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)
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)
Triggerable temperature sensitive microcapsules. Controlled coalescence of inner drops inside double emulsions, drops inside of drops, is useful for triggering chemical reactions inside drops with the outer drop serving as a reaction vessel. Using double emulsions as micro-reactors is one of our reasons for encapsulating different types of inner drops inside another drop.
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)
These boots are made for obstructing. While generating double emulsions using glass capillaries looks easy, there are actually a few tricks for making it work. One is the careful alignment between the injection and collection glass capillaries. The other is preventing the wrong fluids from wetting the surfaces of the capillaries. Here we show wetting in the form of an ‘oil boot’ that pushes down on water-oil- water double emulsions as they exit the device; this is seen in the video.
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)
Why does salt dissolve in water and not oil? Positive and negative charges on water molecules are attracted to sodium and chloride ions as seen in the image above. Water molecules “hydrate” sodium and chloride ions, keeping them separate in water, which, when you think about it, is quite extra-ordinary. And preventing salt’s ions from re-combining is something that water is particularly good at because it forms this protective sheath around the ions. All the more interesting is that while water can keep salt ions separated, even at room temperature, oil cannot. What are the differences between water and oil? Water and oil have many differences; here, we will focus on their very different dielectric constants. Water has a dielectric constant of 80, while oil has a dielectric constant of only 2. This means that it is easier to polarize water than oil. It also means that the energy associated with ‘how attracted’ two ions of opposite charges are to each other in water is much weaker than in oil. The oppositely charged ions do not see each other in water because of this sheath that forms around them, and whether or not there is a sheath depends, in part, on the strength of the dielectric constant of the fluid they are immersed in.
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)
Caterpillar Emulsions. Not all drops pinch off from the orifice of a nozzle as spheres. By loading the inside of a drop of very small volume with drops that do not deform, caterpillar double emulsions emerge as seen in the video. The outer drop is oil with surfactants, and the inner drops are water. The dark and light colored drops are simply water dyed with two different colors, red and blue.
These caterpillars change shape moments after pinching off; they would much rather be spheres because of surface tension which favors minimizing their surface area. (Video credit: L. L. A. Adams)