The Effects of a Surfactant on the Operation of T-Junctions for Mass-Producing Foam Targets

A T-junction microfluidic device consists of one microchannel connected to a second microchannel at 90 deg. The size of the emulsions that form at the junction depends on the dimensions of the channel and the properties of the immiscible fluids flowing through them. Micron-sized emulsions are easily formed in small channels where interfacial tension forces dominate, but it is more difficult to form larger emulsions that could be used to produce inertial confinement fusion (ICF) targets. The concept and feasibility of using this method to mass-produce millimeter-sized ICF targets are presented.

The experimental data presented here will demonstrate the competing contribution of the fluids’ surface tension and fluid velocity to forming and controlling the volume of millimeter-sized oil-in-water emulsions. The oil-in-water emulsion is the first step in the process of making resorcinol-formaldehyde foam targets (1 to 4 mm in diameter). Adding a surfactant to the aqueous phase lowered the aqueous-solid surface energy, which allowed for greater flexibility in manufacturing T-junctions. Equally important, although it also lowered the interfacial surface tension, the emulsions remained encapsulated by adjusting the flow velocities. The effect of the surfactant on the completing shear, viscous, and surface energy forces involved in the microencapsulation mechanism is described. Oil-in-water emulsions, 1.32 to 8.32 mm in diameter, and water-in-oil emulsions, 1.10 to 3.2 mm diameter, were formed. A protocol is presented for tuning the droplet diameter to a desired value based on the capillary number and the relative fluid velocities ratio (which must be below 0.5). A linear regression showed the relationship between the fluid velocities and desired droplet diameter. Control of the outer diameter was demonstrated over a 1.75- to 4.14-mm-diameter range with a 426- to 900-μm wall thickness.