Emulsion droplets with ultralow interfacial tensions can be pulled apart by a pair of optical traps into daughter droplets
that remain connected by an oil thread of nanoscopic thickness. This thread is stabilized by the bending modulus of the
oil-water interface, which opposes the necking that leads to break up into droplets in the Rayleigh-Plateau instability.
Variation in the pressure exerted on the droplets by the optical traps leads to a flow of liquid between the droplets via the
nanothread. The flow has two components: (i) Poiseuille flow within the thread, and (ii) transport of the entire thread
from one droplet to the other, with interface being created on one droplet and destroyed on the other. For typical
viscosities of the oil and water, the dominant contribution to flow arises from the transport of the whole thread with the
flow internal to the thread making a minor contribution.
We present a microfluidic platform for the generation, characterization and optical manipulation of monodisperse oil droplets in water with equilibrium interfacial tensions on the order of 0.1-1μN/m. An oil-in-water emulsion containing the surfactant Aerosol OT, heptane, water and sodium chloride under conditions close to the microemulsion phase transition was used. Through active control of the emulsion salinity and temperature, our microfluidic platform offers the unique capability of tuning the interfacial tension of droplets in the range of 1μN/m to few mN/m according to the operation required. Upon collection in a separate observation chamber, droplets were characterized by using video microscopy-based measurement of thermally-induced capillary waves at the droplet interface. Holographic optical tweezers were used to manipulate the droplets and construct 3D nanofluidic networks consisting of several droplets connected by stable oil threads a few nanometers across.
We discuss the design, implementation and performance of a novel platform for the production and optical control of
ultra-low interfacial tension droplets in the 1-10 micron regime. A custom-designed, integrated microfluidic system
allows the production of oil-in-water emulsion droplets of controllable size. This provides an optimised physical
platform in which individual droplets are selected, trapped and shaped by holographic optical tweezers (HOTs) via
extended optical landscaping. The 3D structure of the shaped droplet is interrogated by a combination of conventional
brightfield imaging and fluorescent structured-illumination sectioning. We detail the problems and limitations of closed-loop holographic control of droplet shape.