Micro-droplet Generation with Dilute Surfactant Concentration in a

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Micro-droplet Generation with Dilute Surfactant Concentration in a Modified T-junction Device Yankai Li, Kai Wang, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02588 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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Industrial & Engineering Chemistry Research

Micro-droplet Generation with Dilute Surfactant Concentration in a Modified T-junction Device Yankai Li, Kai Wang, Guangsheng Luo* The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China

Abstract Emulsions with very low surfactant concentration or without surfactant are highly required in many fields. Microfluidics have proven to be a promising emulsion preparation method. However, for the fresh interface formation during microfluidic droplet generation, excessive amounts of surfactant are also demanded to speed up surfactant transfer and drive low interfacial tensions. Here we present a newly designed capillary embedded T-junction micro-device to intensify surfactant mass transfer for micro-droplet generation. Within the device, surfactant molecules are concentrated onto the tip of droplet break-up very fast by hydrodynamic forces, leading to extremely low dynamic IFTs temporarily. Micro-droplets as small as 15μm are generated with surfactant concentrations significantly lower than their CMC values, when the concentrations of SDS, Tween 20, and F68 are only 0.05wt% (1/6 CMC), 0.20wt% (4/7 CMC), and 0.20wt% (2/3 CMC) individually. The novel method would be promising in fields of biocompatible material, drug delivery and food-grade

Corresponding author. Email: [email protected]. Tel.: +86-10-62783870.

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micro-emulsions.

Key words: microfluidics, dilute concentration, surfactant, micro-droplet, dynamic interfacial tension

Introduction

Monodispersed micron droplets provide confined spaces for biological analysis,1, 2 chemical reactions3 and material preparations.4 Small droplet volumes down to pL reduce the amount of samples, speed up reactions and thus hasten the development of techniques like next-generation sequencing,5 high-throughput screening6 and multi-step synthesis.7 Conventional emulsification methods like rotor-stator homogenizers have been used for droplet production, but the distribution of droplet size is relatively wide. Currently microfluidics provide a new pathway for droplet production. This drop-by-drop method could generate small droplets with extremely high monodispersity, presenting huge potential for food, chemical and biological industries.8

However, microfluidic one-by-one droplet generation always involves dynamic interface formation, inducing surfactant mass transfer as well as dynamic interfacial tensions (IFTs).9-14 As small micro-droplets could only be obtained with low dynamic IFTs, surfactant transfer onto freshly created interface often limits the decrease of IFTs and thus droplet sizes. Excessive amounts of surfactant would thus be added to


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speed up surfactant transfer rate. Researchers have shown that for droplet production with hundreds of micrometers, solutions with surfactant concentration of 1.5 (sodium dodecyl sulfate, SDS) to 400 (polyoxyethylene 20 sorbitan monolaurate, Tween20) times of critical micelle concentrations (CMCs) should be prepared to eliminate dynamic interfacial tensions.11 And if the droplet formation time is very short, the surfactant concentration would even reach 30 (SDS) times of CMC.13 However, over adsorbed surfactants often come with acute toxicity like respiratory distress, renal failure as well as electrolyte imbalances and they burden industry with significant separation cost.15-17 Hence the amounts of surfactants are strictly limited. The shortages hinder the microfluidic droplet generation method from entering business, especially in food and biological industries. 18-20

Therefore, surfactant mass transfer with microfluidic emulsification needs to be intensified and obtain micron droplets with as less surfactants as possible. Even though techniques like microfluidic active droplet generation21, 22 and micromolds20, 23, 24

have been applied to micron droplet formation at low/zero surfactant concentration,

such methods are still tedious and inefficient. The inspiration for surfactant mass transfer intensification in this work derived from a well-known surfactant dynamic interfacial phenomena of tip-streaming.9,

10, 25-27

In condition of tip-streaming,

surfactant molecules would first be adsorbed on interface and be swept to the tip of droplet formation point by flow induced surface movement. Surfactants would then accumulate and drive transient interfacial tension to decrease at the tip. In some cases,



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dynamic interfacial tensions could even decrease to extremely low values at below CMC concentrations by hydrodynamic forces, for the transient surfactant interfacial coverage in the presence of flows is higher than the static surfactant coverage, of which the upper limit is determined by CMC.10, 25, 26 Thus, micron droplets would be produced with intensified surfactant mass transfer. Nevertheless, as former microchannels for tip-streaming were always axisymmetric, it would be difficult to form stable interface with low surfactant concentrations. Thus droplet generation with tip-streaming has always been constrained within a narrow operating range, and the droplet size distribution is relatively wide.

In this work, we realized the surfactant transfer intensification driven by hydrodynamic forces within a newly designed capillary embedded T-junction microfluidic device. In our former works, the T-junction micro-devices have been developed for droplet/bubble generation.28-31 Here the micro-device was applied to liquid/liquid (L/L) dispersions with different surfactants (sodium dodecyl sulfate (SDS), polyoxyethylene20 sorbitan monolaurate (Tween 20) and Pluronic F68 (F68)). We would first study the static interfacial properties with three surfactants. Followed are two phase flow regimes, droplet size variations and droplet size scaling law with SDS as surfactants. Droplet size variations and scaling laws with Tween 20 and F68 as surfactants were compared later and a general scaling law was presented then.

Experimental Section


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As shown in Figure 1A, the step T-junction devices were fabricated on polymethyl methacrylate (PMMA) chips using end mills. The width (W) and the height of the main channel are both 750 µm. A quartz capillary with 530 µm inner diameter (w) and 740 µm outer diameter was embedded in the side channel and extended into the main channel. The devices were sealed using another PMMA chip and cured at 75 °C for 3 min using a high-pressure thermal sealing technique. The distance of the capillary to the opposite side, h, was fixed at 95 µm in this work. Figure 1B presents the mechanism of dynamic surfactant concentrating process within the device.

Figure 1 (A) Capillary embedded step T-junction microchannel. (B) Mechanism of dynamic surfactant concentrating process. It should be noted that there is a dead zone at the upstream of the embedded capillary.

n-hexane and 50% glycerol aqueous solutions with surfactants were used as the dispersed phases (DP) and the continuous phase (CP) respectively. SDS (MW=288, ionic surfactant, Sigma, USA), Tween 20 (MW=1226, nonionic surfactant, Guangming, China), or F68 (MW=7980, nonionic surfactant, BASF, Germany) were added into CP. The interfacial tensions (IFTs) at 25 were measured with a pendent



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drop interfacial tension meter (OCAH200, DataPhysics Instruments GmbH, Germany), and the results were plotted in Figure 2. The viscosities of the glycerol solution and n-hexane were measured with a rotary viscosimeter (BROOKFIELD DV-II+P, USA) as 6.92 mPas and 0.37 mPas at 25. Langmuir-Szyszkowski isothermal equations (eqn.1) and CMCs of the three surfactants could be then obatained, as listed in Table 1. γ = γ − Γ ln + 1 (1)

where γ is the interfacial tension of the pure component, R is the thermodynamic constant number, T is the Kelvin temperature, Γ is the maximum surfactant interfacial adsorption density, a is the Langmuir parameter and c is the surfactant concentration in the bulk phase. Table 1 Langmuir-Szyszkowski parameters and properties of surfactants (/)

! ("#/$ )

%& ! (/)

'("#/()

)*) (+,%)

SDS

28.64

2.60 × 1078

6.69

3.2 × 107;

0.30

Tween20

28.64

1.60 × 1078

3.97

5.2 × 1078

0.35

F68

28.64

7.35 × 107B

1.82

1.7 × 107C

0.30


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Figure 2 (A)-(C) present IFT variations and CMCs at differing surfactant molar concentrations. Fitted curves by Langmuir-Szyszkowski eqn. are also shown. (D) presents IFT variations at differing surfactant mass concentrations.

It should be noted that the Langmuir-Szyszkowski isothermal equations would even predict unrealistic surfactant behaviors. Such as, when surfactant concentrations exceed certain values the IFTs could reach zero, as shown the fitted curves in Figure 2. It could not be realized when a static condition is applied, for the concentration of surfactant monomer remains fixed when bulk surfactant concentration is higher than CMC. But in the presence of flows, surfactant molecules are swept toward the tip of droplet break-up point and the surface coverage would be possible to exceed the values of CMC by hydrodynamic forces (Figure 1B).25,


32

Transient interfacial


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tensions would thus decrease to near zero.

The feed flow rates of DP were controlled from 1 to 10 µL/min, while those of CP were from 100-6000 µL/min. After changing any of the flow parameters, we allowed at least 1 min of equilibration time. The apparent average diameters (dE ) were determined by measuring at least 25 droplets. The microdispersion process was captured by a microscope (BX51, Olympus) and a high-speed CMOS camera (i-SPEED TR, Olympus, capture frame frequencies of 1000-7000 fps). The microchannels were first immersed in each surfactant solutions for at least 3 h to ensure complete adsorption of the surfactant on the channel before experimental.

Results and discussion

Flow regimes, droplet size variations and scaling law with SDS as surfactants In this section, we would present two phase flow regimes at varying surfactant concentrations, in which we take SDS as an example. Droplet size variations with different flow regimes and scaling law in jetting flow with SDS would be analyzed at the same time.

Three flow regimes could be observed within the modified T-junction device at differing CP volume flow rates (Figure 3A-C): squeezing, dripping and jetting. And the transitions between different flow regimes are mainly determined by CP flow rates as well as surfactant concentrations.


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Figure 3 (A)-(C) present squeezing, dripping and jetting flows respectively and all the CP was added with 0.05wt% SDS. Monodispersed droplets with CV

Micro-droplet Generation with Dilute Surfactant Concentration in a

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