1. Introduction

Emulsion is a fine dispersion of small droplet made from two immiscible liquids, where one liquid phase is dispersed into another continuous flow of liquid phase in which both liquids are not soluble with one another [1]. The fundamental liquids combination for emulsion to form is rather simple, i.e., where the multiphase system usually have large liquids density difference (at least >100 kg/m<sup>3</sup> ) using the example of water and cooking oil; the larger the density difference, the better the emulsion can be formed. In addition, the multiphase system of the liquids selection can be classified into Newtonian fluid systems and non-Newtonian fluid systems. In the case of Newtonian fluids such as water, oil, glycerol and salt solutions with low molecular weight, due to the independent of fluid viscosity from shear rate and fluid

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

velocity flow, the shear stress in a steady, laminar flow condition is having a linear relationship with its shear rate. In contrast, non-Newtonian fluids, examples of which in daily life including chocolate, toothpaste, lubricating oils experience a non-linear relationship for its shear stress versus shear rate curve due to the fact that the fluid viscosity is a variable at any given flow condition, i.e., temperature and pressure. Such fluid properties lead to much differences in term of the emulsion productions thus far in all engineering approaches. Conventionally, emulsion is prepared using high-pressure homogenizers and colloid mill. These devices apply high mechanical shear force to break up the large emulsion into smaller ones that are subsequently stabilized by the use of emulsifier [2]. However, such method contributed to large size distribution of emulsions formed [3], leading to material loss as well as emulsion function efficiency issue. This dispersity issued was then resolved by integration of microfluidic technology that was firstly developed in 1950s [4]. Microfluidic technology is defined as a branch of fluid mechanics that focuses on the understanding, designing, fabrications and operations of system that convey liquids inside channels with two of the three geometry length scales in the order of microns [5]. Furthermore, with the length scale associated within the microchannel, the flow regime formed in a microfluidic channel will not develop into turbulent flow, enables fluid to be manipulated that will form emulsion with high monodispersity. The wide range of technology options from decades of microfluidic multiphase system developments has allowed emulsion to be generated and manipulated.

Approaches Breakup

Co-flowing streams

Cross-flowing streams

Elongational strained flows regimes

Flow regime description Diagrams

Microdroplets Advancement in Newtonian and Non-Newtonian Microfluidic Multiphase System

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Dripping Droplet forms at the exit of the capillary tube tip and propagates

Jetting Droplets pinch off from an extended thread downstream of the

Squeezing Dispersed threads block the outlet channel leads to dramatic

Dripping Dispersed thread does not touch the wall of the channel in the

Jetting Droplet breakup point moves progressively downstream of the

Squeezing The tip of the droplet phase effectively blocks the cross-section of

Jetting The tip of the droplet phase extends downstream of the orifice and

Threading Each droplet is formed a long and thin thread of the inner liquid

Reproduced from Ref. [7, 15–17] with permission from the Physics of Fluids, Physical Review Letter and Lab on Chip.

is dragged behind the droplet. This thread subsequently breaks up

small droplets are formed at its terminus.

into a group of tiny secondary droplets

entire droplet formation process.

Dripping Droplets are periodically formed in the orifice.

increase in the dynamic pressure upstream of the droplet, thus forcing the interface to neck and pinch off into a droplet.

downstream with the flow.

capillary tube tip.

outlet channel.

the orifice.

Table 1. Main approaches for droplet breakup in microchannel.

In this chapter, we aim to summarize the main technologies for emulsion formation non-Newtonian microfluidic multiphase system using Newtonian fluid as a comparison. The chapter will start with the review of fundamental two-phase flow in microfluidic followed by discussion of the fundamental flow physics of microdroplets translocation and breakup phenomena in microfluidics. The detailed differences between Newtonian and non-Newtonian flow system will be illustrated and compared. Emphasis will be placed on the advancement of emulsions formed, i.e. encapsulation and fission from single emulsion. Finally, we conclude with an outlook to the future of the field. This chapter is meant to familiarize readers who may be new to the field of microdroplets formation in Newtonian and non-Newtonian fluid systems, as well as those readers who are new to the field of microdroplets formation via encapsulation and fission approaches, and eventually bridge the knowledge gap between the two correlations, disciplinary fields.
