A study on filament breakup process of silicone oil/PMMA suspensions

Abstract

The effect of particles dispersed in a Newtonian medium on the filament breakup of suspensions under extensional flow was investigated. In particular, the final stage of the breakup process was focused because the process in the final stage was more complicated and complex than in the initial stage. In order to study the effect of the particles, 10 μm poly(methyl methacrylate) (PMMA) particles with 0~20 wt% were dispersed in a silicone oil and the effects of the particles on the thinning dynamics and the shape of the filament were observed with time dependent changes in the minimum neck diameter (W: the shortest length of the filament). The particles affected the dynamics of the thinning filament, and the role of the particles was different according to the stages of the breakup process. In the initial stage of the process, the particles decelerated the filament breakup process until the minimum thickness (W) of the filament became 5 D (D=10 μm: particle diameter), and then they accelerated the process rapidly until the pinch-off, which was the state when the filament breaks up perfectly. In order to investigate the effect of the particles on the thinning dynamics, the breakup velocity of 20 wt% PMMA suspension was compared with that of a silicone oil having the same viscosity and surface tension with the 20 wt% PMMA suspension. When W>35 D (350 μm), the time evolutions of minimum neck diameter for the PMMA suspension and the viscosity-matched silicone oil were almost the same. In this regime, the thinning dynamics of the two filaments was governed only by the bulk properties of the fluids. However when the filament became thinner than 35 D, the thinning dynamics of the two fluids began to show differences even though they had the same properties. In addition to the increase in the effective viscosity, the particles decreased the breakup velocity more than the viscosity-matched silicone oil because the particles confined in the narrow filament channel disturbed the flow of the thinning filament under the extensional flow. In this way, the effect of the particles on the thinning filament could be confirmed while eliminating differences in the rheological properties of the two fluids. The effect of the particles on the filament shape was discussed through the comparison of the filament shape between the 20 wt% PMMA suspension and the viscosity-matched silicone oil. When the filaments of the fluids were thicker than W=10 D (100 μm), the filament shape of the fluids was almost the same. It indicates the particles did not affect the shape of the filament in regime of W>10 D. The particles began to affect the filament shape when the filament became thinner than W=10 D. The filament surface of the PMMA suspension was rough and the shape of the filament became random for W<2 D, which led to a random pinch-off of the thinning filament. The randomness was induced because the local concentration of the particles confined in the filament became non-uniform as the filament of the PMMA suspension was thin in the length scale of a particle diameter. The random distribution of the particles induced heterogeneity in the final stage of the filament thinning process, and the random distribution of the particles was quantified using the analysis of light intensity in the central part of the thinning filament. Thus, the effect of the particles in the final stage of filament breakup was found to be more complicated than that in the initial stage, and we demonstrated that the particles dispersed in a Newtonian medium induced complex behaviors in the final stage of filament breakup. This work provides the information about the effect of the particles confined in the thinning filament on the dynamics and the shape of the filament for particulate suspensions. It is expected that the information about the role of the particles in the filament breakup process will be useful in designing the extensional process such as coating, jetting, spraying, among others.