Biomedical subjects

Hsueh-Chia Chang

Publications and source records attributed to Hsueh-Chia Chang.

9 recordsLinked to original sources

PubMed2004

Nonlinear electrokinetics and "superfast" electrophoresis.

Nonlinear and nonequilibrium electrophoresis of spherical particles of radius a is shown to be possible when the solid surface allows field or current penetration. At low particle Peclet numbers, transient capacitative charging occurs until the surface polarization completely screens the external field. For a DC applied field [see text], the resulting electrokinetic velocity reaches Dukhin's maximum value of [formula: see text], where [see text] and mu are the liquid permittivity and viscosity. At high Peclet numbers, electroosmotic convection of the electroneutral bulk stops the transient charging before complete field-line exclusion. For an ion-selective and conducting spherical granule, the polarization is then determined by the steady-state Ohmic current driven by the penetrated external field. The high-Peclet electrokinetic velocity is lower, diffusivity-dependent and scales as [see text].

Electrophoresis↗

PubMed2004

A new ac electrospray mechanism by Maxwell-Wagner polarization and capillary resonance.

We report a new high-frequency (>10 kHz) ac electrospray that is capable of generating micron-sized electroneutral drops. Unlike its dc counterpart, the drops are not ejected continuously from a sharp Taylor cone but intermittently from a resonating meniscus at the orifice. We attribute the resonant frequency to the capillary-inertia vibration time of the meniscus and the drop ejection to the Maxwell-Wagner electric stress at the drop tip, which is observed to reverse its direction across a crossover frequency. Above this frequency, the oppositely directed Maxwell-Wagner force causes the liquid to recede up the microneedle as an apparent electrowetting effect.

Journal Article↗

PubMed2003

Knudsen diffusivity of a hard sphere in a rough slit pore.

An analytic theory for the Knudsen self-diffusivity D(s) of hard spheres in an atomically rough slit-shaped pore is presented which quantitatively matches simulation results. The theory assumes that, due to chaotic molecular trajectories caused by surface morphology, collisions of gas molecules with the wall are partly diffuse and partly specular, the relative magnitude of each depending upon the magnitude of the tangential momentum accommodation coefficient f. The theory thus represents a universal Knudsen fluctuation-dissipation correlation between longitudinal momentum loss and diffusivity that can simplify efforts to estimate D(s). It is also found that D(s) computed using Maxwell's theory of slip, in which collisions with the walls are assumed to be purely diffuse or specular, overpredicts the simulated D(s) by a large margin.

Journal Article↗

PubMed2003

Electrokinetic mixing vortices due to electrolyte depletion at microchannel junctions.

Due to electric field leakage across sharp corners, the irrotational character of Ohmic electroosmotic flow is violated. Instead, we demonstrate experimentally and theoretically evidence of electrolyte depletion and vortex separation in electroosmotic flow around a junction between wide and narrow channels. When the penetration length of the electric field exceeds the width of the narrow channel and if the electric field is directed from the narrow to the wide channel, the electromigration of ions diminishes significantly at the junction end of the narrow channel due to this leakage. Concentration depletion then develops at that location to maintain current balance but it also increases the corner zeta potential and the local electroosmotic slip velocity. A back pressure gradient hence appears to maintain flow balance and, at a sufficient magnitude, generates a pair of vortices.

Journal Article↗

PubMed2003

Dynamic synchronization analysis of venous pressure-driven cardiac output in rainbow trout.

Measurement of venous function in vivo is inherently difficult. In this study, we used the Hilbert transform to examine the dynamic relationships between venous pressure and cardiac output (CO) in rainbow trout whose blood volume was continuously increased and decreased by ramp infusion and withdrawal (I/W). The dorsal aorta and ductus Cuvier were cannulated percutaneously and connected to pressure transducers; a flow probe was placed around the ventral aorta. Whole blood from a donor was then I/W via the dorsal aortic cannula at a rate of 10% of the estimated blood volume per minute, and the duration of I/W was varied from 40, 60, 80, 90, 120, 230, 240, 260, 300, and 340 s. Compliance [change in (delta) blood vol/deltavenous pressure] was 2.8 +/- 0.2 ml x mmHg-1x g-1 (N = 25 measurements; 6 fish with closed pericardium) and 2.8 +/- 0.3 ml. mmHg-1x kg-1 (N = 19 measurements, 4 fish with open pericardium). Compliance was positively correlated with the duration of I/W, indicative of cardiovascular reflex responses at longer I/W durations. In trout with closed pericardium, CO followed venous pressure oscillations with an average time lag of 4.2 +/- 1.0 s (N = 9); heart rate (HR) was inversely correlated with CO. These studies show that CO is entrained by modulation of venous pressure, not by HR. Thus, although trout have a rigid pericardium, venous pressure (vis-a-tergo), not cardiac suction (vis-a-fronte), appears to be the primary determinant of CO. Estimation of venous compliance by ramp-modulation of venous pressure is faster and less traumatic than classical capacitance measurements and appears applicable to a variety of vertebrate species, as does the Hilbert transform, which permits analysis of signals with disparate frequencies.

Animals↗

PubMed2003

Manipulation and characterization of red blood cells with alternating current fields in microdevices.

The motion of a suspension of erythrocytes (red blood cells, RBCs) in response to a high-frequency alternating current (AC) field in a microfluidic device is examined with parallel and orthogonal electrode configurations to delineate the various fundamental driving forces. Cell repulsion from the platinum electrodes due to electrode polarization interacting with cell membrane polarizations is observed to be the strongest force acting on the particles in the first few seconds of field application. We exploit this strong repulsion to concentrate the bioparticles between the microelectrodes to amplify multiparticle aggregation phenomenon and dielectrophoretic (DEP) manipulation in a small and well-characterized region within the microfluidic device. Secondary motions include RBC pearl chain formation along field lines due to particle polarization followed by classical dielectrophoretic motion of the chains across field lines to regions of weaker field. These are driven by far weaker dipole-dipole and field-dipole interactions than the preliminary electrode repulsions. RBC chain length and total aggregated cells are presented for a variety of AC frequencies and are significantly amplified by the electrode repulsion. Motion of particles away from the polarized electrode is found to be species- and age-sensitive and can stand by itself as a promising identification and separation mechanism. In a 0.1 S/m isotonic phosphate buffer saline medium, we observe the largest cell mobilities at an optimal frequency of approximately 1 MHz, corresponding to the inverse diffusion time across the double layer of the cell and across the electrode's polarized layer. This suggests that the dielectric responses of both particles and electrodes in the low MHz frequency range are mostly determined by normal electromigration of ions from the bulk to their interfaces. Sensitivity to RBC age and species suggests that the surface proteins and membrane ion channels can affect the capacitance of the interface to accommodate the ions from the bulk. Such surface ion accumulation and polarization mechanisms are different from the classical dielectric theories. The resonant frequency of electrode polarization at around 1 MHz falls between positive and negative dielectrophoretic resonant frequency peaks - suggesting that the double-layer polarization mechanism is a distinct and potentially important bioparticle manipulation tool.

Electrochemistry↗

PubMed2002

Electrokinetic transport of red blood cells in microcapillaries.

Electrokinetic flow of a suspension of erythrocytes (red blood cells, RBCs) in 20 num cylindrical fused-silica capillaries is examined in the present work. Flow direction anomalies are observed experimentally and tentatively explained by the development of a pH gradient between the cathode well and the anode well due to electrolysis reactions at the electrodes. This pH gradient alters the local zeta potentials of both the capillary and the RBC and thus the local electroendosmotic liquid flow (EOF) velocities and RBC electrophoretic (EP) velocities. The two velocities are opposite in direction but with EOF dominating such that the RBC moves toward the cathode, opposite to the anode migration observed in bulk conditions. The opposing zeta potentials also lead to RBC aggregation at the anode end for low fields less than 25 V/cm. As the electroendosmotic velocity decreases at the anode end due to decreasing pH, pressure-driven back flow develops to oppose the original EOF at the remaining portions of the capillary ensuring constant fluid flux. When the anode EOF velocity is smaller in magnitude than the EP velocity, reversal of blood cell transport is observed after a short transient time in which a pH gradient forms. RBC velocities and pH dependencies on electric field and MgCl(2) concentration are presented along with data showing the accumulation of charge separation across the capillary. Also, a short-term solution to the pH gradient formation is presented that could help thwart development of pH gradients in micro-devices at lower voltages.

Electrophoresis, Capillary↗

PubMed2002

Nanoscale pore formation dynamics during aluminum anodization.

A theoretical analysis of nanoscale pore formation during anodization reveals its fundamental instability mechanism to be a field focusing phenomenon when perturbations on the minima of the two oxide interfaces are in phase. Lateral leakage of the layer potential at high wave number introduces a layer tension effect that balances the previous destabilizing effect to produce a long-wave instability and a selected pore separation that scales linearly with respect to voltage. At pH higher than 1.77, pores do not form due to a very thick barrier layer. A weakly nonlinear theory based on long-wave expansion of double free surface problem yields two coupled interface evolution equations that can be reduced to one without altering the dispersion relationship by assuming an equal and in-phase amplitude for the two interfaces. This interface evolution equation faithfully reproduces the initial pore ordering and their dynamics. A hodograph transformation technique is then used to determine the interior dimension of the well-developed pores in two dimensions. The ratio of pore diameter to pore separation is found to be a factor independent of voltage but varies with the pH of the electrolyte. Both the predicted pH range where pores are formed and the predicted pore dimensions are favorably compared to experimental data. (c) 2002 American Institute of Physics.

Journal Article↗

PubMed1999

Pattern selection during electropolishing due to double-layer effects.

We extend our earlier study of nanoscale pattern formation during electropolishing [Nanotechnology 7, 360 (1996); Phys. Rev. B 56, 12 608 (1997)]. The patterns are attributed to preferential adsorption of organic molecules on the convex portion of the electrode due to its enhanced electric field. This local enhancement occurs because of the effect of surface curvature on the double-layer potential drop. By allowing for transport correction to the double-layer potential drop at thermodynamic equilibrium, we estimate this anodic overpotential to be in the realistic mV range and hence verify the Debye-Huckel approximation used in our model. This small anodic overpotential suggests that pattern formation is a generic electropolishing phenomenon whose only requirement is that the polarizability of the organic additive relative to water must lie within a range specified by our theory. We verify this prediction experimentally with a variety of electrolyte solutions. The voltage ranges for specific hexagonal and ridge patterns are well correlated by our model with only a single parameter. (c) 1999 American Institute of Physics.

Journal Article↗