Characterization of Spontaneous Transformation-Based Droplet Formation during Microchannel Emulsification

Abstract

Recently, we proposed a microchannel (MC) emulsification technique, which is a novel method for making a monodispersed emulsion from a microfabricated channel array. The droplet formation mechanism for MC emulsification is a unique one, in which the distorted dispersed phase is spontaneously transformed into spherical droplets by interfacial tension. The objective of this study was to characterize the flow in MC emulsification. We investigated the emulsification behavior at different flow velocities of the dispersed phase using MCs with different sizes and analogous shapes and found a critical flow velocity over which the character of flow changed drastically. The formed droplet diameters were almost constant below the critical velocity, and monodispersed emulsions were formed. The droplet diameters increased drastically above the critical velocity, and polydispersed emulsions were formed. The critical velocities were independent of MC size. Analysis using a dimensionless number revealed that the critical point, at which the character of the flow of MC emulsification changes, is determined by the Capillary number (Ca), which is the ratio of viscous force to interfacial tension force. Below the critical Ca, the interfacial tension, which is the driving force for droplet formation, is dominant, and the flow in MC emulsification was based on spontaneous transformation. Above the critical Ca, viscous force is dominant, and the flow is similar to laminar flow. We experimentally confirmed this idea using the dispersed and continuous phases with different viscosities, MCs with different sizes, and different interfacial tensions. These results suggested a law of similarity in MC emulsification, and the transition of the flow was determined by Ca. The information obtained in this study includes the essential physics underlying spontaneous transformation flow in MC emulsification and is useful for practical applications.

Manufacture of large uniform droplets using rotating membrane emulsification

“…Therefore, in order to obtain uniform droplets at 350 rpm, the Carbomer content in the aqueous phase should exceed a critical value of 0.1 %. Under these conditions, the average droplet size is some 5-7 % larger than the pore size and the CV is slightly less than 5 %, which is similar to the CVs reported in emulsification using grooved-type silicon microchannels [5].…”

“…Several single-drop technologies have been developed for generating uniform droplets, such as injection of liquid through a capillary into another co-flowing immiscible fluid [3,4], penetration of dispersed phase through microfabricated parallel silicon channels [5] or interconnected channel network in microfluidic devices [6,7], and injection of dispersed phase through microporous membranes of different nature (glass, ceramic, metallic, polymeric) [8][9][10][11][12][13][14]. Production of various particulate products, such as microspheres and microcapsules, using membrane emulsification routes was recently reviewed by Vladisavljević and Williams [15].…”

Membrane emulsification: Droplet size and uniformity in the absence of surface shear

“…The increasing size, and widening, of the distribution occurs most probably because jet instability forms. Table 1 or Capillary number, after a threshold value, as has been found previously in microchannel emulsification [17]. In that work, a critical Capillary number of 0.052 was determined as the threshold between drop diameters remaining stable, as a function of injection rate, and beyond that threshold where average diameters were observed to increase with injection rate.…”

CFD analysis of microchannel emulsification: Droplet generation process and size effect of asymmetric straight flow-through microchannels

“…The (Sugiura et al 2002b). Here, ݀ ҧ drop and its variation were independent of d MC in the range of 100 µm or less (Fig.…”

“…The resultant droplet size is primarily determined by the dimensions of the MC array (Sugiura et al 2002a). The droplet size and its distribution are barely influenced by the velocity of each phase below a critical value (Sugiura et al 2002b;Kobayashi et al 2003), indicating that MC emulsification is useful for practical production of monodisperse emulsions. Moreover, straight flow-through MC arrays are promising for mass production of uniform droplets, since numerous channels (e.g., 10 4 cm -2 ) can be compactly arranged on an MC emulsification device (Kobayashi et al 2002a;2005a).…”