Abstract
Dispensing and manipulation of small droplets is important in bioassays, chemical analysis and patterning of functional inks. So far, dispensing of small droplets has been achieved by squeezing the liquid out of a small orifice similar in size to the droplets. Here we report that instead of squeezing the liquid out, small droplets can also be dispensed advantageously from large orifices by draining the liquid out of a drop suspended from a nozzle. The droplet volume is adjustable from attolitre to microlitre. More importantly, the method can handle suspensions and liquids with viscosities as high as thousands mPa s markedly increasing the range of applicable liquids for controlled dispensing. Furthermore, the movement of the dispensed droplets is controllable by the direction and the strength of an electric field potentially allowing the use of the droplet for extracting analytes from small sample volume or placing a droplet onto a pre-patterned surface.
Highlights
Dispensing and manipulation of small droplets is important in bioassays, chemical analysis and patterning of functional inks
Besides the orifice-based techniques, small droplets can be produced by orifice-free techniques, such as droplets splitting based on surface-wetting properties[28,29], pyroelectrodynamicdriven techniques[30] and nanowire liquid pump techniques[31]
During the pinch-off process, the gravity does not play a significant role since the Bond number Bo 1⁄4 (r1 À r2)gRm2 /s12 is always smaller than 0.1. r1 À r2 is the density difference between the dispensed (1) and surrounding liquid (2), g is the acceleration due to gravity and s12 is the interfacial tension between the two liquids
Summary
Dispensing and manipulation of small droplets is important in bioassays, chemical analysis and patterning of functional inks. Small droplets have been produced by methods based on micro-orifices (or channels) or by orifice-free methods. Despite the effectiveness of the micro-orifice (or channel)-based droplet-producing devices, their typically confined geometry poses several challenges, such as high flow resistance and propensity to clogging. The currently existing micro-orifice (or channel)based techniques (for example, inkjet printing) still encounter problems to generate droplets that are substantially smaller than the orifice from which they are ejected[25]. Instead of squeezing the liquid out of the nozzles, the droplets are formed by sucking back the liquid from a sessile drop initially suspended from the nozzles In this way the size parameter w can be decreased to 0.025 enabling the dispensing of picolitre to microlitre volumes even from a millimetre-sized orifice that is rather immune against clogging when dispensing highly viscous liquids or suspensions. The use of orifices with diameters of several micrometres shows the capacity of dispensing sub-micrometre sized droplets
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