Biomedical subjects

Steven D Hudson

Publications and source records attributed to Steven D Hudson.

6 recordsLinked to original sources

PubMed2006

Microfluidic approach for rapid multicomponent interfacial tensiometry.

We report a microfluidic instrument to rapidly measure the interfacial tension of multi-component immiscible liquids. The measurement principle rests upon the deformation and retraction dynamics of drops under extensional flow and was implemented for the first time in microfluidics (S. D. Hudson et al., Appl. Phys. Lett., 2005, 87, 081905 (ref. )). Here we describe in detail the instrument design and physics and extend this principle to investigate multicomponent mixtures, specifically two-component drops of adjustable composition. This approach provides fast real-time sigma measurements (on the order of 1 s), the possibility of rapidly adjusting drop composition and utilizes small sample volumes (approximately 10 microL). The tensiometer operation is illustrated with water drops and binary drops (water/ethylene glycol mixtures) in silicone oils. The technique should be particularly valuable for high-throughput characterization of complex fluids.

Equipment Design↗

PubMed2005

Breakup of carbon nanotube flocs in microfluidic traps.

The critical stress to break flocs of multiwalled-carbon nanotubes suspended in low-molecular-weight polymer fluid is measured in planar elongational flow. Through image analysis of aggregates and their fragments, the extension rate of the flow and the size and aspect ratio of the aggregates are measured in real time. While trapping an aggregate at the stagnation point of the device, the flow rate is increased continually and breaking events are recorded, establishing a correlation between aggregate size and the critical elongation stress for fragmentation sigma. This relationship resembles that for the breakup of flocs comprising spherical particles, yet it indicates that the strength of these fibrous flocs depends much more strongly on the local particle concentration. Fracture avalanches (or cascades) are also observed.

Journal Article↗

PubMed2004

Frontal photopolymerization for microfluidic applications.

Frontal photopolymerization (FPP) offers numerous advantages for the rapid prototyping of microfluidic devices. Quantitative utilization of this method, however, requires a control of the vertical dimensions of the patterned resist material. To address this fundamental problem, we study the ultraviolet (UV) photopolymerization of a series of multifunctional thiolene resists through a combination of experiments and analytical modeling of the polymerization fronts. We describe this nonlinear spatio-temporal growth process in terms of a "minimal" model involving an order parameter phi(x, t) characterizing the extent of monomer-to-polymer conversion, the optical attenuation T(x, t), and the solid front position h(t). The latter exhibits an induction time (or equivalent critical UV dose) characterizing the onset of frontal propagation. We also observe a novel transition between two logarithmic rates of growth, determined by the Beer-Lambert attenuation constants mu(0) and mu(infinity) of the monomer and fully polymerized material, respectively. The measured frontal kinetics and optical transmission of the thiolene resist materials are consistent with our photopolymerization model, exhibiting both "photodarkening" and "photoinvariant" polymerization. This is apparently the first observation of photodarkening reported in FPP. On the basis of these results, multilevel fluidic devices with controlled height are readily fabricated with modulated illumination. A representative two-level microfluidic device, incorporating a chaotic mixer, a T junction, and a series of controlled flow constrictions, illustrates the practical versatility of this fabrication method.

Journal Article↗

PubMed2004

Self-assembly of amphiphilic dendritic dipeptides into helical pores.

Natural pore-forming proteins act as viral helical coats and transmembrane channels, exhibit antibacterial activity and are used in synthetic systems, such as for reversible encapsulation or stochastic sensing. These diverse functions are intimately linked to protein structure. The close link between protein structure and protein function makes the design of synthetic mimics a formidable challenge, given that structure formation needs to be carefully controlled on all hierarchy levels, in solution and in the bulk. In fact, with few exceptions, synthetic pore structures capable of assembling into periodically ordered assemblies that are stable in solution and in the solid state have not yet been realized. In the case of dendrimers, covalent and non-covalent coating and assembly of a range of different structures has only yielded closed columns. Here we describe a library of amphiphilic dendritic dipeptides that self-assemble in solution and in bulk through a complex recognition process into helical pores. We find that the molecular recognition and self-assembly process is sufficiently robust to tolerate a range of modifications to the amphiphile structure, while preliminary proton transport measurements establish that the pores are functional. We expect that this class of self-assembling dendrimers will allow the design of a variety of biologically inspired systems with functional properties arising from their porous structure.

Biological Transport↗

PubMed2003

The effect of surfactant on the efficiency of shear-induced drop coalescence.

The volume-averaged shear-induced drop-coalescence efficiency epsilonv is measured by in situ videomicroscopy of blends of poly(propylene glycol) and poly(ethylene glycol), emulsified with poly(ethyleneglycol-b-propyleneoxide-b-ethyleneglycol) block copolymer surfactant. Adsorption of copolymer to the immiscible blend interface is indicated by a reduction in the interfacial tension, measured by the drop retraction method. The effects of temperature, copolymer molecular weight, copolymer concentration, and capillary number Ca are explored. At small Ca, epsilonv is essentially independent of shear rate and drop size, and depends mainly on the solubility, diffusivity, and surface pressure of the surfactant, indicating that drop trajectories during flow are perturbed by surfactant Marangoni stresses that are controlled by the diffusion-limited sorption of surfactant. At larger Ca, epsilonv approaches zero. This arrest of coalescence is associated with the onset of slight deformation of the drops during their collision, and drainage of a film of continuous fluid between them. The effect of the surfactant, though significant, saturates even while the amount of surfactant adsorbed to the interface is quite small. Governing dimensionless parameters, associated material parameters and the behavior of more insoluble surfactants are discussed.

Journal Article↗

PubMed2002

Layered droplet microstructures in sheared emulsions: finite-size effects.

We investigate the influence of confinement on the steady state microstructure of emulsions sheared between parallel plates, in a regime where the average droplet dimension is comparable to the gap width between the confining walls. Utilizing droplet velocimetry, we find that the droplets can organize into discrete layers under the influence of shear. The number of layers decreases from two (at relatively higher shear rates) to one (at lower shear rates), as the drops grow slightly larger due to coalescence. We argue that the layering and overall composition profile may be controlled by the interplay of droplet collisions (which can cause separation of droplet centers in the velocity gradient direction), droplet migration toward the centerline (due to wall effects), and droplet packing constraints. We also study the effects of mixture composition on droplet microstructure, and summarize these results in the form of a morphology diagram in the parameter space of mass fraction and shear rate. We find that formation of strings of the suspended phase (reported earlier by our group in flow-visualization studies on confined emulsions) is observed over a broad composition window. We also find a stable (nontransient) morphology wherein the droplets are arranged in highly ordered pearl-necklace chain structures.

Emulsions↗