Glass-Based Devices to Generate Drops and Emulsions

Summary

Here, a protocol to manufacture glass-based microfluidic devices used for generating highly monodisperse emulsions with controlled drop size is presented.

Abstract

In this manuscript, three different step-by-step protocols to generate highly monodisperse emulsion drops using glass-based microfluidics are described. The first device is built for the generation of simple drops driven by gravity. The second device is designed to generate emulsion drops in a coflowing scheme. The third device is an extension of the coflowing device with the addition of a third liquid that acts as an electric ground, allowing the formation of electrified drops that subsequently discharge. In this setup, two of the three liquids have an appreciable electrical conductivity. The third liquid mediates between these two and is a dielectric. A voltage difference applied between the two conducting liquids creates an electric field that couples with hydrodynamic stresses of the coflowing liquids, affecting the jet and drop formation process. The addition of the electric field provides a path to generate smaller drops than in simple coflow devices and for generating particles and fibers with a wide range of sizes.

Introduction

Controlled generation of drops in the micron and nanoscale with a narrow size distribution is a challenging task. These drops are of interest for the engineering of soft materials with many applications in science and technology^1,2,3,4,5,6.

The most common devices for the high production rate of drops are mixers⁷ and ultrasound emulsificators⁸. These methods are simple and low cost, but they typica....

Protocol

1. Making simple drops

1. For making simple drops, use a glass base made with a microscope slide (76.2 mm x 25.4 mm) to build the device. This allows for easy transportation and visualization of the liquids through the glass.
2. Use a round glass capillary for the tip. For this protocol, use 1 mm diameter round capillaries (readily available in a wide range of sizes).
   1. To make a tip with the desired diameter, pull the capillary using a micropipette pulling machine until two half capillaries with a very small tip (~ 1 µm) are obtained.
   2. Use a microforge to cut the tip to the desired diameter (2-80 µm). For larger di....

Results

In this manuscript, three different devices have been designed to generate drops. We have generated drops with a size of (3.29 ± 0.08) mm (Figure 4B) and (1.75 ± 0.04) mm (Figure 4C) using the device described in step 1. The emulsion drops can be generated using the coflow and the electro-coflow devices. For the latter, we show dripping in Figure 9, while cone-jet and whipping modes are shown in Figure 10

Discussion

The protocol to fabricate three different glass-based devices has been described above. In the case of the device to generate simple drops, the flow rate and liquid properties are crucial to generate drops in a controlled manner. Drops will form at the tip in the dripping regime, or at the end of the jet in the jetting regime. The transition from dripping to jetting is parametrized by the dimensionless Weber number, We²³. This number represents the ratio between inertial and surface tension forces.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2-[methoxy(polyethyleneoxy)6-9propyl] trimethoxysilane.	Gelest	SIM6492.7
Ceramic tile	Sutter	CTS
Ethylene glycol	Fisher	BP230	These can be found at other companies like Sigma-Aldrich
Hexane	Sigma- Aldrich	34859	Available in other vendors
ITW Polymers Adhesives Devcon 5 Minute Epoxy Adhesive 25 mL Dev-Tube	Ellsworth adhesives	470740
Microforge	Narishige	MF 830
Micropipette puller	Sutter	P97
Microscope slides	Fisher	12-544-1	Available in other vendors
Needle 20 Gauge, .0255" ID, .0355" OD, 1/2" Long	McMaster	75165A677
SDS	Sigma-aldrich	428015	Surfactant
Silicone oil	Clearco	PSF-10cSt	The catalog number correspond to the 10cSt viscosity oil. Different viscosity oils can be found at this company
Span 80	Fisher	S0060500G	non-ionic surfactant
Square glass capillary 2mm ID (borosillicate 300 or 600 mm long)	VitroCom	S 102
Standard Glass Capillaries, 6 in., 2 / 1.12 OD/ID	World Precision instruments	1B200-6	These can be found at other companies like Sutter or Vitrocom
Syringe pump	Chemyx	FUSION 100-X	This model has a good quality/price ratio
Syringes (it will depend on the compatibility with the liquids)	Fisher		Catalog number will depend on the size
Trimethoxy(octyl)silane	Sigma- Aldrich	376221	Available in other vendors
Tubing ( it will depend on the compatibility with the liquids)	Scientific commodities	BB3165-PE/5	This reference is for polyethylene micro tubing. The size fits the needle size listed here