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November 2022

Tunable Wetting Behavior - Part Two

Last month we introduced the idea of a responsive smart surface - that is, a surface which permits the wetting behavior to be adjusted or fine-tuned using external stimuli and without permanently modifying the surface. We covered several stimuli: magnets, silanization and desilanization, polymer brushes that are controlled by different solvents, and tunable micro-wrinkles.

Another potentially effective stimulus is an electrical field. The phenomenon of electrowetting is based on the principle of electrocapillarity which was first explained and demonstrated by a Frenchman named Gabriel Lippmann in 1873. It wasn't until the 1990s that a thin insulating layer is proposed in order to prevent electrolysis. In a standard electrowetting setup for microfluidic manipulation, electrodes are mounted to a surface. These are then coated with a dielectric layer and then a very thin hydrophobic surface. When a drop is deposited on the surface and voltage is applied to the electrodes under the drop, the surface tension of a drop is reduced which leads to greater wetting and subsequently a lower contact angle when energized. By creating an array of electrodes, it's possible to build a lab-on-a-chip on which a drop can be moved around simply by changing which electrodes are energized.1

Microfluidic Lab-on-a-chip by Charles Grassin
Watch the video

Another class of smart surfaces respond to changes in temperature. Thermoresponsive polymers like Poly-isopropylacrylamide (PNIPAAm) are used to fabricate surfaces which have wetting behavior that can be controlled by the temperature.2 When the temperature is below the lower critical solution temperature (LCST) which is 32°, the intermolecular hydrogen bonding becomes weak and the result is hydrophilic behavior. At higher temperatures, the strong hydrogen bonding promotes hydrophobic behavior. If roughness is added, a thermoresponsive surface can have a wide spectrum of wetting behavior, from complete wetting with a contact angle of 0° to superhydrophobic with a contact angle in the 150° neighborhood.

A smart photoresponsive surface exhibits a change in wetting properties depending on what light or lack of light is shed on it.3 Surfaces made from titanium dioxide (TiO2), for example, have been hydrophilic by applying UV irradiation. Under UV illumination, the thin film of titanium dioxide transitions from a Ti4 state to a Ti3 state making it hydrophilic. When the surface is placed in darkness, the state reverses and the surface becomes hydrophobic once again.

In short, smart responsive surfaces with tunable wettability driven by various external stimuli prove to be useful candidates in a variety of novel applications where continuous and reversible surface wetting is advantageous. These surfaces use the extra energy gained from the external stimulus to change their wetting behavior as needed. Surface wetting properties can be changed from hydrophobic to hydrophilic on smooth surfaces and from superhydrophilic to superhydrophobic on roughened or patterned surfaces. Because each technique has few advantages and disadvantages over others, there is a wide array of options available to researchers. There may be some limitations where scientific understanding is not completely clear and that is currently the subject of additional research.


See https://charleslabs.fr/en/project-Microfluidics+Electrowetting+platform 
2 See https://doi.org/10.3390/ijms20246295 
See https://doi.org/10.1016/j.jphotochemrev.2007.03.001

Tech Tip: Two Ways to Measure Contact Angle
This month we are going to address the question: What is the best way to measure contact angle using ramé-hart DROPimage Advanced? There are two common ways: the Contact Angle Tool and a methods-based experiment.

First, let's address some of the advantages of using the Contact Angle Tool:

  • It's very easy to setup and use.1
  • A setup option is available to help with leveling and snapping to the baseline.
  • Red line and right line options are available to filter out the needle.2
  • It can be used for a single contact angle measurement or a series of equidistant measurements.
  • Contact angle precision is in tenths, XX.X°.
  • In addition to left and right and mean contact angles, the drop height and width are also reported (in mm).
  • The contact angle data can be used to measure surface energy using any number of built-in surface energy tools.3
  • It's possible to take contact angle measurements on previously saved images.4  
  • The Contact Angle Tool can also be used to measure captive bubble.5
  • On-screen reporting of the contact angle makes setup and measurement easy.
  • All contact angle data can be saved (to .CA file) or exported (to .TXT file) for easy manipulation later or using a different program.
  • The optional Overhead Optical Imaging Kit can be used with the Contact Angle Tool to measure the contact angle of drops on hydrophilic surfaces using the overhead method.
  • The "step out and measure" and "step in and measure" options become available when using the Contact Angle Tool with a ramé-hart Automated Dispenser.

In general, the Contact Angle Tool should be your go-to tool for most contact angle measurements. However, if you are doing any type of time dependent study or taking dynamic measurements, then a methods-based experiment may be the way to go. Here are the key advantages of the methods-based method:

  • Contact angle precision increases to hundredths, XX.XX°.
  • An Experiment Wizard is available to easily create a new experiment simply by answering a series of questions.
  • For doing advancing and receding contact angle measurements using the tilting base method, it's possible to integrate the tilting instructions in a methods-based experiment which makes the experiment fully automatic (requires the Automated Tilting Base).6
  • Likewise, control of the Automated Dispensing System can be integrated in a methods-based experiment to dispense and dynamically add or remove volume automatically.
  • Time intervals can be set to equidistant or a time file can be used for specific delay instructions.
  • Individual frames can be saved and reviewed later using a replay option.7
  • A trigger option allows an experiment to begin precisely the moment a drop hits the surface.8 
  • When used with the ramé-hart Hot Plate, Heated Environmental Cell, Environmental Chamber, Advanced Chamber, Elevated Temperature Syringe, or Temperature Logger, the current chamber temperature can be logged during any methods-based experiment.
  • Both reporting and graphing tools are available for any methods-based contact angle study.9
  • A methods-based experiment can be created to measure the rate of absorption.10

To conclude, use the Contact Angle Tool for most applications and when you want to measure the surface energy. If you are doing a dynamic experiment including those that integrate automatic tilting or dispensing, consider creating a methods-based experiment.


See this video: https://youtu.be/5w0wtzTwpeo
2 See this video: https://youtu.be/PKRMzSK3xTU
3 This video https://youtu.be/5MGTb1EDSgE shows the two-liquid surface energy tool in action.
4 See this video: https://youtu.be/qhOW6HT894w
5 See this video: https://youtu.be/lMzdYh3M6co
6 To see an example, check out this video: https://youtu.be/UWMwSppjv6s
7 To see how to create a dynamic experiment and then replay it, check out this video: https://youtu.be/vp2TWXK9_S8
8 See this video: https://youtu.be/LKRLWnKzMl8
9 See this video: https://youtu.be/0vHw3hBVaG0
10 See this video: https://youtu.be/-oaTQQ8IDV4


Carl Clegg
Director of Sales
Phone 973-448-0305
Contact us


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