‘Crystal critter’ research has implications for anti-fouling technology
New research shows new ways for engineers to prevent the build up of minerals in power plants and possibly use salt water instead of fresh water for cooling.
The research, published in the journal Science Advances, shows unusual phenomenon in which crystal structures formed from evaporating drops of water saturated with sodium chloride self-eject from heated, superhydrophobic surfaces. The texture of a materials surface at the micro and nano-meter level changes the shape of salt crystals (liquid marble made of extremely salty water) that form as water evaporates. Researchers call these salt crystal structures “crystal critters” because of their eerie resemblance to land-bound animals.
Other than being weirdly cute and amusing, the crystal critter effect has profound implications for turbomachinery component design. These crystal structures (or critters) have minimal contact with the substrate and thus pre-empt crystal fouling. This revelation can help advance extreme antifouling systems for spray cooling of hot surfaces using concentrated brines produced during desalination. The research can even help power plants to one day use salt water from the ocean instead of precious and increasingly scarce fresh water.
A typical power plant uses billions of gallons of fresh water every year and thermoelectric power production is one of the largest sources of water consumption. In 2015, 41% of all surface water withdrawals in the United States went toward cooling in thermoelectric power plants.
As a coolant, water is placed in contact with hot equipment, such as a pipe, boiler, or reactor. When that water is evaporated, contaminants precipitate at the point of evaporation. Over time, the mineral buildup creates blockages, reducing heat transfer performance. This results in mineral fouling. a leading cause of equipment degradation and failure in heat exchange processes. Fouling phenomena are common in different industrial environments, ranging from ship hulls, natural surfaces in the marine environment and fouling of heat-transfer components.
A lot of resources go into pretreatment of coolant water by using technologies such as ion exchange and reverse osmosis. Because fresh water is so important to society at large, water for thermoelectric cooling is increasingly sourced from natural saline sources or from desalination waste brines.
In recent years, many have researched hydrophobicity and superhydrophobicity wetting properties to eliminate fouling. Mineral fouling remains a multiphase problem. Interactions between the crystal and substrate and between the crystal and liquid are equally important for determining fouling propensity as are the interactions between the liquid and substrate.
The researchers write in Science Advances, “This [crystal critter] effect is also of interest for drop levitation/transport applications, which have traditionally been accomplished by heating surfaces to temperatures far in excess of the fluid boiling point. In such Leidenfrost levitation, evaporative flows create a lubricating vapor cushion between the drop and surface. In contrast, the critter effect occurs at much lower temperatures (60° to 100°C) than previously observed in both the traditional Leidenfrost effect (200°C) and even for cold-regime Leidenfrost on superhydrophobic materials (~130°C) (23–25). We demonstrate that this low-temperature ejection is accomplished via cooperative effects of crystallization, evaporative flows, and nanoscale phenomena.”
(A) Schematic of experiment, where a drop of water containing dissolved salt is evaporated on a hot, superhydrophobic substrate. (B) Scanning electron microscopy (SEM) images showing nanotexture of superhydrophobic nanograss surface. Scale bars, 3 μm. (C) Growth of crystal critters from a 5-μl drop with time at a substrate temperature of 90°C. Scale bar, 0.5 mm. (D) Time for evaporation as a function of temperature. Entire bar represents the total evaporation time, the blue segment is the first stage of evaporation before leg growth, and the orange segment is the second stage of evaporation during which legs grow. (E) Growth of legs with time as a function of temperature, where the lowest temperature (purple, right-most line) is 60°C and hottest (red, left-most line) is 110°C.
(A) Schematic showing drop on nanograss surface. Inset is SEM image showing pore geometry of the nanograss. Scale bar, 2 μm. (B) Illustration of nanograss texture. Most of the drop rests on a layer of plastron within the superhydrophobic texture, but liquid may impinge in small, localized areas. (C) Top view of a drop immediately after placement on a surface showing small, localized areas where liquid is impaled within the texture. Scale bar, 1 mm. (D) SEM image showing detail of a salt stain where the outer perimeter has also been deposited. The outer diameter is ~32 μm, and the inner diameter is ~25 μm. Scale bar, 5 μm. (E) SEM image of a region where salt stains reveal where critter legs previously grew. Scale bar, 200 μm. (F) Optical image of critter legs near surface, where a small tube still connected to the surface is outlined in yellow. (G) SEM image of the bottom of a tube left behind on a surface. Scale bar, 20 μm.
From left to right, images in (A) to (E) show SEM of substrate texture, initial drop contact angle, intermediate step where crystals have begun to form, and final crystalline deposit formed by evaporation on substrates heated to 70°C for (A) hydrophobic smooth silicon, (B) superhydrophobic nanograss (i.e., the same texture used in Figs. 1 and 2), (C) superhydrophobic microposts, (D) superhydrophobic microposts further textured with nanograss, and (E) superhydrophobic microholes. SEM images for (A) to (E) are 50 μm wide. (F) Contact line radius with time for each substrate. Green line shows superhydrophobic nanograss surface, light gold line shows hydrophobic untextured surface, dark gray corresponds to the superhydrophobic micropost surface, black corresponds to the superhydrophobic nano-micro composite surface, and lavender to the superhydrophobic microholes. (G) Illustration showing process of crystal intrusion into microtextures that leads to the Cassie-Wenzel transition on micropost surfaces. (H) SEM image of salt deposit inside the microtexture of the superhydrophobic microposts and nanograss substrate (D).
(A) Images defining model parameters including h (length of the legs), Ro, and Ri (outer and inner diameter of a given leg). (B) Average leg growth rate (millimeters per minute) as a function of substrate temperature, where purple circles indicate experimental values averaged from five to six trials, error bars show SD, and solid line is the model of Eq. 3 (where Ro = 16 μm and Ri = 12.5 μm). (C) Experiment showing critter growth on a substrate with an imposed temperature gradient. Legs grow longer on the side with a higher temperature, causing the crystal critter to become more and more unstable until it eventually tips over and rolls in the direction of the lower temperature. New legs begin to grow at the new position until evaporation is complete. Scale bar, 1 mm.
Read the report here.
