Session Z32: Micro/Nanofluidics II

Electro-coflow as a means to study whipping instabilities in electrified liquid jets

Whipping is a non-axisymmetric instability that appears in electrified jets. In air, it usually manifests in a chaotic fashion preventing its detailed experimental characterization. We use electro-coflow to generate a steady-state whipping structure and quantify its wave-like properties, which we understand from simple force balances.   

Do electroviscous effects impact the hydraulic conductance of xylem? A theoretical inquiry

Experiments show that the hydraulic conductance of plant xylem (K) varies with the ionic-strength (I) and pH of the sap, a behavior usually attributed to the swelling of hydrogels that cover bordered pits---conduits that interconnect individual xylem vessels. These gels are believed to swell at low I or large pH, and thus decrease the flow cross-section and K. But experiments have shown behaviors that contradict this hypothesis, where a decrease in I serves to increase K. Here, we investigate whether these observations could be explained by electroviscous effects in the pores of bordered pits, since the literature suggests that pits are covered by materials that develop electric charge in aqueous solution, e.g. lignin and pectin. We use experimental measurements from the literature, combined with standard electrokinetic theory, to estimate the electroviscous effect of I and pH on K. We find that K varies non-monotonically with I and can drop to a minimum of 0.8 of its maximum value, and that our predictions fit the available experimental data for physiologically relevant conditions in I and pH. We conclude that electrokinetics could explain, at least partially, the observed changes in K, and propose experiments to test this hypothesis.   

Microfluidic route to generation of celloidosomes

Here we present a microfluidic method to generate alginate particles with a liquid core and a shell with yeast cells encapsulated in it. This particular class of celloidosomes with cells embedded into the thin shell region at the surface, allows for easy access of oxygen to the cells improving their viability. The liquid core opens the possibility of encapsulating multiple types of cells into the core and the shell. The microfluidic method involving double emulsion technology employed here ensures robust control over the size of the particles and density of the encapsulated cells. The study has shown that the stability of the inner core is very much dependent on the viscosity of the oil used for collecting the emulsion.   

Dynamics assembly of magnetic microparticles suspended in moving droplets under the influence of magnetic fields

Droplet microfluidics has experienced tremendous growth, particularly since it is well suited for single-cell manipulation and analysis. As mature methods for high throughput droplet manipulation have been developed a technological bottleneck of current droplet microfluidics is that because droplets are separated, sequential chemical reactions are more difficult to achieve. For example, it is very difficult to concentrate target molecules, especially since every reaction step adds volume to the droplets. Our solution to this problem is to employ functionalized magnetic beads inside droplets. The basic idea is that an external magnetic field could be used to concentrate the magnetic beads in one part of the droplet and those could then be extracted by splitting the droplet. Here we present an experimental study of the self-assembly of superparamagnetic microparticles that are suspended in moving droplets and experience a combination of forces due to the internal fluid flow fields and external magnetic fields. We observed that this interplay of flow fields coupled to the formation of particle assemblies leads to the formations of stable patterns depending on the flow speed and magnetic field strength. An understanding of this dynamic assembly is critical in employing external forces for applications in separation and sorting.   

Droplet pairing and coalescence control for generation of combinatorial signals

A co-flowing aqueous phase with an immiscible oil phase in a microchannel generates uniformly spaced, monodisperse droplets, which retain their shape by not touching each other or by being stabilized with surfactants at the oil-water interface. However, droplet coalescence is required in many advanced applications, which can be achieved by a complex channel geometry or size differences in the droplets, and as well as by procedures to reduce the effect of a surfactant. These approaches, again, hinder the stability of droplets further downstream. We designed a microchannel which consistently inserts gas-bubble between droplets so that pairing and coalescence of droplets occurs even in the presence of surfactant, and yet prevents unwanted merging with other droplets. Aqueous droplets placed between the bubbles alter their relative speeds and spacing, and consequently we study the change in the number of droplet pairings in relation to the characteristics of the bubbles and the volume of aqueous droplets. By integrating this approach with droplets of different materials, we can program the output sequence of droplet compositions, and such complex combinatorial signals generated are aimed for concentration gradient generation and dynamic stimulation of biological cells with chemicals.   

Microfluidic Printing and Ablation of Metallic Films by Modulated Capillary and Maxwell Stresses

Liquid dosing strategies for micro/nanofluidic applications normally rely on interior flow driven by external pressure gradients. To maintain a constant flow rate, the effective pressure drop over a given length conduit must scale inversely as the fourth power in the conduit radius, as prescribed by the Hagen-Poiseuille relation. For micron or nanoscale capillaries, this constraint requires enormous pressure gradients and external control mechanisms. This burden, coupled with the likelihood of occlusions due to gas bubbles, contaminants or carrier particles, limits the usefulness of internal flow strategies for applications involving emission of charged droplets or ions. In this talk, we focus on capillary flow in slender V grooves as a more robust and self-regulating fluidic delivery system. When coupled with spatiotemporal modulation of Maxwell stresses induced by an external electric field, beams of droplets or ions can be metered reliably and effectively. Here we explore the steady state, transient and oscillatory flow characteristics of microscale metallic films in V-grooves subject to capillary and Maxwell stresses. The geometry investigated will focus on printing and ion ablation of thin films for electronic circuits and photovoltaic displays.