Snap-off production of monodisperse droplets

Abstract

Abstract. We introduce a novel technique to produce monodisperse droplets through the snap-off mechanism. The methodology is simple, versatile, and requires no specialized or expensive components. The droplets produced have polydispersity < 1% and can be as small as 2.5 µm radius. A convenient feature is that the droplet size is constant over a 100-fold change in flow rate, while at higher flows the droplet size can be continuously adjusted.Microfluidics applications often require emulsions with a wide range of characteristics, prompting the development of several distinct techniques for producing droplets [1,2]. One important parameter is the degree of polydispersity among droplet sizes, where smaller values are preferable for many applications. Droplets with an extremely low polydispersity are particularly desirable for basic science investigations of emulsions [3,4], vessels for tiny experiments [5,6,7], as well as calibration in both academic and industrial settings [8]. Here we present a method we have recently developed using glass capillaries and a surface tension driven 'snap-off' instability to produce droplets. This method is remarkable for its simplicity, ease of implementation, and the high monodispersity of droplets produced. An additional convenience is that there are two distinct regimes of droplet production: 1) the size of droplets is insensitive at low flow rates; while, 2) at high flow rates the droplet size is tunable.The snap-off instability of droplets in cylindrically symmetric capillaries was first described in 1970 [9], and has since been investigated further in the context of understanding the physics behind snap-off [10,11]. We took advantage of this effect to develop a versatile system for production of monodisperse droplets that is easy to assemble and operate. One important consideration is that this setup requires no flow of the continuous phase, since the pinch-off is driven by surface tension forces rather than viscous forces. Although the snap-off process has been used previously to produce droplets in flattened microfluidic geometries [12,13,14], our simple cylindrical configuration is able to produce droplets that are more monodisperse.In order to prepare monodisperse droplets we have utilised a method that is schematically depicted in Fig.

Predicting the size of droplets produced through Laplace pressure induced snap-off

“…1A. The dispersed phase forms a growing spherical droplet that becomes unstable at some critical size and subsequently snaps-off [9]. Figure 1B illustrates schematically the location and relative values of the relevant pressures, which we now describe.…”

“…The instability requires no viscous interaction, but is due entirely to changes in the curvature of the interface between the two phases, affecting the Laplace pressure. We have previously found that the resulting droplets are highly monodisperse [9], as the system becomes unstable immediately after the protrusion of the dispersed phase reaches a critical size. The snap-off phenomenon has been studied and used to produce droplets in many different geometries [4,5,[8][9][10][11].…”

“…We have previously found that the resulting droplets are highly monodisperse [9], as the system becomes unstable immediately after the protrusion of the dispersed phase reaches a critical size. The snap-off phenomenon has been studied and used to produce droplets in many different geometries [4,5,[8][9][10][11]. However, the case of ejecting the dispersed phase from a cylindrically symmetric tube is the simplest from a theoretical standpoint [6,9,12].…”

“…By neglecting pressure gradients within the dispersed phase, several groups have been able to provide a theoretical framework for droplet sizes in a quasi-static regime [5,8,9,12]. The approximation of a static dispersed phase has been shown to be valid in regimes that are useful for droplet production [2,5,9].…”

“…The approximation of a static dispersed phase has been shown to be valid in regimes that are useful for droplet production [2,5,9]. Additionally, it was shown that droplets grow larger before snapping off at higher flow rates, and eventually grow indefinitely for sufficiently large flow rates [6,8,9,13]. The ability to deliberately vary droplet size without changing the apparatus, for instance by changing the flow rate, represents a useful experimental control variable.…”