Research
Ice nucleation. My main research interest is the study of metastable fluids using microfluidic technology. I work with deeply supercooled water, which is still liquid at temperatures around -37 ºC. Producing supercooled water at such temperatures remains challenging despite decades of experimental research, and much of my recent work was aimed at developing better methods for producing and handling supercooled water.
We developed a high-accuracy and high-speed instrument for studying supercooled water by investigating how statistical ensembles (more than 10 000 systems) of ~100-micron diameter drops of water freeze by homogenous nucleation of ice. This experimental platform is faster and more reliable than any other previous instruments for the study of ice nucleation, and it allows high-quality imaging as well: the animated image on this page shows how ice crystals grow from a inside a 150-micron drop of supercooled water.
We measured the rate of homogeneous nucleation of ice in supercooled water with low noise and excellent reproducibility; I envision that our experiment will provide the most accurate measurements of homogeneous ice nucleation rates between -35 ºC and -39 ºC. In another project we investigated whether external electric fields have an effect on the homogenous nucleation of ice; we could apply larger fields than in any previous experiments but we have not observed any changes in the rates of nucleation.
The most promising application of our experimental platform, however, is the investigation of heterogeneous nucleation of ice on foreign particles present in water; the high data throughput of the ice nucleation instrument allows us to measure quickly the probability of heterogeneous nucleation over a wide range of temperatures—a feat that is difficult to achieve using other instruments.
Multiphase microfluidic flow. I also research the production and physical properties of two-phase microfluidic systems such as (i) laminar flow structures which we used as opto-fluidic components, and (ii) drops of liquid or bubbles of gas inside another liquid phase, systems in which we are interested as micro-containers for chemical reactions. These fluidic systems are remarkably stable and reproducible, and I'm interested in using them to investigate problems that require high-precision and high-accuracy measurements.
In one project, we used the smooth interface between two liquids flowing in a microchannel as an optical element. The second image from top shows an "optofluidic" liquid lens formed by flowing two liquids through an expanded region of a microchannel. Diverging light incoming from the left side (green) was focused by the lens; we made the focused light beam (yellow) visible by passing it though a solution of fluorescent dye. The focal length of this liquid lens can be adjusted by varying the rates of flow of the two liquids.
Drops and bubbles produced inside microfluidic channels can be used as microreactors for chemical and biological applications. Microfluidics techniques for producing drops and bubbles are similar to process in which water drips from a leaky faucet, but are more reliable and have a higher rate of repetition. The properties of the liquid that surrounds drops or bubbles, and its viscosity in particular, determine how drops and bubbles are generated and how they move in microchannels.
We used a relatively simple technique for tuning the properties of the surrounding fluid—changing its temperature—to develop methods for the control of the generation or of the positioning of drops and bubbles. These control methods are useful because they are precise, reproducible, and predictable. They also motivated us to research fundamentally the flow of drops and bubbles in microchannels, and we discovered that in microchannels the strength of hydrodynamic lift forces cannot be predicted by a simple extrapolation from larger systems.