on the role of the silane coating in improving demolding in UV-NIL by using different silane coatings. The silane coatings were characterized by x-ray photoelectron spectroscopy, water contact angle, and friction force measurements. Then, the work of demolding was systematically measured for different silane coatings using stamps with the same micropattern but different pattern depths. Comparison of the results to the theoretical model developed for fiber-matrix debonding energy by Sutcu and Hillig [Acta Metall. Mater. 38(12), 2653-2662] indicated that with a hydrophobic silane coating, the main parameter contributing to overall demolding work shifts from adhesion to stored elastic energy and frictional dissipation as surface adhesion keeps decreasing. The results confirm that the main role of the silane coating in reducing the demolding is to reduce surface adhesion rather than friction at the stamp/substrate interface.
Leidenfrost droplets, liquid droplets placed on a hot flat surface above the Leidenfrost temperat... more Leidenfrost droplets, liquid droplets placed on a hot flat surface above the Leidenfrost temperature of the liquid, are an interesting model system to understand and achieve frictionless motion of droplets on a surface. Controlled unidirectional motion of otherwise random Leidenfrost droplets can be achieved by replacing the flat surface by a surface with topological ratchets. In this study, we show how an increase in the vapor layer thickness below the Leidenfrost droplet influences the droplet motion for underlying ratchets with various periods ranging from 1.5mm down to 800nm. This was exploited by systematically studying the Leidenfrost droplet motion of various liquids with low boiling points including acetone, isopropanol, and R134a on the aforementioned various ratchets. For all liquids with boiling points lower than water, no unidirectional motion was observed for 800 nm. This indicates that the asymmetric vapor flow beneath the Leidenfrost droplet becomes negligible due to the large vapor layer thickness relative to the ratchet depth. However, unidirectional droplet motion was still observed for the micron and millimeter scale ratchets even when the ratchet surface temperature was increased up to 360°C and 230°C for acetone and isopropanol, respectively. This can be attributed to the insulating property of the thick vapor layer which prevent the droplet from producing more vapor with increasing temperature. We also report the effect of the ratchet period on the droplet motion at room temperature using R134a droplets.
Volume 7B: Fluids Engineering Systems and Technologies, 2015
Template synthesis of silver nanowires is studied in this paper. Unlike previous studies, highly ... more Template synthesis of silver nanowires is studied in this paper. Unlike previous studies, highly ordered polymer-based template fabricated by nanoimprinting lithography was used instead of porous polycarbonate (PC) and anodic aluminum oxide (AAO) templates. This new template combines the advantages of the traditional templates and overcomes their drawbacks at the same time. Electrodeposition was carried out under different overpotential and time periods. Long single silver nanowires were found in the templates. The average length of the nanowires was much shorter than the length of the template, which can be explained by difficulty of plating solution diffusion. Wettability of the template and how to increase channel filling rate were also discussed in this paper.
Volume 10: Micro- and Nano-Systems Engineering and Packaging, 2015
Nanowires are widely used as sensing components for lab-on-a-chip devices. One major problem in u... more Nanowires are widely used as sensing components for lab-on-a-chip devices. One major problem in utilizing pre-grown nanowires in lab-on-a-chip applications is the agglomeration of nanowires during their preparation process. The common methods to reduce the agglomeration of nanowires include stirring, sonication and using of surfactants. However, these methods break the long nanowires and are not efficient to produce enough single nanowires. This paper shows a new method to improve the deposition process of individual nanowires. An intermediate membrane was used for the deposition of the nanowires after their preparation process. The membrane helps to filter the nanowire agglomerates and to deposit separated individual nanowires over a silicon surface underneath. The study also shows that the number of single nanowires is increased by increasing the tilt angle of the membrane. The method also helps achieving single long nanowires.
Volume 11: Micro and Nano Systems, Parts A and B, 2007
This study presents fabrication of free-standing, perforated membranes in polymer with the pore d... more This study presents fabrication of free-standing, perforated membranes in polymer with the pore diameter down to 500 nm via novel imprint lithography and use of the membrane to selectively immobilize lipid vesicles at the micropores in the membrane. For the fabrication, a combination of imprint lithography and a sacrificial layer technique was employed in order to get a clean, fully released, and mechanically stable membrane with perforated pores. Si molds with microscale pillar structures fabricated via photolithography were used to define patterns in SU-8 resist layer which was spin-coated on lift-off resist (LOR) used as a sacrificial layer. This was followed by a UV curing process to achieve enough mechanical strength in the SU-8 layer to be fee-standing. Release of the SU-8 layer from the LOR sacrificial layer by lift-off results in the free-standing, perforated membranes with pore diameter down to sub-micrometer range. Prior to the application of lipid vesicles, the SU-8 membr...
Volume 12: Micro and Nano Systems, Parts A and B, 2009
This study presents a stable and flexible method for fabricating a free-standing polymer membrane... more This study presents a stable and flexible method for fabricating a free-standing polymer membrane with perforated micro- and nanopores using an imprint lithography combined with a pressed self-perfection method and a sacrificial layer technique. For the fabrication, micropores were initially patterned on a double resist layer: the upper SU-8 resist layer as an active membrane layer and the lower life-off resist used as a sacrificial layer. The membrane with micropores was then pressed with a flat quartz wafer to reduce pore size down to sub-micrometer. Finally, a free-standing SU-8 membrane with perforated micro- and nanopores was successfully lifted-off from the substrate by dissolving the sacrificial layer.
Volume 9: Micro- and Nano-Systems Engineering and Packaging, Parts A and B, 2012
ABSTRACT Nanopores have proven to be an important sensing element in biosensors to detect and ana... more ABSTRACT Nanopores have proven to be an important sensing element in biosensors to detect and analyze single biomolecules such as DNAs, RNAs, or proteins. The charged biomolecules are driven by an electric field and detected as transient current blocks associated with their translocation through the pores. While protein nanopores, such as alpha-hemolysin and MspA protein nanopores embedded within a lipid bilayer membrane [1], promise to be a rapid, sensitive and label-free sensing paradigm, their duration of usage is too short to perform repetitive experiments due to the mechanical instability of the lipid bilayer. A variety of methods have been developed to prepare synthetic nanopores, which can substrate for protein nanopores, including a direct milling with a focused high-energy electron or ion beam in insulating substrates, an ion track etching in polymer substrates, and an anodizing in aluminum substrates. However, those methods do not allow for control over both the size and location of pores and the high yield of production.
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Papers by Junseo Choi