Welcome From The Chair

Message From The Director of Graduate Admissions

Faculty

Hsueh-Chia Chang, Bayer Professor

Education

B.S. Caltech (1976)
M.S. Princeton (1977)
Ph.D. Princeton (1980)

 

Professional Experience

Assistant Professor, University of California, Santa Barbara (1980-1983)
Associate Professor, University of California, Santa Barbara (1983-84)
Associate Professor, University of Houston (1984-1987)
Professor, University of Notre Dame (1987-1998)
Chair, Chemical Engineering, University of Notre Dame (1989-1995)
Senior Visitor, DAMTP, Cambridge University (1993)
Bayer Professor, Univresity of Notre Dame (1998-Present)
Director, Center for Microfluidics and Medical Diagnostics (2003-Present)

Research Interests

We explore and apply electrokinetic phenomena to develop new diagnostic and microfluidic devices. We are particularly interested in AC and non-equilibrium electrokinetics due to charging of double layers by external electric fields. These phenomena require a new theoretical formulation as they cannot be described by the classical Maxwell-Wagner dielectric polarization and equilibrium Poisson-Boltzmann theories. Current projects include dielectrophoresis and self-assembly of nanocolloids and nanowires, super-charging at field critical points, charge accumulation in an AC Taylor cones and the resulting electrospray modes, electro-osmosis in nanoporous materials and assembly of charged nanocolloids. Devices and techniques under development include bacteria sorters and concentrators, rapid hybridization kits with colloidal DNA probes and continuous DNA stretching, directed immuno-colloid manipulation and concentration, nano-encapsulaton and precision coating by AC sprays, AC spray mass spectrometry, diseased blood cell detection, rapid immuno-assays and an array of micro-pumps, micro-mixers and micro-valves.

Publications

H.-C. Chang and E. A. Demehkin. (2002). Complex Wave Dynamics on Thin Films. Elsevier Scientific. view abstract // link Wave evolution on a falling film is a classical hydrodynamic instability whose rich wave dynamics have been carefully recorded in the last fifty years. Such waves are known to profoundly affect the mass and heat transfer of multi-phase industrial units.

This book describes the collective effort of both authors and their students in constructing a comprehensive theory to describe the complex wave evolution from nearly harmonic waves at the inlet to complex spatio-temporal patterns involving solitary waves downstream. The mathematical theory represents a significant breakthrough from classical linear stability theories, which can only describe the inlet harmonic waves and also extends classical soliton theory for integrable systems to real solitrary wave dynamics with dissipation. One unique feature of falling-film solitary wave dynamics, which drives much of the spatio-temporal wave evolution, is the irreversible coalescence of such localized wave structures. It represents the first full description of a hydrodynamic instability from inception to developed chaos. This approach should prove useful for other complex hydrodynamic instabilities and would allow industrial engineers to better design their multi-phase apparati by exploiting the deciphered wave dynamics. This publication gives a comprehensive review of all experimental records and existing theories and significantly advances state of the art on the subject and are complimented by complex and attractive graphics from computational fluid mechanics.

Y. Ben and H.-C. Chang. Nonlinear Smoluchowski Slip Velocity and Vortex Generation. Journal of Fluid Mechanics, 461:229-238, 2002. view abstract // link When an electric field is applied across a conducting and ion-selective porous granule in an electrolyte solution, a polarized surface layer with excess counter-ions is created. The depth of this layer and the overpotential V across this layer are functions of the normal electric field j on the granule surface. By transforming the ionic flux equations and the Poisson equation into the Painlevé equation of the second type and by analysing the latter's asymptotic solutions, we derive a linear universal j–V correlation at large flux with an electrokinetic slip length [beta]. The flux-induced surface polarization produces a nonlinear Smoluchowski slip velocity that can couple with the granule curvature to produce micro-vortices in micro-devices. Such vortices are impossible in irrotational electrokinetic flow with a constant zeta-potential and a linear slip velocity.

S. Thamida and H.-C. Chang. Nonlinear Electrokinetic Ejection and Entrainment due to Polarization at Nearly Insulated Wedges. Physics of Fluids, 14:4315-4328, 2002. view abstract // link We examine a singular electrokinetic flow around a corner or a wedge in micro-channels constructed from dielectric materials whose permittivity is small but finite compared to that of the electrolyte. When the wedge angle is less than 180°, the applied electric field, which is tangential far from the corner, develops a normal surface component that becomes singular at the corner. This normal field leakage causes opposite polarization at the two sides of the wedge and produces a converging singular tangential electrokinetic flow that ejects liquid from the tip. By expanding in cylindrical harmonics, we estimate this ejecting flow as a function of the permittivity ratio, applied electric field, angle of the wedge and the microscopic corner curvature that suppresses the singularity. The ejecting flow entrains tangential flow on the front side of the wedge and produces a vortex on the downstream side. This entrainment offers a long-range attractive hydrodynamic force that complements short-range electrostatic DLVO (Derjaguin–Landau–Verwey–Overbeek) and dielectrophoretic forces to enhance corner deposition and aggregation of colloids and proteins during electrophoresis/electro-osmosis.

L. Yeo, D. Lastochkin, S.-C. Wang and H.-C. Chang. A New AC Electrospray Mechanism by Maxwell-Wagner Polarization and Capillary Resonance. Physical Review Letters, 92:133902-133904, 2004. view abstract // link 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.

R. Zhou, P. Wang and H.-C. Chang. Bacteria Capture, Concentration and Detection by Alternating Current Dielectrophoresis and Self-Assembly of Dispersed Single-Wall Carbon Nanotubes. Electrophoresis, 27:1376-1385, 2006. view abstract // link The high polarizability and dielectrophoretic mobility of single-walled carbon nanotubes (SWNT) are utilized to capture and detect low numbers of bacteria and submicron particles in milliliter-sized samples. Concentrated SWNT solutions are mixed with the sample and a high-frequency (>100 kHz) alternating current (AC) field is applied by a microelectrode array to enhance bulk absorption of the particles (bacteria and nanoparticle substitutes) by the SWNTs via dipole-dipole interaction. The same AC field then drives the SWNT-bacteria aggregates to the microelectrode array by positive-AC dielectrophoresis (DEP), with enhanced and reversed bacteria DEP mobility due to the attached SWNTs. Since the field frequency exceeds the inverse RC time of the electrode double layer, the AC field penetrates deeply into the bulk and across the electrode gap. Consequently, the SWNTs and absorbed bacteria assemble rapidly (

P. Wang, S. Maheshwari and H.-C. Chang. Polyhedra Formation and Transient Cone Ejection of a Resonant Microdrop Forced by an AC Electric Field. Physical Review Letters, 96:254502, 2006. view abstract // link New deformation or fission phenomena are reported for microdrops driven by an ac electric field at their resonant frequencies. The Maxwell forces that pull out the vertices from a drop can be enhanced when the ac frequency is comparable to both the drop resonant frequency and the inverse charge relaxation time of the diffuse layer. The selected polyhedra possess symmetries that ensure a global force balance of the Maxwell forces and a linear dimension consistent with a sphere whose nth harmonic (n is up to six in the observation) coincides with the applied ac frequency. At high voltages, the resonant focusing of charges by the vibration modes produces evenly distributed and transient Taylor cones that can eject charged nanodrops.

Awards

Regent's Junior Faculty Award

Given on June 15, 1980 by University of California, Santa Barbara

Presidential Young Investigator

Given on May 31, 1985 by National Science Foundation

Sigma Xi Outstanding Research Award

Given on April 27, 1990 by University of Notre Dame

Francois N. Frenkiel Award

Given on November 24, 1991 by American Physical Society

Fellow of the Americal Physical Society

Given on September 29, 1997 by American Physical Society

Courses

Images

Sorting and Trapping Bacteria on the Diagnostic Chip
Continuous trapping of Lactobacillus, an intestinal bacteria, after they have been sorted from a polymicrobial mixture within the diagnostic chip.
 Integrated Diagnostic Chip
This is an image of a micro-fluidic diagnostic chip for sorting and concentrating bacteria and immuno-colloids at speeds upto 500 particles per second. The chip is only 1 cm wide and 3 cm long but it can process a realistic medical sample of 100 microliter in about 15 minutes. Each chip contains a filtering stage, a focusing stage, a sorting stage and a trapping stage. All stages involve micro-fabricated electrodes which work on the principle of dielectrophoresis. The chips can be linked in series and in parallel with recycles to resemble a multi-unit miniature plant.