Physics in Drops

We add surfactants to the aqueous dispersed phase to generate ultra-low surface tension inner drops that are encapsulated inside of oil drops with a microfluidic device. As the outer oil drop pinches off, due to the Plateau-Rayleigh instability, a trapped water drop inside the oil neck deforms and stretches before splitting into two.  (Gif and emulsion credits: L. L. A. Adams)

As the shells of the bubbles come into contact with each other, they transition from spherical shapes to polyhedral shapes. The boundaries between the polyhedral shapes are called Plateau borders. Plateau borders are important for fluid drainage and are used to describe the stability of foams. Heat from a hot air gun causes the expansion of the bubbles.  (Gif and bubble credits: L. L. A. Adams)

 Self Assembly: Trimers.   Microfluidics offers a robust and tunable platform for the generation of new artificial materials that can be soft and squishy or rigid and non-deformable.  Three aqueous drops containing  either red or blue dye are encapsulated inside an ultra-thin sheath of oil and self-assemble into trimers as seen in the gif above. The drops re-arrange to minimize their interfacial energy. The core water drops maintain their spherical shape because the surface tension of water is much higher than the surface tension of the surrounding oil.  Other shapes are also possible.  (Gif and Trimer Credit: L. L. A. Adams)

Controlled Release. Double emulsions, drops inside of drops, are useful as carriers of drugs and other macromolecules.  The mechanical properties of the shells, such as their elasticity, permeability and selectivity,  are tunable using different material compositions during their microfluidic production. Here we show a microcapsule with a bio-compatible wax shell composed of different triglycerides. As the shell is heated, the wax shell melts and expands releasing cargo containing different aqueous drops as seen in the gif above. (Gif and emulsion credit: L. L. A. Adams)

Packing spheres inside a sphere. Confined water drops inside an oil drop easily form a hexagonal closed packed crystal structure when they are of the same size. We generate monodispersed water drops inside oil drops with microfluidics. This gif is produced from confocal microscope images taken at different heights of the oil drop; the combined vertical images are called a z-stack.  Since water is denser than oil, the water drops sink to the bottom of the oil drop. We also, in somewhat related work, study how crystals grow inside the oil drop and address the question of what radius does the hexagonal close packed lattice first appear. Also of special interest is understanding the order-to-disorder transition as the water drops change their arrangement from a hexagonal closed packed lattice at the center to a disordered lattice at the spherical boundary, which is not so obvious above but can be seen in other related work. (Gif and double emulsion credit: L.L. A. Adams)

Swimming Micron-size Robots? Flowbots? These cylindrically-shaped double emulsions, drops inside of drops, wiggle and buckle after they detach from the nozzle and  flow through a microfluidic capillary device.  Our goal is to utilize them to detect and cure diseases in the human body, like something from the science fiction movie Fantastic Voyage, by loading them with drugs and sensors. (Flowbots and gif credit: L. L. A. Adams; you tube video here)

Monodispersed Multi-Component Double Emulsions. These drops inside of drops are generated using glass microfluidics. By controlling the stoichiometric ratio of different inner drops and housing reagents separately inside, chemical reactions inside of drops can be triggered with temperature using a temperature sensitive shell or with pressure using an elastic shell. (Soft Matter, Image credit: L. L. A. Adams)

Blowing bubbles: Encapsulating gas inside drops with microfluidic technology.  These bubbles are remarkably stable, lasting more than 6 months since the outer shell becomes rigid with solvent evaporation. These bubbles are used for acoustic contrast measurements. (Gif credit: L. L. A. Adams)

Dimers with soft, liquid interfaces.   Not all double emulsions, drops inside of drops, are spherical.  When microfluidic conditions are right, it is possible to make two component dimers in large quantities. The two inner drops are water, dyed two different colors, and the outer drop is an ultra-thin layer of oil. If you look closely you can see satellite drops trailing the dimers. These satellite drops are composed of oil and surfactants and are a result of the pinch-off of the double emulsion from the injection capillary.  The two inner drops do not coalesce with each other and are very stable as a result of the ultra-thin layer of oil and surfactants. The ultra-thin layer of oil mechanically stabilizes the drops by the lubrication effect.  (Video credit: L. L. A. Adams)

Coalescence of inner drops is triggered with heat from a heat gun as seen in this video.  The outer drop is made of wax using a melt emulsification technique. As the wax melts,  the inner aqueous droplets are free to rotate and move around until they come into contact with one another. Prior to drops coalescing, a small bridge is formed between drops. This is noticeable in the video.  The video is taken with a high speed camera and played back at a much slower rate.  (Credits: L. L. A. Adams , Soft Matter)

Another squishy bubble. This double emulsion has a gas core and a polymer shell. It was generated using microfluidic techniques. By compressing the double emulsion between two glass slides, we can test its elasticity as shown in the video above. (Credits: L. L. A. Adams)

Pressure responsive microbubbles. Deforming air bubbles is easy with compression, but their life time is very short because compression forces are strong enough to overcome  surface tension forces causing bubbles to burst. One way to increase their lifetime is to encapsulate air bubbles in a protective polymer shell  using microfluidics.  By sandwiching 'polymer encapsulated bubbles' in between two glass slides and pressing on the top slide, we demonstrate not only its elasticity, but also its robustness as seen in the video above and also seen here. We can tune the elastic properties of the shell by adding nanoparticles to the polymer matrix.  Moreover, a shell with only nanoparticles and no polymers is extremely rigid and easily cracks.  (Gif credit: L. L. A. Adams)

Chiral Double Emulsions. Double emulsions with four different components are generated with microfluidics as are their isomers to form enantiomers. Pairs of stereoisomer emulsions that are enantiomers are mirror images of each other but they cannot be superimposed onto each other. Moreover, since the shape, size, and composition are control parameters that govern the properties of these structures and their subsequent self-assembly into larger aggregates, using microfluidics to tune these parameters facilitates a rich area of study of their bulk properties as well as their properties at the single emulsion level. For example, while the optical properties of chiral molecules have been extensively investigated and remarkably demonstrate the ability to rotate plane polarized light, no one knows how chiral double emulsions will interact with microwaves. (Image credit: L. L. A. Adams)

Wetting can interfere with the flow dynamics making it impossible to generate double emulsions or very difficult to produce complex double emulsions. How fluids wet the glass capillary surface depends on the interaction between the fluids with glass. Often we treat the surface of the capillaries with hydrophobic or hydrophilic silanes; the choice of silanes we use depends on whether or not we are making water-oil-water or oil-water-oil double emulsions. And with some fluids, particularly when encapsulating gas, it is better not to treat the capillaries at all. (Video credit: L. L. A. Adams)

Mirror Symmetry. The word chiral is derived from the Greek word meaning hand. If the two mirror images do not coincide with any possible rotation then they are said to be chiral.  If the two mirror images can be superimposed onto each other, then the emulsions are not chiral as shown in the images above. By flipping or rotating the "mirror image" emulsions out of plane, they do, in fact, coincide; these emulsions are not chiral.   (Image credits: L. L. A. Adams)

It turns out that if the thermal energy, kT, is small enough, then salt will not dissolve because the attractive energy has to be less than kT for ions to remain dissociated. There is also a length scale to think about; this is called the Bjerrum length and it defines the distance between ions required for stable dissociation. For water, at room temperature,  this distance is 0.7 nm which is the size of their hydrated sheaths, but for oil the distance is 28 nm.  That means that salt ions cannot hide in oil unless there is a sheath of 28 nm in diameter to hide in. There are polymers or copolymers that are capable of sequestering ions into sheaths that are 28 nm; these sheaths with ions are called reverse micelles. (Image credit: L. L. A. Adams)