# Evolution of Water-in-Oil Droplets in T-Junction Microchannel by Micro-PIV
## 1 Introduction  
Over the years, microfluidic devices have been developed significantly for micro-electromechanical systems (MEMS), microchemical technology and micrototal analysis systems (\(\upmu\)TAS). They are also used to a great extent in biotechnology applications. Generally, multiphase flow, especially two-phase flow, occurs frequently in these applications [1]. For instance, liquid-liquid multiphase flow has numerous applications in the mixing process [2, 3], chemical reactions [4] and also emulsion technology [5, 6]. One important application of microfluidic devices is in the generation of monodisperse emulsions and particles that have precise size and composition [7, 8, 9]. They rely on the co-axial flow of these immiscible liquids and on breaking up the disperse phase into droplets suspended in the outer continuous liquid, along with the resulting shear forces through geometric constriction [10]. The common geometries used for the microfluidic droplet generation are flow-focusing, co-flow and cross-flow devices. For example, [PERSON], [PERSON] [11] used a polydimethylsiloxane (PDMS)-based microfluidic flow-focusing channel to investigate the size, gap and generation rate of oil-in-water droplets. They highlighted the abilities of asynchronous oil droplet generations, with the gap varying from a few microns to a few hundred microns in successive and rapid cycles. [PERSON], [PERSON] [12] studied the hydrodynamics of rising droplets, i.e., soybean oil and toluene in quiescent water, using a co-flowing microfluidicdevice. The results of the experimentations indicated the rigid sphere-like behaviour of a single micro-droplet in terms of its terminal velocity; however, the swarm of droplets moved with a higher terminal velocity compared to a single droplet. [PERSON], [PERSON] [13] investigated the effect of different viscosities of carrier oil on water-in-oil emulsion using a cross-flow T-junction microchannel. The results showed that regardless of the flow pressure levels, the droplet size and droplet generation rate decreased as the oil viscosity increased. Based on these three types of microfluidic droplet generation geometries, cross flow is the simplest and most commonly used geometry to generate droplets in a controlled way [14]. In addition, the offset T-junction microchannel is proven to have better performance for immiscible dissimilar liquids than the conventional T-junction microchannel [15]. Producing oil-in-water droplets is easier than developing water-in-oil droplets [16]. Suspended water droplets are significant in encapsulating the bioactive compounds for controlled release in fat-based edible products [17]. As in the droplet-based microfluidics experiment, [PERSON], [PERSON] [13] investigated the effect of different viscosities of carrier oil on water-in-oil emulsion using a T-junction microchannel. The results indicated that with the increase in oil viscosity, the size of the droplet decreased, and this occurred regardless of pressure level of flow. The micro particle image velocimetry (micro-PIV) method has been a widely known technique in the application of microfluidics [18; 19; 20; 21; 22]. Moreover, the droplet internal flow details in the microchannels have been characterized mainly by this technique due to its capability to obtain instantaneous velocity measurements and related properties in fluids [23; 24; 25; 26; 27; 28]. There are a number of studies involving the use of this method to study the formation of droplets. [PERSON], [PERSON] [28] studied the internal flow field of water-in-oil droplets traveling in a T-junction microchannel by means of the micro-PIV method. One important observation is the impact of the capillary number on the flow physics, including its critical value, while the geometry of the droplet impacts the axial as well as transverse velocity components. [PERSON], [PERSON] [29] measured and visualized the internal flow of a moving water/glycerol-in-oil droplet in the PDMS T-junction microchannel using a confocal micro-PIV system.