Alvin U. Chen

A new method for significantly reducing drop radius without reducing nozzle radius in drop-on-demand drop production

The lack of a simple method for generating drops whose radii (Rd) are much smaller than those (R) of nozzles which produce them has heretofore been a major limitation of the drop-on-demand technique. Therefore, the only reliable way to reduce Rd to date has been to reduce R. A new method is reported which allows an order of magnitude reduction in drop volume while using the same nozzle. It is shown that the key to forming drops with Rd<R is to judiciously control the capillary, viscous, and inertial time scales that govern the flow within the nozzle and the forming drop. The time scales are controlled in experiments by appropriately driving a piezoelectric sleeve surrounding a microcapillary tube, and the interplay between them is elucidated through computation.

Imaging of fluorescent yield and lifetime from multiply scattered light reemitted from random media.

The feasibility of employing fluorescent contrast agents to perform optical imaging in tissues and other scattering media has been examined through computational studies. Fluorescence lifetime and yield can give crucial information about local metabolite concentrations or environmental conditions within tissues. This information can be employed toward disease detection, diagnosis, and treatment if noninvasively quantitated from reemitted optical signals. However, the problem of inverse image reconstruction of fluorescence yield and lifetime is complicated because of the highly scattering nature of the tissue. Here a light propagation model employing the diffusion equation is used to account for the scattering of both the excitation and fluorescent light. Simulated measurements of frequency-domain parameters of fluorescent modulated ac amplitude and phase lag are used as inputs to an inverse image-reconstruction algorithm, which employs the diffusion model to predict frequency-domain measurements resulting from a modulated input at the phantom periphery. In the inverse image-reconstruction algorithm, a Newton-Raphson technique combined with a Marquardt algorithm is employed to converge on the fluorescent properties within the medium. The successful reconstruction of both the fluorescence yield and lifetime in the case of a heterogeneous fluorophore distribution within a scattering medium has been demonstrated without a priori information or without the necessity of obtaining absence images.

Satellite drops: Unexpected dynamics and change of scaling during pinch-off

During drop formation from a tube, a thin liquid thread—the precursor to satellites—connects an about-to-form primary drop to the remainder of the liquid hanging from the tube at the incipience of breakup. Whether the thread, once it detaches from the primary and pendant drops, evolves into a sphere or breaks into several subsatellites has heretofore been inadequately explored due to experimental and theoretical difficulties. These challenges are resolved here with an ultrafast digital imaging system and a novel computational algorithm. New findings range from the discovery of unexpected dynamics to the first demonstration of the transition from one scaling law governing interface rupture to another.

Solvent exchange method: a novel microencapsulation technique using dual microdispensers.

AbstractPurpose. A new microencapsulation method called the “solvent exchange method” was developed using a dual microdispenser system. The objective of this research is to demonstrate the new method and understand how the microcapsule size is controlled by different instrumental parameters. Methods. The solvent exchange method was carried out using a dual microdispenser system consisting of two ink-jet nozzles. Reservoir-type microcapsules were generated by collision of microdrops of an aqueous and a polymer solution and subsequent formation of polymer films at the interface between the two solutions. The prepared microcapsules were characterized by microscopic methods. Results. The ink-jet nozzles produced drops of different sizes with high accuracy according to orifice size of a nozzle, flow rate of the jetted solutions, and forcing frequency of the piezoelectric transducers. In an individual microcapsule, an aqueous core was surrounded by a thin polymer membrane; thus, the size of the collected microcapsules was equivalent to that of single drops. Conclusions. The solvent exchange method based on a dual microdispenser system produces reservoir-type microcapsules in a homogeneous and predictable manner. Given the unique geometry of the microcapsules and mildness of the encapsulation process, this method is expected to provide a useful alternative to existing techniques in protein microencapsulation.

Use of phosphorescent and fluorescent dyes for lifetime-based imaging within tissues

The use of fluorescent and phosphorescent dyes has long been established in microscopy and bio-assays for rapid and accurate biochemical spectroscopy. In this work, we explore extension of these dyes for in vivo tissue spectroscopy. Using a theoretical model for the time-dependent light propagation through scattering media, we investigate the coupling of light propagation and re-emission kinetics for florescent and phosphorescent dyes delivered deep within tissues. Our results show that the use of phosphorescent dyes can provide spectroscopy or localization of specific tissue compartments, but may be information-poor for localized spectroscopy or imaging. While re-emitted fluorescence signals contain contributions owing to both light scattering and kinetics, they do contain the necessary information for localized spectroscopy.