(491a) Nanofluid-Induced Droplet Pinch-Off during Liquid-Liquid Flow in Mesoscale

1. Introduction

The advent of miniaturized devices has revolutionized process intensification. These devices are designed to maximize surface area, ensuring efficient mass transfer, effective mixing, minimized waste generation, and easy scalability. Miniaturization for process intensification is applicable to exothermic liquid-liquid reactions, fast or instantaneous reactions, etc. Accordingly, several studies are reported for biphasic liquid flow in reduced dimensions. However, the majority of the studies [1] are reported in microchannels. Biphasic flow in mesoscale has not received much attention where mesoscale is quantified [2] as 0.57 < Co < 4.46 (Co, confinement number= 1/D√σ_i/gΔρ, and Δρ, D, σ_i and g are difference in liquid densities, conduit diameter, interfacial tension and gravitational acceleration respectively). Further, previous researchers have shown that reduced dimensions offer a controlled environment for continuous production of droplets in flow system. Droplet formation in a controlled and reproducible manner is relevant for material science applications, e.g., in pharmaceutical, fine chemicals and food industries. Most of these applications require uniform droplet size for consistent and predictable results. Monodispersed droplets are also essential to improve productivity and product quality. Till date, the past studies have reported formation of micro droplets in microchannels. In industrial applications, achieving uniform droplet sizes at higher throughput remains a challenge. Furthermore, elongated droplets (length ≥ twice the conduit diameter) are desirable for enhancing the rate of transport processes due to increased interfacial area and internal convection.

With these considerations, the present study proposes a unique technique to produce elongated monodispersed droplets at higher throughput during biphasic liquid flow in the mesoscale. The technique uses an aqueous nanofluid (NF) to pinch-off droplets of the immiscible organic liquid during biphasic liquid flow in a 2 mm diameter glass conduit. The droplets are produced without contaminating the organic phase.

2. Experimental setup

The nanofluid (NF) comprises of Al₂O₃ nanoparticles (20-50nm), sodium dodecyl sulfate (surfactant), and de-ionized water. Two nanofluids NF-1 and NF-2 are prepared with 0.01 vol% and 0.02 vol% Al₂O₃ respectively. The surfactant proportion is same (0.4 wt.%) in the two cases. The NF (carrier phase) is introduced with toluene, colored red with iodine (dispersed phase) in a 2mm diameter, 1m long transparent vertical tube by programmable dual syringe pumps. The flow rates of the two fluids are varied from 1 ml/min to 80 ml/min. Initially, the flow rate of the carrier phase is kept constant at 1ml/min and the flow of the dispersed phase is gradually increased from 1 to 80ml/min. The process is then repeated at higher flow rates of the carrier phase. The visualization studies are performed by a high-speed camera (Nikon Micro LC 320S) at the entry to understand the underlying mechanisms of droplet pinch-off and at 40 cm from the entry, where the flow is fully developed. Additional experiments are performed with water-toluene (W-T) and surfactant-toluene (S-T) to understand the influence of surfactant and nanoparticles on the flow distribution.

3. Results and discussions

The flow patterns observed are droplet, inverted droplet, elongated droplet, thread, and transition between thread and elongated droplet flow (Figure 1). During droplet flow, the organic phase is dispersed in the carrier phase, while the reverse occurs during inverted droplet flow. Elongated droplets are characterised by L_d ≥ 2(conduit diameter), where the average droplet length is estimated from 50 droplets under the same flow conditions. Our observations at the entry section further reveal that the droplets are generated by squeezing, dripping, and jetting mechanisms. Squeezing occurs at low phase flow rates while dripping occurs at high aqueous and low organic flow rates. The most predominant mechanism is jetting. It occurs at high phase flow rates.

The range of existence of the different flow patterns is presented as flow pattern maps in Figure 2 for a) water(W)-toluene(T), b) NF1-T and c) NF2-T. Droplet, elongated droplet, inverted droplet, thread flow and transition are represented by Δ, ο, *, □ and x respectively. The mechanism of droplet generation is also superimposed in the figures and denoted by magenta (squeezing), yellow (dripping) and cyan (jetting). Additionally, the range of monodispersity is displayed by a red curve in all the figures. The experiments reveal -

1. Nanofluids enhance the range of elongated droplet flow and promote inverted droplet flow in lieu of thread flow.
2. An increased concentration of nanoparticles at a constant proportion of surfactant (0.4 wt%) in NF increase range of elongated droplet flow. A further increase in nanoparticle and/or SDS concentration reduces the range (not shown in the figure).
3. Nanofluids enhance the squeezing and jetting regime while decreasing the dripping regime. This is important as squeezing always produces monodispersed elongated droplets.
4. Interestingly, NFs produce a large range of monodispersed elongated droplets in the jetting regime There have been several attempts [3] to obtain monodispersed droplets in the jetting regime, since it predominates droplet formation over a wide range. The past researchers [4] have used different entry sections for this. In our study, nanofluid-induced droplet pinch-off produces monodispersed elongated droplets at high throughput without contaminating the dispersed phase.
5. Further, the higher range of monodispersity in Figure 2(c) compared to Figures 2(a) and 2(b) suggests that by tuning the proportion of nanoparticle concentration, monodispersed elongated droplets can be obtained over a wide range in the jetting regime.
6. In the domain of study, experiments with surfactant (S) solution (without NPs) and toluene reveal an enhanced range of transition and thread flow in S-T compared to the W-T (not shown in the figure).

In order to investigate the effect of aqueous and organic phase properties on flow distribution, we have measured density, viscosity and surface tension of the test fluids and the interfacial tension of the different fluid pairs (W-T, S-T, NF1-T and NF2-T). Table 1 indicates that NFs have practically the same density and viscosity as water. Therefore, changes in flow morphology can be associated with change of interfacial tension. Introducing nanoparticles to the surfactant solution reduces interfacial tension to some extent, but not below that of surfactant solution. Additionally, interfacial tension of NF decreases with increase in nanoparticle concentration at a constant proportion of surfactant. This suggests that the observed flow phenomena cannot be solely explained by changes in interfacial tension. Previous studies [5] have reported that nanoparticles and surfactants form a complex structure (NPS), which settles at the interface. We show this to be responsible for the production of monodispersed elongated droplets over a wide range of flow conditions in the present study.

4. Conclusion

The study offers valuable insights into the influence of nanoparticles on the flow distribution in biphasic liquid flow at the mesoscale. The salient conclusions are

1. Nanofluids induce monodispersed elongated droplet flow in the mesodomain, which offers higher throughput.
2. The extent of elongated droplet flow depends on the concentration of nanoparticles and SDS in the nanofluids.
3. Nanofluids enhance the range of squeezing and jetting while simultaneously reducing droplet formation by the dripping mechanism.

5. References

1. Al-Azzawi, Marwah, Farouq Sabri Mjalli, Afzal Husain and Muthanna H. Al-Dahhan. “A Review on the Hydrodynamics of the Liquid-Liquid Two-Phase Flow in the Microchannels.” Industrial & Engineering Chemistry Research, (2021) 60, 14, 5049-5075, https://doi.org/10.1021/acs.iecr.0c05858

2. M. Li, B.X. Wang, Size effect on two-phase regime for condensation in micro/mini tubes, Heat Transf. - Asian Res. 32 (2003) 65–71. https://doi.org/10.1002/htj.10076

3. Da Ling, J. Zhang, Z. Chen, W. Ma, Y. Du, J. Xu, Generation of monodisperse micro-droplets within the stable narrowing jetting regime: effects of viscosity and interfacial tension, Microfluid. Nanofluidics 26, 53 (2022), https://doi.org/10.1007/s10404-022-02558-8

4. Opalski, Adam S., Karol Makuch, Ladislav Derzsi and Piotr Garstecki. “Split or slip – passive generation of monodisperse double emulsions with cores of varying viscosity in microfluidic tandem step emulsification system.” RSC Advances10 (2020): 23058 – 23065, https://doi.org/10.1039/D0RA03007D

5. Chai, J. Hasnain, K. Bahl, M. Wong, D. Li, P. Geissler, P.Y. Kim, Y. Jiang, P. Gu, S. Li, D. Lei, B.A. Helms, T.P. Russell, P.D. Ashby, Direct observation of nanoparticle-surfactant assembly and jamming at the water-oil interface, Sci. Adv.6,eabb8675(2020), https://doi.org/10.1126/sciadv.abb8675