Researchers use optical and scanning electron microscopy to describe the phenomenon of self-ejecting crystals from heated, nanotextured superhydrophobic materials
Water is a vital part of our every day lives. In developed countries, water also plays a critical role in cooling thermoelectric power plants. Evaporation is part of the cooling process and can result in mineral precipitation on equipment surfaces, also called mineral fouling. Mineral fouling can have devastating and costly effects on equipment.
Dr. Varanasi, along with his former graduate student, Dr. Samantha McBride (now a presidential post-doctoral fellow at Princeton University, USA), recently published a paper which describes how so called “crystal critters” can self-eject from heated, nanotextured superhydrophobic materials during evaporation of saline water drops. The crystal critter effect has potential applications for improving sustainability in spray cooling heat exchange.
Tell us about your laboratory at the Massachusetts Institute of Technology (MIT)
The Varanasi Lab focuses on how micro and nanoscale processes at the interfacial level can be manipulated to tackle big industrial problems. For example, fouling (the accumulation of contaminants in pipes, reactors and/or membranes) is a critical global challenge affecting the energy, desalination and chemical industries. Monetary losses due to fouling account for 0.25% of the GDP of industrialized countries and fouling is currently a limiting factor in the efficiency, material lifetimes and overall sustainability of many processes. Although the impacts are large, the onset of fouling is an inherently interfacial process that must be understood on a microscopic level in order to be prevented.
Our approach is to develop a rigorous thermodynamic framework to identify the true bottlenecks and then figure out efficient kinetic pathways to impart the solution.
By understanding the phenomena, it becomes possible to reduce even the largest problems down to a few nondimensional parameters and thus collapse complex processes into manageable formulas and phase diagrams.
These can then be used to design new processes, new products and zero-tradeoff solutions. These strategies can be applied to any problem where interfacial effects lead to critical bottlenecks. We work on problems ranging from aircraft anti-icing, drug delivery, efficient application of pesticides to crops and water harvesting.
What is the crystal critter phenomenon?
In this publication, we were interested in how the length-scale of superhydrophobic surfaces influences crystal fouling from evaporating drops. Superhydrophobic surfaces are created by a combination of hydrophobic chemistry with a surface topology on the micro- or nano- length scale. Many superhydrophobic surfaces utilize micro-scale topologies with characteristic length scales of about 5 to 30 microns. However, a number of previous investigations have found that such superhydrophobic surfaces fail to prevent crystal fouling and that hydrophobic materials often out-perform superhydrophobic ones.
We hypothesized that this failure was a function of the topology length scale and that nano-scale confinement effects would prevent crystal intrusion and therefore eliminate crystal fouling.
In addition to confirming this hypothesis, our experiments on superhydrophobic nanotextured surfaces (characteristic length scale <1 micron) also revealed that these materials enable a dynamic self-ejection of crystal structures during evaporation. This self-ejection is a rather unusual consequence of the combined effects of crystallization, evaporative flux and nanoscale confinement of growing crystal forms. The lifelike motion of the crystal structures during growth as well as their final morphology composed of a crystalline “body” balanced on a set of “legs” led us to name these structures “Crystal Critters.”
How did you use microscopy in this publication?
We used both optical microscopy and scanning electron microscopy in this publication.
During experiments of critter growth, we used a ZEISS light microscope to record the dynamics. These videos allowed us to identify local regions of fluid impingement into the nanoscale texture and demonstrate that both the position and size of those regions correlated with the position and diameter of critter legs. This was critical in understanding where and how critter legs first began to form, which was key to understanding the self-ejection phenomena.