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Biomedical subjects

Howard A Stone

Publications and source records attributed to Howard A Stone.

31 records · Page 2Linked to original sources
PubMed2005

Mechanism for flow-rate controlled breakup in confined geometries: a route to monodisperse emulsions.

This Letter describes a quasistationary breakup of an immiscible, inviscid fluid at low capillary numbers. The breakup proceeds in a coflowing, viscous liquid, in a confined geometry of a long and narrow orifice. In contrast to the capillary instability in an unbounded fluid, the collapse proceeds through a series of equilibria, each yielding the minimum interfacial energy of the fluid-fluid interface. The process is slow in comparison to typical relaxation speeds of the interface, and it is reversible. Its quasistatic character of collapse forms the basis for controlled, high-throughput generation of monodisperse fluid dispersions.

Journal Article↗
PubMed2005

Microfluidic flow focusing: drop size and scaling in pressure versus flow-rate-driven pumping.

We experimentally study the production of micrometer-sized droplets using microfluidic technology and a flow-focusing geometry. Two distinct methods of flow control are compared: (i) control of the flow rates of the two phases and (ii) control of the inlet pressures of the two phases. In each type of experiment, the drop size l, velocity U and production frequency f are measured and compared as either functions of the flow-rate ratio or the inlet pressure ratio. The minimum drop size in each experiment is on the order of the flow focusing contraction width a. The variation in drop size as the flow control parameters are varied is significantly different between the flow-rate and inlet pressure controlled experiments.

Microfluidic Analytical Techniques↗
PubMed2005

Clinical implications of a mathematical model of benign paroxysmal positional vertigo.

Benign paroxysmal positional vertigo (BPPV) is the most commonly diagnosed vertigo syndrome. It is caused by movement of detached otoconia within the inner ear (canalithiasis) or otoconia adherent to the cupula (cupulolithiasis). A mathematical model incorporating fluid dynamics of BPPV, which results in the following insights, has been developed recently: (1) The characteristic latency of BPPV is explained by movement of detached otoconia through the ampulla, as pressure caused by moving otoconia is negligible until otoconia enter the narrow duct of the semicircular canal. Typical otoconia move at a rate of 0.2 mm/s, or about 1% of the circumference of the canal each second. (2) Particle-wall interactions can account for the considerable variability in duration and latency of BPPV. (3) Dispersion of a clump of otoconia creates more rather than less nystagmus. Thus, dispersion is not a viable explanation of fatigability. (4) Cupulolithiasis is predicted to cause a far weaker nystagmus than canalithiasis. (5) Inertial effects of treatment maneuvers cause negligible movement of otoconia.

Ear, Inner↗
PubMed2004

Inhaling to mitigate exhaled bioaerosols.

Humans commonly exhale aerosols comprised of small droplets of airway-lining fluid during normal breathing. These "exhaled bioaerosols" may carry airborne pathogens and thereby magnify the spread of certain infectious diseases, such as influenza, tuberculosis, and severe acute respiratory syndrome. We hypothesize that, by altering lung airway surface properties through an inhaled nontoxic aerosol, we might substantially diminish the number of exhaled bioaerosol droplets and thereby provide a simple means to potentially mitigate the spread of airborne infectious disease independently of the identity of the airborne pathogen or the nature of any specific therapy. We find that some normal human subjects expire many more bioaerosol particles than other individuals during quiet breathing and therefore bear the burden of production of exhaled bioaerosols. Administering nebulized isotonic saline to these "high-producer" individuals diminishes the number of exhaled bioaerosol particles expired by 72.10 +/- 8.19% for up to 6 h. In vitro and in vivo experiments with saline and surfactants suggest that the mechanism of action of the nebulized saline relates to modification of the physical properties of the airway-lining fluid, notably surface tension.

Administration, Inhalation↗
PubMed2004

Colloidal crystallization and banding in a cylindrical geometry.

Colloidal crystallization takes advantage of the strong interfacial forces and tunable interactions that organize particles into regular structures at small scales. Thus, colloidal crystallization and patterning provide a powerful and simple method to functionalize planar surfaces with applications to optical, catalytic, sensing, and cleansing materials. Nevertheless, the ability to pattern topologically more complex surfaces such as curved, confined, or soft substrates can open new avenues for novel, "intelligent", and responsive materials. We present one step in this direction by characterizing colloidal crystallization inside circular capillaries: a nearly periodic banding is observed, and the colloidal packing is dictated by confinement produced by the wedge-like region formed by a capillary confined meniscus. The packing consists of a succession of hexagonally close-packed regions, which are separated by narrow regions of "buckled phase crystals".

Journal Article↗
PubMed2004

Two-peak and three-peak optimal complex networks.

A central issue in complex networks is tolerance of random failures and intentional attacks. Current literature emphasizes the dichotomy between networks with a power-law node connectivity distribution, which are robust to random failures but fragile to targeted attacks, versus networks with an exponentially decaying connectivity distribution, which are less tolerant to failures but more resilient to attacks. We prove analytically that the optimal network configuration under a classic measure of robustness is altogether different from both of the above: in all cases, failure and/or attack, there are no more than three distinct node connectivities in the optimal network.

Models, Biological↗
PubMed2004

A mathematical model for top-shelf vertigo: the role of sedimenting otoconia in BPPV.

Benign paroxysmal positional vertigo (BPPV) is a mechanical disorder of the vestibular system in which calcite particles called otoconia interfere with the mechanical functioning of the fluid-filled semicircular canals normally used to sense rotation. Using hydrodynamic models, we examine the two mechanisms proposed by the medical community for BPPV: cupulolithiasis, in which otoconia attach directly to the cupula (a sensory membrane), and canalithiasis, in which otoconia settle through the canals and exert a fluid pressure across the cupula. We utilize known hydrodynamic calculations and make reasonable geometric and physical approximations to derive an expression for the transcupular pressure DeltaPc exerted by a settling solid particle in canalithiasis. By tracking settling otoconia in a two-dimensional model geometry, the cupular volume displacement and associated eye response (nystagmus) can be calculated quantitatively. Several important features emerge: (1) a pressure amplification occurs as otoconia enter a narrowing duct; (2) an average-sized otoconium requires approximately 5 s to settle through the wide ampulla, where DeltaPc is not amplified, which suggests a mechanism for the observed latency of BPPV; and (3) an average-sized otoconium beginning below the center of the cupula can cause a volumetric cupular displacement on the order of 30 pL, with nystagmus of order 2 degrees/s, which is approximately the threshold for sensation. Larger cupular volume displacement and nystagmus could result from larger and/or multiple otoconia.

Calculi↗
PubMed2003

Self-assembly of gears at a fluid/air interface.

This paper describes a dynamic system-a system that develops order only when dissipating energy-comprising millimeter to centimeter scale gears that self-assemble into a simple machine at a fluid/air interface. The gears are driven externally and indirectly by magnetic interactions; they are made of poly(dimethylsiloxane) (PDMS) or magnetically doped PDMS, and fabricated by soft lithography. Transfer of torque between gears can take place through three different mechanisms: mechanical interaction, hydrodynamic shear, and capillarity/overlap of menisci. Interplay between these forces allows interactions and motions that are not possible with conventional systems of gears.

Journal Article↗
PubMed2003

Recent advances in the development of an inhaled insulin product.

Inhaled insulin first entered clinical human testing in the mid 1990s. Since then, the commercial potential and technical challenges of an inhaled insulin product have grown increasingly clear, with several pharmaceutical partnerships now targeting treatment of diabetes mellitus through inhalation products in clinical development. While clinical results to date show the therapy to be generally promising, recent data have raised questions related to human safety and slowed progress toward a commercial product. Answering these questions positively in the coming years will be critical to making inhalation therapy a practical diabetes-care reality.

Administration, Inhalation↗
PubMed2002

Drainage of single Plateau borders: direct observation of rigid and mobile interfaces.

Foam drainage varies with surfactant. We present direct measurements of the flow velocity profiles across single Plateau borders, which make up the interconnected channel-like network for liquid flow. For protein foams the interface is rigid, whereas small-surfactant foams show significant interfacial mobility. The results agree with a model that takes into account the shearing of the liquid-gas interface transverse to the flow direction. A significant consequence is that bubble size and liquid volume fraction in a foam affect the relative importance of surface rheology on the drainage behavior.

Journal Article↗
PubMed2002

Dynamics of self assembly of magnetized disks rotating at the liquid-air interface.

This paper is a theoretical study of dynamic self assembly in a system of millimeter-sized magnetized disks floating at a liquid-air interface and spinning under the influence of a rotating magnetic field. Equations of motions are derived that account for the hydrodynamic and magnetic forces acting in the system. Numerical integration of these equations predicts formation of ordered structures of spinning disks; the simulated structures reproduce the patterns observed experimentally.

Journal Article↗
PubMed2002

Chaotic mixer for microchannels.

It is difficult to mix solutions in microchannels. Under typical operating conditions, flows in these channels are laminar-the spontaneous fluctuations of velocity that tend to homogenize fluids in turbulent flows are absent, and molecular diffusion across the channels is slow. We present a passive method for mixing streams of steady pressure-driven flows in microchannels at low Reynolds number. Using this method, the length of the channel required for mixing grows only logarithmically with the Péclet number, and hydrodynamic dispersion along the channel is reduced relative to that in a simple, smooth channel. This method uses bas-relief structures on the floor of the channel that are easily fabricated with commonly used methods of planar lithography.

Chemical Phenomena↗