Abstract
Open microfluidics is an emerging field of bio/medical applications that need direct energy/matter exchange between microfluids and environment. This paper presents the design, simulation, fabrication, and test of a microfluidic chip for a water-in-oil (WiO) two-phase flow in closed-open-closed microchannels. The chip, fabricated from PDMS using soft lithography, consists of a flow-focusing structure for WiO droplet generation and a long closed-open-closed channel for droplet flow. A negative pressure applied to the end of the channel is used as the driving force for WiO droplets to flow through the open channel. It is found that the negative pressure that is capable of driving a steady flow for a given flow rate, without overflow and air suction, falls into a pressure range instead of being an exact value. The mechanism for the pressure range is investigated theoretically and experimentally and is attributed to the surface tension. Yeast cells have been incorporated in the droplets, and the successful flow through the open channels verifies the function of the chips.
Highlights
Open microfluidics, in which fluids flow in a microchannel with at least a part of the sealing walls being absent, is an emerging area in biological and medical applications for direct exchange of energy or matter between fluids and environment.1,2 As open microfluidics provides the accessibility to fluids during flowing, a device-to-world interface can be achieved in dynamic modes, and direct fluid treatment or manipulation that is impossible in conventional sealed microfluidics can be performed
An open microfluidic chip consisting of a droplet generator and closed-open-closed channels has been designed, fabricated, and characterized for a water-in-oil two-phase flow
By applying a negative pressure to the outlet of the channel end, a steady two-phase flow has been successfully obtained without an overflow at the open channel
Summary
In which fluids flow in a microchannel with at least a part of the sealing walls being absent, is an emerging area in biological and medical applications for direct exchange of energy or matter between fluids and environment. As open microfluidics provides the accessibility to fluids during flowing, a device-to-world interface can be achieved in dynamic modes, and direct fluid treatment or manipulation that is impossible in conventional sealed microfluidics can be performed. Besides direct energy/matter exchange, the open area makes it easy to remove detrimental air bubbles that are inevitable in sealed microfluidics, which are difficult to predict and hard to remove.. Besides direct energy/matter exchange, the open area makes it easy to remove detrimental air bubbles that are inevitable in sealed microfluidics, which are difficult to predict and hard to remove.5 Such unique features make open microfluidics an enabling technology in a wide range of applications such as cell culture, rare cell isolation, cell signal studying, metabolomics, etc. The third challenge in the implementation of open channels is the fast evaporation of liquids at the open areas because of the large surface-to-volume ratio at the air-liquid interface, in particular, for aqueous solutions that are indispensable for cell culture.. This device demonstrates the following features for open microfluidics. (1) A negative pressure is applied to the outlet of the channel such that a high flow speed, steady and continuous flow, and good tolerance to liquid properties (such as wettability) can be achieved. (2) By using flow focusing, WiO two-phase flows have been implemented, which use nonvolatile oil that has low evaporation at the open area to protect the volatile aqueous liquid
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