Microfluidic generator of sub-10-micron hydrosomes Zhenghao Ding, Lunjun Liu, Gabriel C. Spalding* Physics Department, Illinois Wesleyan University Abstract Known as the fundamental “lab on a chip” technology, microfluidics is a thriving young research area. It is touted as reducing the amount of material required for biochemical analysis, such DNA sequencing. The proximity to walls within narrow flow channels overcomes any tendency towards turbulence, even in high-speed fluid flow, facilitating a more controllable, systematic processing. By leveraging lithographic techniques of miniaturization developed for the semiconductor industry, enormous capability can be integrated into a single microfluidic chip. We have utilized templates designed by students in our lab, for fabrication of a series of microfluidic chips that, at high speeds, can produce many droplets of nearly identical, microscopic dimensions. We aim to apply standard techniques to stabilize these droplets against coalescence, and will then optically trap and manipulate these droplets within the confines of the original microfluidic chip. Subsequently, we plan to incorporate entrained nanocomponents into each droplet generated (quantum dots at first, and, later on, DNA origami, or diamond microparticles containing Nitrogen vacancy defects). The collected droplets can be manipulated with much higher precision by optical traps than would be possible with direct trapping of the nanocomponents. The long-term potential of our research purposes seems significant (ranging from development of DNA-based information storage technologies to weak magnetic field detection for basic physics research). Making a microchip Microchip manufacture is the first and most critical step in this project. Our microchips contain tiny channels for processing liquids flows and any entrained matter. With different designs, we can split droplets, mix solvents, and even control chemical reactions at the micro-scale. Our designs are molded into a transparent silicone rubber (PDMS) that is not expensive and easy to buy online. Before curing, it is a viscous fluid. When mixed with a cross-linking agent and heated, the PDMS cures into a soft solid that is chemically inert. We use standard photolithography techniques developed for computer chips to transfer our designs onto silicon wafers (as shown at left). Then we pour the PDMS and its curing agent onto these molds, and proceed to cure the PDMS. In this way, our designed patterns are transferred to the bottom of the silicone rubber. After pealing the PDMS from the mold, we use a plasma machine (shown at right) to modify their surface properties. Treatment with an oxygen plasma makes the surface tacky, so that it can be bonded to standard microscope coverslips, creating a leak-tight seal for the fluidic channels designed into our microchips. Our last step is to punch holes on the inlet and outlet spots, and connect to syringe pumps. Forming droplets After connecting our microchip inlets and outlets to syringe pumps, we can control the flow of various liquids and solvents. Careful control of the relative flow speeds of aqueous streams surrounded by oil streams produces droplets of reproducible dimension. The size of the droplets is related to the size of the channel, the viscosity of the liquids and, critically, the relative flow speeds.