Behrouz Abedian

Silk Fibroin Solution Properties Related to Assembly and Structure

The impact of physiological factors on silk fibroin solution properties was studied. Specifically, the impact of fibroin concentration, protein purity, cation type and concentration, and pH on aqueous solution viscosity, shear behavior, and surface tension were assessed in the context of silk protein assembly. The results demonstrate that in vitro results could be correlated to in vivo processing events during silk spinning. Rheological properties with reference to the amphiphilic block structure of the protein are described, pH dependency of shear response was quantitatively correlated to the predicted pI values of the fibroin protein, and cooperativity among environmental factors such as pH and salts was identified. Stabilization of silk fibroin solution states by calcium was identified as a mode to control shear sensitivity of the fibroin solution. The cooperativities identified suggest tight control of fibroin aqueous solution rheological properties to gain a window of protection against premature crystallization of the fibroin during processing, assuring safe storage, transport, and finally successful fiber spinning.

Electric currents generated by turbulent flows of liquid hydrocarbons in smooth pipes: Experiment vs. theory

Abstract Available data on the electric convection current generated by turbulent liquid hydrocarbon flows in smooth-walled pipes are reviewed and compared with the theory of Abedian and Sonin (1982, J. Fluid Mech. 120 199–217). A good comparison is obtained by assuming a single value of the zeta-potential for all experiments.

Low-voltage electrowetting on a lipid bilayer formed on hafnium oxide

We present a class of electrowetting systems in which lipid bilayers on thin hafnium oxide films function as reversibly wettable dielectrics, eliminating the need for solid organic dielectrics (e.g., fluoropolymers) in electrowetting systems. These bilayers self-assemble on hafnium oxide in oil, yielding a dielectric stack that can withstand high electric fields, enabling high contact angle changes at unprecedentedly low voltages. We demonstrate that these electrowetting systems can be virtually free of contact angle saturation with low oil-water surface energies (<1 mJ/m2), allowing for a reversible contact angle change from over 140° to under 10° using less than 1 V actuation.

New Generation of Digital Microfluidic Devices

This paper reports on the design, fabrication, and performance of micro-sized fluidic devices that use electrowetting to control and transport liquids. Using standard microfabrication techniques, new pumping systems are developed with significantly more capability than open digital microfluidic systems that are often associated with electrowetting. This paper demonstrates that, by integrating closed microchannels with different channel heights and using electrowetting actuation, liquid interfaces can be controlled, and pressure work can be done, resulting in fluid pumping. The operation of two different on-chip pumps and devices that can form water drops is described. In addition, a theory is presented to explain the details of single-electrode actuation in a closed channel.

Temperature effects on the electrical conductivity of dielectric liquids

The effect of temperature on electrical conductivity of low-conductivity liquid solutions is discussed theoretically and experimentally. A well-known expression for liquid viscosity is used to derive an empirical correlation for temperature dependency of electrical conductivity of insulating liquids. Five different solutions all composed of polyol (hindered) esters were tested over a wide temperature range. Viscosity measurements were used to extract the constants in the viscosity correlation. The measured electrical conductivity of the tested fluids varied over 1000-fold for the temperature range. It is shown that the suggested correlation could predict reasonably well the temperature dependency of electrical conductivity for single-phased solutions.

Validation of the trapped charge model of electrowetting contact angle saturation on lipid bilayers

The problem of modeling contact angle saturation in electrowetting has resisted a number of concentrated efforts by leading researchers. Several models have been proposed, from charge trapping, to droplet ejection, to thermodynamic instability, but no consensus has been reached as to which model better describes the effect. In this paper, we validate the charge trapping based model of contact angle saturation in electrowetting on lipid bilayers, through careful analysis of charge movement between the liquid charge states and trapped charge states at the solid dielectric interface. We also describe a powerful new methodology for studying electrowetting systems by modeling them with an equivalent circuit and simulating the circuit using the SPICE circuit simulator.

An Effective Method for Spreading Flowable Composites in Resin-based Restorations

UNLABELLED Cavity lining with flowable composites has been suggested for better marginal adaptation and a reduction in interfacial stress and post-operative sensitivity. The following in vitro study compares the spreading of flowable composite that lines the inner wall of a test cavity using an explorer and a specially designed smooth, oscillating, off-center rotating bur spun at 300 to 600 rpm. Two commercial flowable materials were used in this study. RESULTS With use of the rotating bur, a more consistent, uniform cavity lining was achieved. This was not possible using the dental explorer. CONCLUSION The specially designed rotating bur provides an effective way for clinicians to form a uniform cavity lining.

Electrical conductance across self-assembled lipid bilayers

This study investigates electrical conduction across lipid bilayers that self-assemble in oil between a water drop and a hafnium oxide surface. Morphology and electrical properties of two bilayers formed from equal molar concentrations of sorbitan monooleate (Span 80) and sorbitan trioleate (Span 85) in dodecane are quantified and compared. The molecular structures of the two surfactants are quite similar, except that sorbitan monooleate contains one unsaturated lipid tail per molecule, whereas sorbitan trioleate contains three. We find that the leakage current density across both bilayers increases exponentially with electric field. The relative leakage current densities across the two bilayers scale with the packing density of lipid tails in the bilayers. This correlation provides evidence that the lipid tail interactions through thermal fluctuations provide a pathway of electrical conduction across the membrane.

Resonant frequency of a polyvinylidene flouride piezoelectric bimorph: the effect of surrounding fluid

This work presents experimental and theoretical results on the dynamic behavior of piezoelectric cantilever bimorph in the presence of surrounding air. The bimorph is composed of a pair of piezoelectric sheets bonded by a uniform elastic layer of adhesive in the center. The transverse motion of the bimorph is generated by a sequential application of two opposing electric fields on the piezoelectric sheets. Theoretically, the tip deflection and the natural frequency of the bimorph are obtained making use of an energy balance technique. The fluid in modeled as inviscid and incompressible whose motion induces locally additional mass in the transverse direction. An expression for the kinetic energy of the system is derived based on this additional mass from which the natural frequency of the combined system is obtained. Tests were performed on the piezoelectric bimorphs with similar geometries and varying adhesive thickness in a vacuum chamber. The air pressure in the chamber was varied from 10 kPa to one atmosphere. Good agreements between the theoretical predictions and the observed values were obtained. This study could have applications in the use of piezoelectric materials for fluid property measurements.

Electric charging in flow of low-conductivity liquids through screens: A comparison of theory and experiments

Abstract This paper presents an experimental study of the charging of low-conductivity liquids by screens. Measurements were performed over a wide range of fluid conductivities and screen types. The relationship that was obtained between the current generated and the velocity of the fluid closely matched the theory. Potential measurements across the screen indicated that the charging process can be characterized by a charging electromotive force.