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ramé-hart instrument co. August 2015 Newsletter
|Book Review: Wetting of Real Surfaces|
Review by Jenalyn Clegg
Edward Yu. Bormashenko’s Wetting of Real Surfaces provides a new look at some concepts that are already well understood by surface scientists, such as the models of Young, Wenzel, Cassie-Baxter and others. The explanations are straight-forward and crisp, combining familiarity with newer and less well-defined areas such as liquid marbles, electro-wetting, and nonstick droplets. Wetting of Real Surfaces combines readability and simplicity (as much as possible) with mathematics and logical proof, making this book accessible for a range of people from undergraduate and graduate students to scientists and researchers interested in wetting behavior on solid surfaces.
True to the book’s name, Bormashenko walks the reader through ideal and non-ideal surfaces, explaining surface tension, wettability factors (e.g., line tension, disjoining pressure, fiber wetting and capillary rise), and why hysteresis (or the difference between the advancing and receding contact angles) happens and what it means. The author answers the questions why solid-liquid interactions deviate from the ideal, why some surfaces have greater hysteresis, and how the triple line (where the liquid, solid and gas meet) behaves. The treatment of triple line pinning and behavior is impressive and helps the reader gain insight on how real solids deviate from the ideal surface. By the way, an ideal surface is "atomically flat, chemically homogeneous, isotropic, insoluble, nonreactive and nonstretched".
Bormashenko discusses the methods used to measure the surface tension of liquids. We concur that the pendant drop method is one of the most precise. He also discusses factors that affect wetting - absorbed liquid layers and vapors, gravity, wetting transitions, even distortion caused by electrical fields.
A couple of chapters are also devoted to superhydrophobicity and non-stick drops. The author clearly distinguishes between true superhydrophobic surfaces (which have contact angles of greater than 150°, a low sliding, or roll-off, angle and are self-cleaning) and pseudo superhydrophobic surfaces (i.e., the Rose petal effect) which do not easily slide. Bormashenko's analysis of the wetting transitions (the change from initial partial wetting to complete wetting) of these drops is important for those who are attempting to create stably self-cleaning, superhydrophobic surfaces.
The last chapter is devoted to the new area of nonstick drops, both Leidenfrost and liquid marbles. Leidenfrost drops are highly mobile and even self-propelling on heated (sometimes ratcheted) surfaces; they are formed when a liquid comes in contact with a substrate at a temperature significantly higher than its boiling point and a vapor layer is formed between substrate and liquid. Liquid marbles are nonstick droplets coated with nano- or micrometrically scaled particles and may have applications in areas such as miniaturized chemical processes, gas sensing and blood typing. Up-to-date with the latest research, Wetting of Real Surfaces is a must-read for all those in both academia and industry interested in liquid-solid interactions, contact angle, and wetting behaviors.
Note: Wetting of Real Surfaces is available from Amazon and other book sellers.
A customer recently asked us about
inverting phases. They wanted to measure the surface tension of water,
but instead of using a pendant drop of water in an air phase, they
wanted to inject an inverted bubble of air into an external water phase.
We said, yes, of course.
To illustrate how this works, we created the video above. What is illustrated in the video can be replicated with any ramé-hart instrument that has DROPimage Advanced installed. The only additional options you will need are a Quartz Cell (http://www.ramehart.us/quartz-cell/) and Inverted Needle (http://www.ramehart.us/inverted-304-ss-needle/). If you are using a Microsyringe to dispense, you will want to fill it with water and then retract into the needle a small volume of air (just slightly more than your desired bubble volume). This will allow you to control the volume very precisely. If the Microsyringe were filled completely with air, then dispensing would be very spongy and it would be difficult to dispense a precise amount of air. Likewise, if you are using an Automated Dispensing System, keep the syringe and tubing hydraulic and pull a small volume of air into the needle prior to dispensing. When selecting phase data, be sure to change the droplet phase to air and the external phase to water. When you are ready to measure, ensure that the baseline is BELOW the midway point which is defined by the two little red tick marks on the left and right sides of the static image window. (Note that for pendant drops, when the baseline is above the tick marks, DROPimage is looking for a pendant drop; when the baseline is below, it's looking for an inverted pendant drop.)
Why would anyone want to go to the extra effort to measure surface tension using inverted phases? Here are five advantages:
1. If you are using surface tension to test the purity of a liquid, the inverted phase method allows you to test a larger quantity (e.g., 50ml at a time contrasted with 20µl for a liquid pendant drop).
2. If you are interested in measuring the change in contact angle as another liquid is added (e.g., a surfactant), it's much easier to do this in real time with an inverted phase setup. You can quickly determine the change in surface tension as a function of concentration of additives.
3. If you want to capture the change in surface tension as a function of temperature, the inverted phase method allows the liquid phase to be more reliably heated and controlled.
4. If you are interested in measuring the surface tension between a liquid and a number of different gases, the inverted phase method can be implemented without a gas tight chamber.
5. An inverted bubble is less sensitive to vibration and environmental conditions (such as air flow) than an exposed pendant drop.
While the traditional pendant drop is
still most practical for most applications, an inverted phase setup
offers some compelling advantages for special tasks.