Amir Hirsa - Academia.edu (original) (raw)
Papers by Amir Hirsa
Microgravity Science and Technology, 2016
Microgravity is potentially a powerful tool for investigating processes that are sensitive to the... more Microgravity is potentially a powerful tool for investigating processes that are sensitive to the presence of solid walls, since fluid containment can be achieved by surface tension. One such process is the transformation of protein in solution into amyloid fibrils; these are protein aggregates associated with neurodegenerative diseases such as Alzheimer's and Parkinson's. In addition to solid walls, experiments with gravity are also subject to influences from sedimentation of aggregates and buoyancydriven convection. The ring-sheared drop (RSD) module is a flow apparatus currently under development to study formation of amyloid fibrils aboard the International Space Station (ISS). A 25 mm diameter drop of protein solution will be contained by surface tension and constrained by a pair of sharp-edged tubes, forming two contact rings. Shear can be imparted by rotating one ring with the other ring kept stationary. Here we report on parabolic flights conducted to test the growth and pinning of 10 mm diameter drops of water in under 10 s of microgravity. Finite element method (FEM) based fluid dynamics computations using a commercial package (COMSOL) assisted in the design of the parabolic flight experiments. Prior to the parabolic flights, the code was validated against experiments in the lab (1 g), on the growth of sessile and pendant droplets. The simulations show good agreement with the experiments. This modeling capability will enable the development of the RSD at the 25 mm scale for the ISS.
The ability to describe and utilize protein structure has made possible the rational design of ne... more The ability to describe and utilize protein structure has made possible the rational design of new drugs and pharmacological agents. Proteins must be first crystallized in order to utilize the techniques that yield a precise description of their structure. Two-dimensional protein crystallization at the air-water interface entails the specific binding of a protein to a lipid monolayer containing a ligand. There are several classic crystallization strategies such as varying the ionic strength, pH, and temperature of the protein solution; in all of these, fluid motion is reported to be detrimental. However, we have discovered that fluid dynamics can be used to advantage. The flow system utilized consists of a stationary open cylinder driven by the constant rotation of the floor, in the axisymmetric flow regime with inertia. We show that by applying shear stress to the protein-ligand complexes at the interface, it is possible not only to control the protein-protein interaction and induc...
The use of constrained droplets (droplets with pinned contact lines on solid surfaces) is propose... more The use of constrained droplets (droplets with pinned contact lines on solid surfaces) is proposed here as a method for sample support in optical microscopy studies. Capillarity acts to contain the liquid sample, allowing access for observations in the bulk and at the gas/liquid interface. At the capillary length scale, surface tension forms stable interfaces, virtually immune to gravity and with curvatures that can be adjusted. This is particularly useful when studying the gas/liquid interface and its vicinity under high resolution optical microscopy. Such observations are normally performed using oil immersion objectives which must be positioned within distances only tens of microns from the region of interest. Constrained droplets can also be used at small scales, requiring minute volumes of analyte. The use of the constrained droplet method is demonstrated by studying the aggregation of insulin into amyloid fibrils in the solution and at the gas/liquid interface, where proteins ...
ABSTRACT The presence of surfactants, ubiquitous at most gas/liquid interfaces and particularly a... more ABSTRACT The presence of surfactants, ubiquitous at most gas/liquid interfaces and particularly at the air/water interface, has a pronounced effect on the stress balance at the interface, and this in turn is nonlinearly coupled to the bulk flow. Forces acting on the interface include surface tension gradients and the viscous resistance to shear and dilation. These viscoelastic properties vary with the surfactant concentration at the interface. The surface tension and surface shear viscosity were measured directly over a range of surface concentrations of hemicyanine (an insoluble surfactant). Here, we present a fundamental description of the interface and its coupling to the bulk flow, and develop an axisymmetric Navier-Stokes numerical model. Numerical studies in a cylindrical annular channel, driven by the constant rotation of its bottom endwall, are presented comparing contaminated interface cases with that of a clean interface and a rigid no-slip surface. The computations provide insight into the dynamics that result from the surfactant with a nonlinear equation of state and surface shear viscosity. In this presentation, we focus on the low initial surfactant concentration cases where a contamination front is observed and find that its location varies linearly with the initial concentration.
ABSTRACT We report on measurements of insoluble monolayers on water showing patterning at macrosc... more ABSTRACT We report on measurements of insoluble monolayers on water showing patterning at macroscopic scale (i.e., at centimeter scale of the flow apparatus) as well as mesoscopic scale (i.e., down to micron scale resolvable via optical microscopy). A simple flow system is utilized, consisting of a stationary open cylinder driven by the constant rotation of the floor. Velocity field is measured via particle image velocimetry and the monolayer is imaged using a Brewster angle microscope (BAM). The macroscale pattern results from a hydrodynamic instability of the flow, which breaks the symmetry and produces a rotating wave. BAM images show a macroscopically patterned monolayer steadily precessing with the rotating wave flow structure. BAM images also show mesoscale patterning of the monolayer by the flow, which strongly depends on the local shear of the flow, ranging from solid-body rotation near the axis to an interface that is highly sheared by the meridional bulk flow at large radii. For surfactant monolayers, macroscale pattern is lost by the action of elasticity if the flow is ceased, whereas the mesoscale pattern is irreversible. Computations using axisymmetric Navier-Stokes provide a good prediction of the mean location of the monolayer front.
WIT transactions on engineering sciences, 1970
Analytical and experimental methods are used to study the presence of vortex breakdown states app... more Analytical and experimental methods are used to study the presence of vortex breakdown states appearing on slender vortices in slightly diverging streamtubes. Asymptotic methods are used to obtain analytical solutions of the steady and axisymmetric Euler equations for near-critical rotating flows. Multiple flow states which describe either a small perturbation of the base rotating flow or a larger-amplitude disturbance may appear. The stability of the various flow states around the critical swirl is also studied. It is found that, for swirling flows below the critical value, the slightly perturbed states are linearly stable whereas the large-disturbance states are unstable to a certain mode of disturbance. The analysis suggests the transition to vortex breakdown as the swirl level is increased. To demonstrate the applicability of these relations, LDV measurements were made of the leading-edge vortices that develop over a delta wing with a 67.6° sweep back angle. The measured axial a...
Aps Division of Fluid Dynamics Meeting Abstracts, 2002
Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2009
Submitted for the DFD09 Meeting of The American Physical Society Dynamics of pinned-contact oscil... more Submitted for the DFD09 Meeting of The American Physical Society Dynamics of pinned-contact oscillating gas/liquid lenses AMIR HIRSA, BERNARD MALOUIN, JOSEPH OLLES, Rensselaer Polytechnic Institute, CARLOS LOPEZ, Intel Corp., MICHAEL VOGEL, Cornell University-Liquid lenses are a natural solution for applications in adaptive optics requiring a fast response. Existing liquid lenses use large disturbances to overcome liquid inertia and subsequently utilize the lens after the oscillations have dampened. An alternate strategy was demonstrated recently: a harmonically-driven liquid lens with an oscillating focal length that allows the capture of any plane in a given range by grabbing the image 'in sync' with the oscillations. Hence, by continuously oscillating the lens, the task of changing the focal length is effectively transformed from a mechanical manipulation to the electronic timing of image capture by the sensor, which can be achieved much faster. Good optical quality is possible by designing the liquid lens to traverse states where capillarity produces spherical interfaces. Energy efficiency is achieved through pinned contact lines and operation at resonance. We present experimental results along with predictions for the dynamics of such oscillatory driven lenses, including the effects of liquid volume, driving frequency and amplitude on droplet shapes and resultant optical characteristics. High fidelity imaging was demonstrated at 100 Hz for a millimeter scale liquid lens.
Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 1999
Consistent measurements of surface dilatational viscosity, kappa_s, obtained with different techn... more Consistent measurements of surface dilatational viscosity, kappa_s, obtained with different techniques have yet to appear in the literature for a given surfactant system. Measurements of kappas for insoluble monolayers are generally obtained through the tangential stress balance at the interface (e.g., the longitudinal wave method). The coupling between the bulk (shear) stress and the interfacial stress also involves surface tension gradients (Marangoni stress), which can dominate the effect of kappas for many flows. A new flow geometry with planar stationary walls, an oscillatory moving floor, and a flat gas/liquid interface with an insoluble monolayer is introduced for the determination of kappa_s. The flow parameters are selected to make kappas contribution important compared to Marangoni stress. Navier-Stokes computations with the Boussinesq-Scriven surface model are presented utilizing the equation of state measured for a vitamin K1 monolayer on the air/water interface. The res...
In the presence of surfactants, a surface barrier can trap a monolayer on a flowing liquid. The t... more In the presence of surfactants, a surface barrier can trap a monolayer on a flowing liquid. The transition between this monolayer-covered region and the upstream surfactant-free region is marked by a Reynolds ridge. Previous results have shown that the monolayer is essentially stagnant and the flow closely resembles that of a solid wall at the surface. Presently, the nonlinear optical technique of second-harmonic generation is utilized to measure surfactant concentration profiles for insoluble monolayers on water in a planar flow. By also considering the interfacial velocities obtained via Boundary-Fitted DPIV, the results are related to the interfacial boundary conditions which account for Marangoni stress, interfacial viscosities, and surfactant diffusivity. Also, families of functions were measured and modeled for various surfactants, showing the interdependence of three essential variables: (1) the length of the monolayer-covered region, (2) the freestream velocity of the bulk l...
A capillary surface is a liquid/liquid or liquid/gas interface whose shapes is determined by surf... more A capillary surface is a liquid/liquid or liquid/gas interface whose shapes is determined by surface tension. For typical liquids (e.g., water) against gas, capillary surfaces occur in systems of millimeter scale and smaller. Use of capillary surfaces has emerged as a useful approach for manipulating liquids in microfluidic devices, however, practitioners have yet to take advantage of capillary instability to improve energy efficiency in their design of such devices. Activation strategies for a bi-stable switch, comprised of a pair of capillary elements, will be discussed. Laboratory experiments were conducted at millimeter scale in aqueous systems. Actuation via pressure disturbances (parallel action) shows that the energy barrier computed for a static system provides a good estimate for the response of dynamic systems. Addressable actuation was achieved using electrochemical activation via a redox surfactant solution. This device also constitutes a micro-lens with a controllable f...
Through the use of an oscillating double droplet system (DDS), an adaptive varifocal fluidic lens... more Through the use of an oscillating double droplet system (DDS), an adaptive varifocal fluidic lens is created. Pinning the gas/liquid contact lines of the DDS eliminates viscous losses from moving contact lines and aids in simplifying the geometric parameter space. The use of millimeter scale diameters allows capillary effects to form stable droplets with spherical interfaces. We present a range of dependent parameters for the DDS as a fluidic lens which is made to oscillate by a sinusoidal pressure in millisecond cycles and various amplitudes. The lens is characterized through the following optical parameters throughout a cycle: sphericity, radius of curvature, focal length, and field stop. A non-linearity in the DDS's resonant frequency is identified at various volumes; slight changes in the amplitude of the driving pressure produces a substantial shift in the resonant frequency of the system.
By designing pinned-contact, coupled droplet pairs at the appropriate length scale to promote sur... more By designing pinned-contact, coupled droplet pairs at the appropriate length scale to promote surface tension as the dominant force, one can create bi-stable capillary switches and natural oscillators. These systems have been triggered by pressure pulses, electrochemistry, and electroosmosis. These methods are typically accompanied by bulky setups or slow response times. An alternate approach exploits electromagnetic activation. Our device consists of a millimeter scale orifice, overfilled with an aqueous ferrofluid, in proximity to a wire coil that generates a magnetic field. Experimental evidence of such capillary switches and energy efficient oscillators is presented here. Comparisons to a simplified model are also presented. This activation method is shown to have relatively fast response times, low driving voltages, and individual addressability. Electromagnetically activated capillary switches and oscillators offer many applications ranging from high-speed adaptive optics to m...
The interaction of a columnar vortex with a solid wall normal to its axis is investigated here bo... more The interaction of a columnar vortex with a solid wall normal to its axis is investigated here both experimentally and numerically. The interaction results in the production of a secondary flow in a thin boundary layer into which the vortex lines are bent. Within the vortex core radius r_c, the velocity profiles in the boundary layer are spatially oscillatory. The secondary flow in the boundary layer for radii larger than rc consists of a radial inflow which turns into the axial direction for r
Microgravity Science and Technology, 2016
Microgravity is potentially a powerful tool for investigating processes that are sensitive to the... more Microgravity is potentially a powerful tool for investigating processes that are sensitive to the presence of solid walls, since fluid containment can be achieved by surface tension. One such process is the transformation of protein in solution into amyloid fibrils; these are protein aggregates associated with neurodegenerative diseases such as Alzheimer's and Parkinson's. In addition to solid walls, experiments with gravity are also subject to influences from sedimentation of aggregates and buoyancydriven convection. The ring-sheared drop (RSD) module is a flow apparatus currently under development to study formation of amyloid fibrils aboard the International Space Station (ISS). A 25 mm diameter drop of protein solution will be contained by surface tension and constrained by a pair of sharp-edged tubes, forming two contact rings. Shear can be imparted by rotating one ring with the other ring kept stationary. Here we report on parabolic flights conducted to test the growth and pinning of 10 mm diameter drops of water in under 10 s of microgravity. Finite element method (FEM) based fluid dynamics computations using a commercial package (COMSOL) assisted in the design of the parabolic flight experiments. Prior to the parabolic flights, the code was validated against experiments in the lab (1 g), on the growth of sessile and pendant droplets. The simulations show good agreement with the experiments. This modeling capability will enable the development of the RSD at the 25 mm scale for the ISS.
The ability to describe and utilize protein structure has made possible the rational design of ne... more The ability to describe and utilize protein structure has made possible the rational design of new drugs and pharmacological agents. Proteins must be first crystallized in order to utilize the techniques that yield a precise description of their structure. Two-dimensional protein crystallization at the air-water interface entails the specific binding of a protein to a lipid monolayer containing a ligand. There are several classic crystallization strategies such as varying the ionic strength, pH, and temperature of the protein solution; in all of these, fluid motion is reported to be detrimental. However, we have discovered that fluid dynamics can be used to advantage. The flow system utilized consists of a stationary open cylinder driven by the constant rotation of the floor, in the axisymmetric flow regime with inertia. We show that by applying shear stress to the protein-ligand complexes at the interface, it is possible not only to control the protein-protein interaction and induc...
The use of constrained droplets (droplets with pinned contact lines on solid surfaces) is propose... more The use of constrained droplets (droplets with pinned contact lines on solid surfaces) is proposed here as a method for sample support in optical microscopy studies. Capillarity acts to contain the liquid sample, allowing access for observations in the bulk and at the gas/liquid interface. At the capillary length scale, surface tension forms stable interfaces, virtually immune to gravity and with curvatures that can be adjusted. This is particularly useful when studying the gas/liquid interface and its vicinity under high resolution optical microscopy. Such observations are normally performed using oil immersion objectives which must be positioned within distances only tens of microns from the region of interest. Constrained droplets can also be used at small scales, requiring minute volumes of analyte. The use of the constrained droplet method is demonstrated by studying the aggregation of insulin into amyloid fibrils in the solution and at the gas/liquid interface, where proteins ...
ABSTRACT The presence of surfactants, ubiquitous at most gas/liquid interfaces and particularly a... more ABSTRACT The presence of surfactants, ubiquitous at most gas/liquid interfaces and particularly at the air/water interface, has a pronounced effect on the stress balance at the interface, and this in turn is nonlinearly coupled to the bulk flow. Forces acting on the interface include surface tension gradients and the viscous resistance to shear and dilation. These viscoelastic properties vary with the surfactant concentration at the interface. The surface tension and surface shear viscosity were measured directly over a range of surface concentrations of hemicyanine (an insoluble surfactant). Here, we present a fundamental description of the interface and its coupling to the bulk flow, and develop an axisymmetric Navier-Stokes numerical model. Numerical studies in a cylindrical annular channel, driven by the constant rotation of its bottom endwall, are presented comparing contaminated interface cases with that of a clean interface and a rigid no-slip surface. The computations provide insight into the dynamics that result from the surfactant with a nonlinear equation of state and surface shear viscosity. In this presentation, we focus on the low initial surfactant concentration cases where a contamination front is observed and find that its location varies linearly with the initial concentration.
ABSTRACT We report on measurements of insoluble monolayers on water showing patterning at macrosc... more ABSTRACT We report on measurements of insoluble monolayers on water showing patterning at macroscopic scale (i.e., at centimeter scale of the flow apparatus) as well as mesoscopic scale (i.e., down to micron scale resolvable via optical microscopy). A simple flow system is utilized, consisting of a stationary open cylinder driven by the constant rotation of the floor. Velocity field is measured via particle image velocimetry and the monolayer is imaged using a Brewster angle microscope (BAM). The macroscale pattern results from a hydrodynamic instability of the flow, which breaks the symmetry and produces a rotating wave. BAM images show a macroscopically patterned monolayer steadily precessing with the rotating wave flow structure. BAM images also show mesoscale patterning of the monolayer by the flow, which strongly depends on the local shear of the flow, ranging from solid-body rotation near the axis to an interface that is highly sheared by the meridional bulk flow at large radii. For surfactant monolayers, macroscale pattern is lost by the action of elasticity if the flow is ceased, whereas the mesoscale pattern is irreversible. Computations using axisymmetric Navier-Stokes provide a good prediction of the mean location of the monolayer front.
WIT transactions on engineering sciences, 1970
Analytical and experimental methods are used to study the presence of vortex breakdown states app... more Analytical and experimental methods are used to study the presence of vortex breakdown states appearing on slender vortices in slightly diverging streamtubes. Asymptotic methods are used to obtain analytical solutions of the steady and axisymmetric Euler equations for near-critical rotating flows. Multiple flow states which describe either a small perturbation of the base rotating flow or a larger-amplitude disturbance may appear. The stability of the various flow states around the critical swirl is also studied. It is found that, for swirling flows below the critical value, the slightly perturbed states are linearly stable whereas the large-disturbance states are unstable to a certain mode of disturbance. The analysis suggests the transition to vortex breakdown as the swirl level is increased. To demonstrate the applicability of these relations, LDV measurements were made of the leading-edge vortices that develop over a delta wing with a 67.6° sweep back angle. The measured axial a...
Aps Division of Fluid Dynamics Meeting Abstracts, 2002
Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2009
Submitted for the DFD09 Meeting of The American Physical Society Dynamics of pinned-contact oscil... more Submitted for the DFD09 Meeting of The American Physical Society Dynamics of pinned-contact oscillating gas/liquid lenses AMIR HIRSA, BERNARD MALOUIN, JOSEPH OLLES, Rensselaer Polytechnic Institute, CARLOS LOPEZ, Intel Corp., MICHAEL VOGEL, Cornell University-Liquid lenses are a natural solution for applications in adaptive optics requiring a fast response. Existing liquid lenses use large disturbances to overcome liquid inertia and subsequently utilize the lens after the oscillations have dampened. An alternate strategy was demonstrated recently: a harmonically-driven liquid lens with an oscillating focal length that allows the capture of any plane in a given range by grabbing the image 'in sync' with the oscillations. Hence, by continuously oscillating the lens, the task of changing the focal length is effectively transformed from a mechanical manipulation to the electronic timing of image capture by the sensor, which can be achieved much faster. Good optical quality is possible by designing the liquid lens to traverse states where capillarity produces spherical interfaces. Energy efficiency is achieved through pinned contact lines and operation at resonance. We present experimental results along with predictions for the dynamics of such oscillatory driven lenses, including the effects of liquid volume, driving frequency and amplitude on droplet shapes and resultant optical characteristics. High fidelity imaging was demonstrated at 100 Hz for a millimeter scale liquid lens.
Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 1999
Consistent measurements of surface dilatational viscosity, kappa_s, obtained with different techn... more Consistent measurements of surface dilatational viscosity, kappa_s, obtained with different techniques have yet to appear in the literature for a given surfactant system. Measurements of kappas for insoluble monolayers are generally obtained through the tangential stress balance at the interface (e.g., the longitudinal wave method). The coupling between the bulk (shear) stress and the interfacial stress also involves surface tension gradients (Marangoni stress), which can dominate the effect of kappas for many flows. A new flow geometry with planar stationary walls, an oscillatory moving floor, and a flat gas/liquid interface with an insoluble monolayer is introduced for the determination of kappa_s. The flow parameters are selected to make kappas contribution important compared to Marangoni stress. Navier-Stokes computations with the Boussinesq-Scriven surface model are presented utilizing the equation of state measured for a vitamin K1 monolayer on the air/water interface. The res...
In the presence of surfactants, a surface barrier can trap a monolayer on a flowing liquid. The t... more In the presence of surfactants, a surface barrier can trap a monolayer on a flowing liquid. The transition between this monolayer-covered region and the upstream surfactant-free region is marked by a Reynolds ridge. Previous results have shown that the monolayer is essentially stagnant and the flow closely resembles that of a solid wall at the surface. Presently, the nonlinear optical technique of second-harmonic generation is utilized to measure surfactant concentration profiles for insoluble monolayers on water in a planar flow. By also considering the interfacial velocities obtained via Boundary-Fitted DPIV, the results are related to the interfacial boundary conditions which account for Marangoni stress, interfacial viscosities, and surfactant diffusivity. Also, families of functions were measured and modeled for various surfactants, showing the interdependence of three essential variables: (1) the length of the monolayer-covered region, (2) the freestream velocity of the bulk l...
A capillary surface is a liquid/liquid or liquid/gas interface whose shapes is determined by surf... more A capillary surface is a liquid/liquid or liquid/gas interface whose shapes is determined by surface tension. For typical liquids (e.g., water) against gas, capillary surfaces occur in systems of millimeter scale and smaller. Use of capillary surfaces has emerged as a useful approach for manipulating liquids in microfluidic devices, however, practitioners have yet to take advantage of capillary instability to improve energy efficiency in their design of such devices. Activation strategies for a bi-stable switch, comprised of a pair of capillary elements, will be discussed. Laboratory experiments were conducted at millimeter scale in aqueous systems. Actuation via pressure disturbances (parallel action) shows that the energy barrier computed for a static system provides a good estimate for the response of dynamic systems. Addressable actuation was achieved using electrochemical activation via a redox surfactant solution. This device also constitutes a micro-lens with a controllable f...
Through the use of an oscillating double droplet system (DDS), an adaptive varifocal fluidic lens... more Through the use of an oscillating double droplet system (DDS), an adaptive varifocal fluidic lens is created. Pinning the gas/liquid contact lines of the DDS eliminates viscous losses from moving contact lines and aids in simplifying the geometric parameter space. The use of millimeter scale diameters allows capillary effects to form stable droplets with spherical interfaces. We present a range of dependent parameters for the DDS as a fluidic lens which is made to oscillate by a sinusoidal pressure in millisecond cycles and various amplitudes. The lens is characterized through the following optical parameters throughout a cycle: sphericity, radius of curvature, focal length, and field stop. A non-linearity in the DDS's resonant frequency is identified at various volumes; slight changes in the amplitude of the driving pressure produces a substantial shift in the resonant frequency of the system.
By designing pinned-contact, coupled droplet pairs at the appropriate length scale to promote sur... more By designing pinned-contact, coupled droplet pairs at the appropriate length scale to promote surface tension as the dominant force, one can create bi-stable capillary switches and natural oscillators. These systems have been triggered by pressure pulses, electrochemistry, and electroosmosis. These methods are typically accompanied by bulky setups or slow response times. An alternate approach exploits electromagnetic activation. Our device consists of a millimeter scale orifice, overfilled with an aqueous ferrofluid, in proximity to a wire coil that generates a magnetic field. Experimental evidence of such capillary switches and energy efficient oscillators is presented here. Comparisons to a simplified model are also presented. This activation method is shown to have relatively fast response times, low driving voltages, and individual addressability. Electromagnetically activated capillary switches and oscillators offer many applications ranging from high-speed adaptive optics to m...
The interaction of a columnar vortex with a solid wall normal to its axis is investigated here bo... more The interaction of a columnar vortex with a solid wall normal to its axis is investigated here both experimentally and numerically. The interaction results in the production of a secondary flow in a thin boundary layer into which the vortex lines are bent. Within the vortex core radius r_c, the velocity profiles in the boundary layer are spatially oscillatory. The secondary flow in the boundary layer for radii larger than rc consists of a radial inflow which turns into the axial direction for r