saif khan - Academia.edu (original) (raw)
Papers by saif khan
Applied Physics Letters, 2014
RSC Advances, 2013
A poly(vinylidene fluoride) nanohybrid membrane has been prepared using high energy swift heavy i... more A poly(vinylidene fluoride) nanohybrid membrane has been prepared using high energy swift heavy ions (SHI) followed by chemical functionalization. Through-channels were generated by selective etching of the amorphous latent track created during irradiation of SHI, whose average dimensions are varied from 52 nm in pure PVDF to 40 nm for the nanohybrid in the presence of nanoclay, indicating suitable membrane formation using the nanohybrid, as compared to pure PVDF. The nanochannels were suitably grafted with conducting polymer (pyrrole) using the active free radicals caused by SHI irradiation. Spectroscopic and gel permeation chromatographic experiments have been performed to understand the effect of irradiation on filled polymer and subsequent functionalization. Doping and sulphonation on the grafted species has been done to introduce a variety of functionalities. The effect of fluence on the channel width and functionalization study indicates a larger diameter of the channels and formation of a greater ionomer at higher fluence, leading to enhancement of the bulk electrical conductivity up to 10 23 S cm 21 (12 orders higher in magnitude than pure PVDF). The activation energy of the modified nanohybrid membrane is calculated to be 26.9 kJ mol 21 , compared to 10.6 kJ mol 21 of pure PVDF, indicating superior stability of the nanohybrid membrane at higher temperature as compared to pure PVDF or functionalized PVDF under similar conditions. The nanohybrid conducting membrane produces a current of 46 nA compared to a value of almost zero for pristine PVDF or NH measured through I-V characteristic curves against a bias voltage of 10 V, making the nanohybrid suitable for electrochemical applications.
Particle & Particle Systems Characterization, 2014
Sputtering of CaF Thin Films. [AIP Conference Proceedings 1349, 651 (2011)]. Ratnesh K. Pandey, S... more Sputtering of CaF Thin Films. [AIP Conference Proceedings 1349, 651 (2011)]. Ratnesh K. Pandey, Saif A. Khan, DK Avasthi, Avinash C. Pandey. Abstract. In the present work thin films of different thicknesses deposited on Si substrate ...
Vacuum, 2009
Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by dual implantation... more Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by dual implantation of nitrogen ( 14 N þ ) and oxygen ( 16 O þ ) ions sequentially into single crystal silicon in the ratio 1:1 at 150 keV to ionfluences ranging from 1 Â 10 17 to 5 Â 10 17 cm À2 . Heavy ion elastic recoil analysis (HI-ERDA) studies of as implanted samples show Gaussian like distributions of nitrogen and oxygen. After annealing at 800 C, both the nitrogen and oxygen distributions appear as flat plateau like regions near projected range showing the formation of a continuous buried oxynitride layer. Micro-Raman study of as implanted samples shows a broad peak at 480 cm À1 for all fluences. It signifies a complete amorphization of silicon due to high fluence implantation. The annealing at 800 C results in the reduction of the intensity of the broad peak observed at 480 cm À1 and also gives rise to an additional peak at 517 cm À1 . It shows partial recrystallization of damaged silicon due to annealing. The X-ray rocking curves studies from highresolution X-ray diffraction (HRXRD) of the samples implanted with different fluences have also further confirmed partial recrystallization of damaged silicon on annealing.
Vacuum, 2009
Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by implantation of n... more Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by implantation of nitrogen ( 14 N þ ) and oxygen ( 16 O þ ) ions sequentially in the ratio 1:1 at 150 keV to ion-fluences ranging from 1 Â 10 17 to 5 Â 10 17 cm À2 to prepare silicon on insulator (SOI) structures. The as implanted samples were held at 270 C and irradiated to total fluence of 1 Â 10 14 cm À2 by 60 MeV Ni þ5 to study the structural changes/ recrystallization of SOI structures induced by swift heavy ion (SHI) irradiation. Fourier transform infrared (FTIR) measurements on the as implanted samples ( 1 Â 10 18 cm À2 ) show a single absorption band in the wavenumber range 1300-750 cm À1 attributed to the formation of silicon oxynitride (Si-O-N) bonds in the implanted silicon. It is observed that a nitrogen rich silicon oxynitride structure is formed after SHI irradiation. The study of X-ray rocking curves on the samples show the formation of small silicon crystallites due to swift heavy ion irradiation.
Soft Matter, 2012
RSC Publishing Logo View PDF Version DOI:10.1039/C2SM07373K (Paper) Soft Matter, 2012, Advance Ar... more RSC Publishing Logo View PDF Version DOI:10.1039/C2SM07373K (Paper) Soft Matter, 2012, Advance Article. Hierarchical materials synthesis at soft all-aqueous interfaces†. Su Hui Sophia Leea, MK Dawoodb, WK Choibc ...
Particle & Particle Systems Characterization, 2014
ABSTRACT
Nanotechnology, 2011
Quasi-aligned gold nanodots with a periodicity of ∼40 nm have been synthesized on a silica substr... more Quasi-aligned gold nanodots with a periodicity of ∼40 nm have been synthesized on a silica substrate by oblique deposition of gold on fast argon atom-beam-created nanoripples of wavelength 40 nm and subsequent annealing. The size distribution of these aligned nanodots resulting from oblique deposition at 85 • of 0.5 nm Au film perpendicular to ripples is narrower than the similar deposition on a flat surface. The deposition and annealing process was simulated with a three-dimensional kinetic lattice Monte Carlo technique in order to understand the formation of aligned nanodots. The atomistic simulation and the experimental results suggest that there is an optimal thickness which can result in nanodots aligned along the ripples in the case of depositions perpendicular to the ripples. The nanodots formed after annealing of the films deposited parallel to ripples or on flat surface lack alignment.
Nano Letters, 2010
The availability of robust, scalable, and automated nanoparticle manufacturing processes is cruci... more The availability of robust, scalable, and automated nanoparticle manufacturing processes is crucial for the viability of emerging nanotechnologies. Metallic nanoparticles of diverse shape and composition are commonly manufactured by solution-phase colloidal chemistry methods, where rapid reaction kinetics and physical processes such as mixing are inextricably coupled, and scale-up often poses insurmountable problems. Here we present the first continuous flow process to synthesize thin gold "nanoshells" and "nanoislands" on colloidal silica surfaces, which are nanoparticle motifs of considerable interest in plasmonics-based applications. We assemble an ordered, flowing composite foam lattice in a simple microfluidic device, where the lattice cells are alternately aqueous drops containing reagents for nanoparticle synthesis or gas bubbles. Microfluidic foam generation enables precisely controlled reagent dispensing and mixing, and the ordered foam structure facilitates compartmentalized nanoparticle growth. This is a general method for aqueous colloidal synthesis, enabling continuous, inherently digital, scalable, and automated production processes for plasmonic nanomaterials.
Langmuir, 2013
We present simple, inexpensive microfluidics-based fabrication of highly monodisperse poly(ionic ... more We present simple, inexpensive microfluidics-based fabrication of highly monodisperse poly(ionic liquid) microgel beads with a multitude of functionalities that can be chemically switched in facile fashion by anion exchange and further enhanced by molecular inclusion. Specifically, we show how the exquisite control over bead size and shape enables extremely precise, quantitative measurements of anion-and solvent-induced volume transitions in these materials, a crucial feature driving several important applications. Next, by exchanging diverse anions into the synthesized microgel beads, we demonstrate stimuli responsiveness and a multitude of novel functionalities including redox response, controlled release of chemical payloads, magnetization, toxic metal removal from water, and robust, reversible pH sensing. These chemically switchable stimulus-responsive beads are envisioned to open up a vast array of potential applications in portable and preparative chemical analysis, separations and spatially addressed sensing.
Langmuir, 2012
Recently, liquid crystal (LC) droplets in aqueous solutions have become a new platform for chemic... more Recently, liquid crystal (LC) droplets in aqueous solutions have become a new platform for chemical and biological sensing applications. In this work, we present a two-step method to generate monodisperse LC droplets in aqueous solutions for sensing applications. In the first step, we exploit inkjet printing to dispense uniform LC droplets on a solid surface. Uniform LC droplets, ranging from 35 to 136 μm in diameter, can be prepared by printing multiple times on the same spot. In the second step, we flush the LC droplets with a stream of aqueous solution in an open rectangular channel. Factors that determine the polydispersity of the LC droplets include flow rates and surface wettability. Under appropriate experimental conditions (i.e., when the surface is glass and the flow rate is sufficiently high), the LC droplets can be lifted off completely and carried away by the solution, forming free LC droplets (15−62 μm in diameter). These free LC droplets can respond to a chemical reaction and change their optical textures uniformly.
Lab on a Chip, 2009
We demonstrate a new class of microfluidic emulsion where the &am... more We demonstrate a new class of microfluidic emulsion where the 'drops' of the emulsion are dynamic reversible bubble-drop pairs, with potential applications in microfluidic technology for chemical synthesis, molecular separations and screening.
Lab on a Chip, 2012
We report how a nominally symmetric microfluidic junction can be used to sort all bubbles of an i... more We report how a nominally symmetric microfluidic junction can be used to sort all bubbles of an incoming train exclusively into one of its arms. The existence of this "filter" regime is unexpected, given that the junction is symmetric. We analyze this behavior by quantifying how bubbles modulate the hydrodynamic resistance in microchannels and show how speeding up a bubble train whilst preserving its spatial periodicity can lead to filtering at a nominally symmetric junction. We further show how such an asymmetric traffic of bubble trains can be triggered in symmetric geometries by identifying conditions wherein the resistance to flow decreases with an increase in the number of bubbles in the microchannel and derive an exact criterion to predict the same.
Lab on a Chip, 2012
Microfluidic wet-chemical synthesis of nanoparticles is a growing area of research in chemical mi... more Microfluidic wet-chemical synthesis of nanoparticles is a growing area of research in chemical microfluidics, enabling the development of continuous manufacturing processes that overcome the drawbacks of conventional batch-based synthesis methods. The synthesis of ultra-small (<5 nm) metallic nanocrystals is an interesting area with many applications in diverse fields, but is typically very challenging to accomplish in a microfluidics-based system due to the use of a strong gas-evolving reducing agent, aqueous sodium borohydride (NaBH(4)), which causes uncontrolled out-gassing and bubble formation, flow disruption and ultimately reactor failure. Here we present a simple method, rooted in the concepts of multiphase mass transfer that completely overcomes this challenge-we simply inject a stream of inert gas bubbles into our channels that essentially capture the evolving gas from the reactive aqueous solution, thereby preventing aqueous dissolved gas concentration from reaching the solubility threshold for bubble nucleation. We present a simple model for coupled mass transfer and chemical reaction that adequately captures device behaviour. We demonstrate the applicability of our method by synthesizing ultra-small gold nanocrystals (<5 nm); the quality of nanocrystals thus synthesized is further demonstrated by their use in an off-chip synthesis of high-quality gold nanorods. This is a general approach that can be extended to a variety of metallic nanomaterials.
Journal of Applied Physics, 2011
Atomistic simulations of Au-silica nanocomposite film growth. [Journal of Applied Physics 109, 09... more Atomistic simulations of Au-silica nanocomposite film growth. [Journal of Applied Physics 109, 094312 (2011)]. Saif A. Khan, K.-H. Heinig, DK Avasthi. Abstract. The growth of Au-silica nanocomposite film is simulated in the framework ...
Crystal Growth & Design, 2012
Emulsion-based crystallization to produce spherical crystalline agglomerates (SAs) is an attracti... more Emulsion-based crystallization to produce spherical crystalline agglomerates (SAs) is an attractive route to control crystal size during downstream processing of active pharmaceutical ingredients (APIs). However, conventional methods of emulsification in stirred vessels pose several problems that limit the utility of emulsion-based crystallization. In this paper, we use capillary microfluidics to generate monodisperse water-in-oil emulsions. Capillary microfluidics, in conjunction with evaporative crystallization on a flat heated surface, enables controllable production of uniformly sized SAs of glycine in the 35−150 μm size range. We report detailed characterization of particle size, size distribution, structure, and polymorphic form. Further, online high-speed stereomicroscopic observations reveal several clearly demarcated stages in the dynamics of glycine crystallization from emulsion droplets. Rapid droplet shrinkage is followed by crystal nucleation within individual droplets. Once a nucleus is formed within a droplet, crystal growth is very rapid (<0.1 s) and occurs linearly along radially advancing fronts at speeds of up to 1 mm/s, similar to spherulitic crystal growth from impure melts. The spherulitic aggregate thus formed ages to yield the final SA morphology. Overall crystallization times are on the order of minutes, as compared to hours in conventional batch processes. We discuss these phenomena and their implications for the development of more generalized processes applicable to a variety of drug molecules. This work paves the way for microfluidics-enabled continuous spherical crystallization processes.
Applied Surface Science, 2014
In the present work thin films of CaF 2 deposited on Si substrate by electron beam evaporation ha... more In the present work thin films of CaF 2 deposited on Si substrate by electron beam evaporation have been investigated for swift heavy ions induced sputtering and surface modifications. Glancing angle X-ray Diffraction (GAXRD) measurements show that the pristine films are polycrystalline in nature and the grain size increases with increase in film thickness. Rutherford backscattering spectrometry (RBS) of pristine as well as irradiated films was performed to determine the sputter yield of CaF 2 and a decrease in sputter yield has been observed with increase in film thickness. Thermal spike model has been applied to explain this. The confinement of energy in the grains having size smaller than the electron mean free path ( ) results in a higher sputtering yield. Atomic force microscopy (AFM) studies of irradiated CaF 2 thin films show formation of cracks on film surface at a fluence of 5 × 10 12 ions/cm 2 . Also RBS results confirm the removal of film from the surface and more exposure of substrate with increasing dose of ions.
Advanced Materials, 2009
Immunoassays developed in microfluidic systems have attracted a lot of attention in recent years,... more Immunoassays developed in microfluidic systems have attracted a lot of attention in recent years, because they require small sample volumes and have faster reaction times. However, the major challenge in miniaturizing current microfluidic immunoassays into useful lab-on-chip devices is the detection mechanism. Since most microfluidic systems still rely heavily on enzyme-catalyzed reactions (e.g., enzyme-linked immunosorbent assays, ELISA) or fluorescence (e.g., immunofluorescence assays) for detection, the use of bulky equipment such as spectrometers or fluorescent microscopes preclude the use of microfluidic immunoassays for point-of-care (POC) applications. Furthermore, labeling antibodies with enzymes or fluorescence probes requires additional preparation steps, which further limit the possibility of preparing a fully integrated system. To address this issue, we report a label-free detection mechanism based on the interactions between liquid crystals (LCs) and antibodies. We show that by covering microfluidic immunoassays with a thin layer of liquid crystal, antibody concentrations can be quantified in the length of the bright LC region in the microfluidic channels, and test results can be readily observed with the naked eye, without any instrumentation.
Applied Physics Letters, 2014
RSC Advances, 2013
A poly(vinylidene fluoride) nanohybrid membrane has been prepared using high energy swift heavy i... more A poly(vinylidene fluoride) nanohybrid membrane has been prepared using high energy swift heavy ions (SHI) followed by chemical functionalization. Through-channels were generated by selective etching of the amorphous latent track created during irradiation of SHI, whose average dimensions are varied from 52 nm in pure PVDF to 40 nm for the nanohybrid in the presence of nanoclay, indicating suitable membrane formation using the nanohybrid, as compared to pure PVDF. The nanochannels were suitably grafted with conducting polymer (pyrrole) using the active free radicals caused by SHI irradiation. Spectroscopic and gel permeation chromatographic experiments have been performed to understand the effect of irradiation on filled polymer and subsequent functionalization. Doping and sulphonation on the grafted species has been done to introduce a variety of functionalities. The effect of fluence on the channel width and functionalization study indicates a larger diameter of the channels and formation of a greater ionomer at higher fluence, leading to enhancement of the bulk electrical conductivity up to 10 23 S cm 21 (12 orders higher in magnitude than pure PVDF). The activation energy of the modified nanohybrid membrane is calculated to be 26.9 kJ mol 21 , compared to 10.6 kJ mol 21 of pure PVDF, indicating superior stability of the nanohybrid membrane at higher temperature as compared to pure PVDF or functionalized PVDF under similar conditions. The nanohybrid conducting membrane produces a current of 46 nA compared to a value of almost zero for pristine PVDF or NH measured through I-V characteristic curves against a bias voltage of 10 V, making the nanohybrid suitable for electrochemical applications.
Particle & Particle Systems Characterization, 2014
Sputtering of CaF Thin Films. [AIP Conference Proceedings 1349, 651 (2011)]. Ratnesh K. Pandey, S... more Sputtering of CaF Thin Films. [AIP Conference Proceedings 1349, 651 (2011)]. Ratnesh K. Pandey, Saif A. Khan, DK Avasthi, Avinash C. Pandey. Abstract. In the present work thin films of different thicknesses deposited on Si substrate ...
Vacuum, 2009
Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by dual implantation... more Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by dual implantation of nitrogen ( 14 N þ ) and oxygen ( 16 O þ ) ions sequentially into single crystal silicon in the ratio 1:1 at 150 keV to ionfluences ranging from 1 Â 10 17 to 5 Â 10 17 cm À2 . Heavy ion elastic recoil analysis (HI-ERDA) studies of as implanted samples show Gaussian like distributions of nitrogen and oxygen. After annealing at 800 C, both the nitrogen and oxygen distributions appear as flat plateau like regions near projected range showing the formation of a continuous buried oxynitride layer. Micro-Raman study of as implanted samples shows a broad peak at 480 cm À1 for all fluences. It signifies a complete amorphization of silicon due to high fluence implantation. The annealing at 800 C results in the reduction of the intensity of the broad peak observed at 480 cm À1 and also gives rise to an additional peak at 517 cm À1 . It shows partial recrystallization of damaged silicon due to annealing. The X-ray rocking curves studies from highresolution X-ray diffraction (HRXRD) of the samples implanted with different fluences have also further confirmed partial recrystallization of damaged silicon on annealing.
Vacuum, 2009
Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by implantation of n... more Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by implantation of nitrogen ( 14 N þ ) and oxygen ( 16 O þ ) ions sequentially in the ratio 1:1 at 150 keV to ion-fluences ranging from 1 Â 10 17 to 5 Â 10 17 cm À2 to prepare silicon on insulator (SOI) structures. The as implanted samples were held at 270 C and irradiated to total fluence of 1 Â 10 14 cm À2 by 60 MeV Ni þ5 to study the structural changes/ recrystallization of SOI structures induced by swift heavy ion (SHI) irradiation. Fourier transform infrared (FTIR) measurements on the as implanted samples ( 1 Â 10 18 cm À2 ) show a single absorption band in the wavenumber range 1300-750 cm À1 attributed to the formation of silicon oxynitride (Si-O-N) bonds in the implanted silicon. It is observed that a nitrogen rich silicon oxynitride structure is formed after SHI irradiation. The study of X-ray rocking curves on the samples show the formation of small silicon crystallites due to swift heavy ion irradiation.
Soft Matter, 2012
RSC Publishing Logo View PDF Version DOI:10.1039/C2SM07373K (Paper) Soft Matter, 2012, Advance Ar... more RSC Publishing Logo View PDF Version DOI:10.1039/C2SM07373K (Paper) Soft Matter, 2012, Advance Article. Hierarchical materials synthesis at soft all-aqueous interfaces†. Su Hui Sophia Leea, MK Dawoodb, WK Choibc ...
Particle & Particle Systems Characterization, 2014
ABSTRACT
Nanotechnology, 2011
Quasi-aligned gold nanodots with a periodicity of ∼40 nm have been synthesized on a silica substr... more Quasi-aligned gold nanodots with a periodicity of ∼40 nm have been synthesized on a silica substrate by oblique deposition of gold on fast argon atom-beam-created nanoripples of wavelength 40 nm and subsequent annealing. The size distribution of these aligned nanodots resulting from oblique deposition at 85 • of 0.5 nm Au film perpendicular to ripples is narrower than the similar deposition on a flat surface. The deposition and annealing process was simulated with a three-dimensional kinetic lattice Monte Carlo technique in order to understand the formation of aligned nanodots. The atomistic simulation and the experimental results suggest that there is an optimal thickness which can result in nanodots aligned along the ripples in the case of depositions perpendicular to the ripples. The nanodots formed after annealing of the films deposited parallel to ripples or on flat surface lack alignment.
Nano Letters, 2010
The availability of robust, scalable, and automated nanoparticle manufacturing processes is cruci... more The availability of robust, scalable, and automated nanoparticle manufacturing processes is crucial for the viability of emerging nanotechnologies. Metallic nanoparticles of diverse shape and composition are commonly manufactured by solution-phase colloidal chemistry methods, where rapid reaction kinetics and physical processes such as mixing are inextricably coupled, and scale-up often poses insurmountable problems. Here we present the first continuous flow process to synthesize thin gold &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;nanoshells&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; and &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;nanoislands&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; on colloidal silica surfaces, which are nanoparticle motifs of considerable interest in plasmonics-based applications. We assemble an ordered, flowing composite foam lattice in a simple microfluidic device, where the lattice cells are alternately aqueous drops containing reagents for nanoparticle synthesis or gas bubbles. Microfluidic foam generation enables precisely controlled reagent dispensing and mixing, and the ordered foam structure facilitates compartmentalized nanoparticle growth. This is a general method for aqueous colloidal synthesis, enabling continuous, inherently digital, scalable, and automated production processes for plasmonic nanomaterials.
Langmuir, 2013
We present simple, inexpensive microfluidics-based fabrication of highly monodisperse poly(ionic ... more We present simple, inexpensive microfluidics-based fabrication of highly monodisperse poly(ionic liquid) microgel beads with a multitude of functionalities that can be chemically switched in facile fashion by anion exchange and further enhanced by molecular inclusion. Specifically, we show how the exquisite control over bead size and shape enables extremely precise, quantitative measurements of anion-and solvent-induced volume transitions in these materials, a crucial feature driving several important applications. Next, by exchanging diverse anions into the synthesized microgel beads, we demonstrate stimuli responsiveness and a multitude of novel functionalities including redox response, controlled release of chemical payloads, magnetization, toxic metal removal from water, and robust, reversible pH sensing. These chemically switchable stimulus-responsive beads are envisioned to open up a vast array of potential applications in portable and preparative chemical analysis, separations and spatially addressed sensing.
Langmuir, 2012
Recently, liquid crystal (LC) droplets in aqueous solutions have become a new platform for chemic... more Recently, liquid crystal (LC) droplets in aqueous solutions have become a new platform for chemical and biological sensing applications. In this work, we present a two-step method to generate monodisperse LC droplets in aqueous solutions for sensing applications. In the first step, we exploit inkjet printing to dispense uniform LC droplets on a solid surface. Uniform LC droplets, ranging from 35 to 136 μm in diameter, can be prepared by printing multiple times on the same spot. In the second step, we flush the LC droplets with a stream of aqueous solution in an open rectangular channel. Factors that determine the polydispersity of the LC droplets include flow rates and surface wettability. Under appropriate experimental conditions (i.e., when the surface is glass and the flow rate is sufficiently high), the LC droplets can be lifted off completely and carried away by the solution, forming free LC droplets (15−62 μm in diameter). These free LC droplets can respond to a chemical reaction and change their optical textures uniformly.
Lab on a Chip, 2009
We demonstrate a new class of microfluidic emulsion where the &amp;amp;amp;amp;amp;amp;am... more We demonstrate a new class of microfluidic emulsion where the &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;drops&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; of the emulsion are dynamic reversible bubble-drop pairs, with potential applications in microfluidic technology for chemical synthesis, molecular separations and screening.
Lab on a Chip, 2012
We report how a nominally symmetric microfluidic junction can be used to sort all bubbles of an i... more We report how a nominally symmetric microfluidic junction can be used to sort all bubbles of an incoming train exclusively into one of its arms. The existence of this &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;filter&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; regime is unexpected, given that the junction is symmetric. We analyze this behavior by quantifying how bubbles modulate the hydrodynamic resistance in microchannels and show how speeding up a bubble train whilst preserving its spatial periodicity can lead to filtering at a nominally symmetric junction. We further show how such an asymmetric traffic of bubble trains can be triggered in symmetric geometries by identifying conditions wherein the resistance to flow decreases with an increase in the number of bubbles in the microchannel and derive an exact criterion to predict the same.
Lab on a Chip, 2012
Microfluidic wet-chemical synthesis of nanoparticles is a growing area of research in chemical mi... more Microfluidic wet-chemical synthesis of nanoparticles is a growing area of research in chemical microfluidics, enabling the development of continuous manufacturing processes that overcome the drawbacks of conventional batch-based synthesis methods. The synthesis of ultra-small (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;5 nm) metallic nanocrystals is an interesting area with many applications in diverse fields, but is typically very challenging to accomplish in a microfluidics-based system due to the use of a strong gas-evolving reducing agent, aqueous sodium borohydride (NaBH(4)), which causes uncontrolled out-gassing and bubble formation, flow disruption and ultimately reactor failure. Here we present a simple method, rooted in the concepts of multiphase mass transfer that completely overcomes this challenge-we simply inject a stream of inert gas bubbles into our channels that essentially capture the evolving gas from the reactive aqueous solution, thereby preventing aqueous dissolved gas concentration from reaching the solubility threshold for bubble nucleation. We present a simple model for coupled mass transfer and chemical reaction that adequately captures device behaviour. We demonstrate the applicability of our method by synthesizing ultra-small gold nanocrystals (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;5 nm); the quality of nanocrystals thus synthesized is further demonstrated by their use in an off-chip synthesis of high-quality gold nanorods. This is a general approach that can be extended to a variety of metallic nanomaterials.
Journal of Applied Physics, 2011
Atomistic simulations of Au-silica nanocomposite film growth. [Journal of Applied Physics 109, 09... more Atomistic simulations of Au-silica nanocomposite film growth. [Journal of Applied Physics 109, 094312 (2011)]. Saif A. Khan, K.-H. Heinig, DK Avasthi. Abstract. The growth of Au-silica nanocomposite film is simulated in the framework ...
Crystal Growth & Design, 2012
Emulsion-based crystallization to produce spherical crystalline agglomerates (SAs) is an attracti... more Emulsion-based crystallization to produce spherical crystalline agglomerates (SAs) is an attractive route to control crystal size during downstream processing of active pharmaceutical ingredients (APIs). However, conventional methods of emulsification in stirred vessels pose several problems that limit the utility of emulsion-based crystallization. In this paper, we use capillary microfluidics to generate monodisperse water-in-oil emulsions. Capillary microfluidics, in conjunction with evaporative crystallization on a flat heated surface, enables controllable production of uniformly sized SAs of glycine in the 35−150 μm size range. We report detailed characterization of particle size, size distribution, structure, and polymorphic form. Further, online high-speed stereomicroscopic observations reveal several clearly demarcated stages in the dynamics of glycine crystallization from emulsion droplets. Rapid droplet shrinkage is followed by crystal nucleation within individual droplets. Once a nucleus is formed within a droplet, crystal growth is very rapid (<0.1 s) and occurs linearly along radially advancing fronts at speeds of up to 1 mm/s, similar to spherulitic crystal growth from impure melts. The spherulitic aggregate thus formed ages to yield the final SA morphology. Overall crystallization times are on the order of minutes, as compared to hours in conventional batch processes. We discuss these phenomena and their implications for the development of more generalized processes applicable to a variety of drug molecules. This work paves the way for microfluidics-enabled continuous spherical crystallization processes.
Applied Surface Science, 2014
In the present work thin films of CaF 2 deposited on Si substrate by electron beam evaporation ha... more In the present work thin films of CaF 2 deposited on Si substrate by electron beam evaporation have been investigated for swift heavy ions induced sputtering and surface modifications. Glancing angle X-ray Diffraction (GAXRD) measurements show that the pristine films are polycrystalline in nature and the grain size increases with increase in film thickness. Rutherford backscattering spectrometry (RBS) of pristine as well as irradiated films was performed to determine the sputter yield of CaF 2 and a decrease in sputter yield has been observed with increase in film thickness. Thermal spike model has been applied to explain this. The confinement of energy in the grains having size smaller than the electron mean free path ( ) results in a higher sputtering yield. Atomic force microscopy (AFM) studies of irradiated CaF 2 thin films show formation of cracks on film surface at a fluence of 5 × 10 12 ions/cm 2 . Also RBS results confirm the removal of film from the surface and more exposure of substrate with increasing dose of ions.
Advanced Materials, 2009
Immunoassays developed in microfluidic systems have attracted a lot of attention in recent years,... more Immunoassays developed in microfluidic systems have attracted a lot of attention in recent years, because they require small sample volumes and have faster reaction times. However, the major challenge in miniaturizing current microfluidic immunoassays into useful lab-on-chip devices is the detection mechanism. Since most microfluidic systems still rely heavily on enzyme-catalyzed reactions (e.g., enzyme-linked immunosorbent assays, ELISA) or fluorescence (e.g., immunofluorescence assays) for detection, the use of bulky equipment such as spectrometers or fluorescent microscopes preclude the use of microfluidic immunoassays for point-of-care (POC) applications. Furthermore, labeling antibodies with enzymes or fluorescence probes requires additional preparation steps, which further limit the possibility of preparing a fully integrated system. To address this issue, we report a label-free detection mechanism based on the interactions between liquid crystals (LCs) and antibodies. We show that by covering microfluidic immunoassays with a thin layer of liquid crystal, antibody concentrations can be quantified in the length of the bright LC region in the microfluidic channels, and test results can be readily observed with the naked eye, without any instrumentation.