Stability of parallel flows in a microchannel after a T junction

In this work, the flow of immiscible fluids in microchannels is studied. Flow pattern diagrams obtained in microfluidic chips are presented. Monodisperse droplets or parallel flows are obtained depending on the flow rate values of the aqueous phase and the oil phase. Transition from droplet regime to parallel flows cannot be described in terms of capillary numbers. Using confocal microscopy and high speed imaging, it was shown that droplets are formed through a blocking-pinching mechanism ruled by flow rate conservation. Conditions for parallel flow stability are quantified.

Figure 1

Flow pattern diagram as a function of the aqueous phase flow rate $\left(Q_{\text{water}}\right)$ and of the oil phase flow rate $\left(Q_{\text{oil}}\right)$ in a $100\mu \mathrm{m}\times 100\mu \mathrm{m}$ microchannel. The aqueous phase is a mixture of water with SDS (2 CMC) $50\mathrm{wt}\%$ and of glycerine $50\mathrm{wt}\%$. The oil phase is hexadecane. The 엯 refer to droplets formed at the T junction (DTJ), the ◇ to parallel flows (PF), and the ● to parallel flows that break into droplets inside the channel (DC).

Figure 2

The aqueous phase is a mixture of water with SDS (2 CMC) $50\mathrm{wt}\%$ and glycerine $50\mathrm{wt}\%$. The thick lines correspond to the transition between DC and PF and the dotted lines to the transition between DTJ and DC. (a) Effect of the aspect ratio of the microchannel. The oil phase is hexadecane. The + refers to a $100\mu \mathrm{m}\times 100\mu \mathrm{m}$ channel and the 엯 to a $200\mu \mathrm{m}$ $\text{wide}\times 100\mu \mathrm{m}$ high channel. (b) Effect of the viscosity of the oil phase in a $100\mu \mathrm{m}\times 100\mu \mathrm{m}$ microchannel. The ▿ refers to a $20\mathrm{cP}$ silicone oil and the ◇ to a $100\mathrm{cP}$ silicone oil.

Figure 3

Cross section picture of parallel flow between hexadecane (O) and aqueous phase with rhodamine (W) in a $100\mu \mathrm{m}\times 100\mu \mathrm{m}$ microchannel. (a) Inside the funnel. (b) In the outlet channel.

Figure 4

Droplet formation mechanism in a channel. Example with hexadecane at $900\mu \mathrm{L}∕\mathrm{h}$ and water with SDS at $1200\mu \mathrm{L}∕\mathrm{h}$ in a $100\mu \mathrm{m}\times 100\mu \mathrm{m}$ channel. (a) The tip of the jet begins to expand in the channel, (b) the tip clogs the channel, (c) oil phase starts pinching the water jet, and (d) just before the drop formation. Time period is $3.1\mathrm{ms}$.

Figure 5

Capillary number at the transition DC-PF as a function of the water flow rate. ● corresponds to the SDS solution in water at CMC with hexadecane in a $100\mu \mathrm{m}\times 200\mu \mathrm{m}$ PDMS microchannel. +, ▿, and ◇ are obtained in a $100\mu \mathrm{m}\times 100\mu \mathrm{m}$ PDMS-glass channel with water glycerine and hexadecane for +, $20\mathrm{cP}$ silicone oil for ▿, and $100\mathrm{cP}$ silicone oil for ◇. 엯 corresponds to a water-glycerine solution with hexadecane in a $200\mu \mathrm{m}$ $\text{wide}\times 100\mu \mathrm{m}$ high PDMS-glass channel.

Figure 6

Schema of the cross section inside the jet and the tip before the pinching.

Figure 7

Lines correspond to theoretical DC-PF transition. ● correspond to SDS solution in water at CMC with hexadecane in a $100\mu \mathrm{m}\times 200\mu \mathrm{m}$ PDMS microchannel. +, ▿, and ◇ are obtained in a $100\mu \mathrm{m}\times 100\mu \mathrm{m}$ PDMS-glass channel with water, glycerine, and hexadecane for +, $20\mathrm{cP}$ silicone oil for ▿ and $100\mathrm{cP}$ silicone oil for ◇. 엯 corresponds, to water glycerine solution with hexadecane in a $200\mu \mathrm{m}$ $\text{wide}\times 100\mu \mathrm{m}$ high PDMS-glass channel. $\theta =45°$ for glass-PDMS channel and $\theta =0°$ for PDMS channel.