A study of extensional flow induced coalescence in microfluidic geometries with lateral channels

Abstract

The coupled mechanisms of extensional coalescence and subsequent shape relaxation can lead to catastrophic destabilization of moderately concentrated emulsions. We demonstrate that application of local extensional flow through the use of small lateral channels allows controlled, systematic investigation of both single drop pair and propagating (avalanche) coalescence through a chain of drops. Drop-drop collisions and separations were controlled independently, and did not significantly disturb the primary flow. The probability of the first coalescence event was controlled by bulk flow parameters, allowing for systematic investigation of these phenomena. Simulations with COMSOLÒ were used in order to quantify and thus validate various assumptions relating to the flow characteristics of our setup. For the configurations tested, the droplet pair separation speed increased linearly with the lateral channel infusion rate. Flows were laminar and collision conditions remained stable until a first coalescence event between a pair of drops was triggered by the superposed local extensional flow field close to the lateral channels. Results are described in terms of coalescence probability versus separation capillary number (Ca sep ). For all systems tested, an upper limit value Ca * sep was observed, above which coalescence did not occur. The probability and length of upstream coalescence propagation induced by the drop shape relaxation following the initial, triggered event are reported. Drop-drop contact times were varied by injecting fluid using different combinations of lateral channels. Ca * sep shifted to a higher value for a given system as the lubricating film drained for a longer time, which, in addition, increased the probability and length of an avalanche of events. The present results demonstrate how microfluidic tools can be used for systematically mapping the most probable behavior of complex systems with respect to coalescence under well controlled hydrodynamic conditions. In general we observe that larger drops, slower separation and higher surfactant concentration favour extensional coalescence and its propagation, in agreement with earlier published experimental studies.

Forces between oil drops in polymer-surfactant systems: Linking direct force measurements to microfluidic observations

“…As shown in Figure 7, even a relative velocity difference of 5 microns/s is sufficient for hydrodynamic drainage effects to mask the depletion forces encountered from the PVP/SDS complex. We developed a device where the drops come into close contact via the introduction of side channels, similar to those used in drop coalescence studies by Gunes et al (17), enabling the removal of excess continuous phase (see movie, supplemental information, SV1), driving the drops into close separation with the same relative velocity, but without coalescence. The drops continue in this geometry moving through the device (see movie, supplemental information, SV2-6), and exit the device where the persistence of the drop chain is expected to correlate with the attractive interactions between the drops.…”

“…Studies of interactions between drops and bubbles usually focus on the transition between repulsion and attraction, defining precise conditions whereby drops/bubbles may coalesce. These measurements are usually made using direct force measurement methods such as atomic force microscopy (AFM) (1)(2)(3)(4)(5)(6)(7)(8), larger capillary based drop and bubble methods sensitive to interfacial separation (9-13), insightful and novel microfluidic devices (14)(15)(16)(17)(18), or other geometries yielding elongational flow such as the four roll mill (19). However, there are conditions under which drop/bubble interactions are attractive over certain separation distances yet sufficiently stable at closer separations such that aggregation occurs, rather than coalescence (20).…”

“…Microfluidic devices are an obvious candidate to systematically probe drop interactions, yet previous studies that have had such success have been limited to studying phenomena that are either dominated by hydrodynamic interactions between drops or exhibit some coupling of hydrodynamic behaviour and surface forces. The exciting studies by Bremond & Bibbette, (14,15) and Gunes et al, (16,17) systematically explored drop coalescence in microfluidic devices, most notably where drops coalesced upon separation. However, the drop interaction outcomes in these studies were driven largely by hydrodynamic drainage effects until very close drop separations.…”

Mimicking coalescence using a pressure-controlled dynamic thin film balance

“…The models, despite the fact that they consider only the overall droplet deformation and not the film area and its retraction dynamics, were able to capture the general aspects of the AFM and microfluidic experiments that followed. 31,32 However, a more detailed study of the local film dynamics is needed to improve our understanding and modelling of separation-driven coalescence. 29,32 Because of the critical importance of capillarity to film retraction, it is imperative that this process is studied under direct film visualisation for two equally deformable surfaces.…”

Statistics of droplet sizes generated by a microfluidic device

“…Emulsion droplets in microfluidics are useful as discrete sample volumes for micro‐reactors, rheology measurements, and protein crystallization . Precise control of both location and size of droplets within a microfluidic device allows further manipulation such as coalescence of droplets from separate streams or at specific channel locations …”