Parallel Generation of Monodisperse Water Droplets in Hexadecane

By AZoNano

Table of Contents

Introduction
System Configuration
Test Performance
Droplet Size and Rate Estimation
Analysis
Conclusion
About Dolomite

Introduction

The performance of a high-throughput droplet generation system is described in this application note. High density microfluidic connections and advanced fabrication processes enable the design of a chip with 6 flow focusing junctions in parallel that can attain generation rates of up to 30kHz in test conditions.

The system is based on a standard Dolomite chip with a series of parallel droplet junctions producing water-in-hexadecane droplets. Flow rates of both fluids are controlled with Dolomite P-pumps that provide pulse-less flow. The resulting frequency and droplet size is varied in order to explore the system’s operating boundary. Droplet generation system applications include but are not restricted to food and drug manufacturing. Emulsion product volumes can be attained through 'numbering up' strategies which allow monodisperse droplet production at uncompromised rates.

The method shown here makes use of six junctions in parallel all etched on one chip. The concept that is successfully demonstrated here ensures a higher junction density per chip, which is restricted only by the physical layout requirement of the microfluidic architecture.

Key features include:

  • Monodisperse droplets
  • Up to 30 kHz droplet generation frequency
  • Droplet diameters in the range of 20 µm to 60µm
  • More than 600 emulsion production capacity per 24 h period

System Configuration

The system configuration is done in the following manner:

  • The 6 junction hydrophobic droplet chip (part no. 3200289) is used with a multiflux-2 linear connector 7-way (part no. 3200148) and a multiflux-2 double top interface (part no. 3200296) to interface the fluidic connection between tubing and chip.

  • Unused ports are blocked on the multiflex connector with a PTFE plug 0.8 mm (part no. 3200305).

  • Fluid is equally distributed into all six inlet ports from a single pump source through a double flow splitting manifold 7-way (Part No. 3200308). In the experimental set-up shown, the two Mitos P-Pumps (Part No. 3200016) deliver hexadecane and water streams to the parallel droplet chip.

  • The hexadecane flow rate is recorded by a flow sensor interface, (Part No. 3200200) which is equipped with a sensor capable of 30-1000 µL/min measurement range (Part No. 3200097).

  • Flow sensor interface (Part No. 3200200) equipped with a sensor capable of 1-50 µL measurement range (Part No. 3200098) records the water flow rate. FEP tubing (Part No. 3200302) and PEEK tubing (Part No. 3200303) are used to deliver fluids across the system.

  • The appropriate selection of flow resistors helps the droplet system to accommodate fluids with a wide range of viscosity. Suitable tubing lengths of tubing are cut as flow resistors and connected in the setup as shown in the block diagram below. The resistances are selected based on calculations performed using fluid properties of Hexadecane (with 1% SPAN 80) and water.

  • With a change of fluids, the tubing is easily modified to change the flow resistance, thereby enabling quick parametric variation.

Figure 1. Schematic of system configuration

Test Performance

It is possible to generate a wide range of droplet sizes at frequencies up to 30 kHz. The flow ratio that dictates performance is controlled with P-Pumps. The change in the relative pressure causes a change in the relative flow yielding consequently smaller or larger objects. Generally, higher total flows generate higher droplet generation rates. Backflow is caused by the increase in pressure on the hexadecane pump. Droplet coalescence marks the other extreme.

Droplets are generated simultaneously by six separate junctions on the chip. The flow splitting manifolds ensure consistent fluid pressure and flow rates across all six junctions. The graph shows the size variation over a pressure range of 0-5 bar on both carrier and droplet phase.

At very low pressures very large droplets are possible. At high pressures, there is an increase in the generation frequency however this comes at the cost of reduced range of droplet sizes. It is possible to vary the vapor pressure from very low to very high by maintaining a fixed hexadecane carrier pressure.

Larger or smaller droplets are restricted to lower frequency. The band of achievable droplet sizes is also based strongly on the operating frequency. This band narrows with progressively higher generation rates. Jetting phenomena or unstable droplets take place at conditions outside the operating boundary depicted by the grey curve.

Figure 2. Droplet sizes generated at varying pressures of water and hexadecane

Droplet Size and Rate Estimation

Pixel analysis is performed on the captured images to estimate frequency and droplet size. The droplet size with respect to the absolute size of the 150 Mm wide channel provides the droplet volume. The flow rates are divided by the droplet volume to obtain the droplet generation rate. Flow rates are directly recorded from the flow sensor reading.

Figure 3. Comparison of droplet sizes across parallel junctions

Analysis

Test images illustrate at least three different flow regimes dynamically that are accurately controlled with changing fluid supply pressures.

  • Droplet regime - This takes place at lower flow rates and results in monodisperse droplet formation.
  • Chaotic - This occurs when the water flow rate is significantly higher than the hexadecane flow rate. Droplet size is generally polydisperse.
  • Backflow - This is characterized by the hexadecane stream flowing back into the water feed channel. This occurs when the backpressure generated in the output channel and output pipe is greater than the pressure set on the water Mitos P-Pump. To avoid backflow the resistance of the flow resistor on the water input stream should be increased.

Conclusion

The parallel droplet generation system is shown to be a powerful and versatile system to generate monodisperse droplets at an extremely high rate. Test conditions generating water droplets in Hexadecane were able to produce droplet sizes varying from 20 µm to 60 µm diameter and frequency ranges of up to 30 kHz. It can produce a a total emulsion volume of approximately 688 ml (droplet phase = 76 ml, carrier phase = 612 ml) over a 24 h period at stable peak performance.

Operating characteristics of the setup are presented that make use of P-Pumps and flow sensors. The P-Pump pressure is used to tune droplet size. Monodisperse droplet generation takes place over a wide flow rate range of approximately 20 to 450ul/min, with a range of flow rate ratio of 2 to 200 (continuous phase flow/droplet phase flow). Operating characteristics presented are dependent on fluid properties. A change in fluid viscosity directly affects generation frequency. Viscous fluids such as alginates, polymers and hydrogels may result in comparatively lower production rates.

About Dolomite

Microfluidics, also known as “lab-on-a-chip”, enables small scale fluid control and analysis, and is an emerging technology that is changing the future of instrument design. Connecting microfluidic devices to macro-scale systems presents many challenges. To help ensure success, Dolomite provides several microfluidic solutions including chips, pumps, flow sensors and other microfluidic accessories.

In addition to the wide range of standard components, Dolomite also offers the design, development and manufacturing of bespoke solutions, including custom devices, turnkey solutions and fully automated systems.

By combining specialist glass, quartz and ceramic technologies with knowledge of high performance microfluidics, Dolomite is able to provide solutions for a broad range of industries enabling manufacturers to develop more compact, cost-effective and powerful instruments.

This information has been sourced, reviewed and adapted from materials provided by Dolomite.

For more information on this source, please visit Dolomite.

Date Added: Jun 3, 2013 | Updated: Jun 11, 2013
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