Superhydrophobic Surfaces

Home > Labs > Measuring Contact Angles

Measuring Contact Angles

Procedure by George Lisensky, Nicholas Kaung Shein, and Alex Knapp, Beloit College

Turn on lamp. Check the bubble level. Fine positioning of the microscope will be adjusted by turning the knobs.

Start the "Celestron Micro-Capture Pro" software.
Select Options/Resolution/2592*1944 (5M)

Put sample on stage

Add a 4µL drop of water.

If the drop does not stick, use a corral to keep the drop from rolling off.

Close up of corral and drop.

Slide the box for coarse positioning and use the knobs for fine positioning to put the drop at the focal point and produce a sharp image on the monitor.

Click on the upper left icon to take a photo.

To remove a drop, touch with a Kimwipe

Drop Shape Analysis

The effects of gravity make the water drop deviate from spherical so a plug-in for Image J is used to match the Young-Laplace second-order differential equation to the image data. See http://bigwww.epfl.ch/demo/dropanalysis/ and Aurélien F. Stalder, Tobias Melchior, Michael Müller, Daniel Sage, Thierry Blu, and Michael Unser, "Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops," Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 72-81. This analysis assumes the left and right halves are identical so it is important that the surface be level. Given the equation for the drop-shape, the contact angle is the tangent where the drop meets the surface.

Steps using Image-J with the Low-Bond Axisymmetric Drop Shape Analysis plug-in

Use the line tool to find horizontal angle (command-M to display)

Image/Transform/Rotate by the found angle

Image/Type/32 bit (or you will get an error that says you cannot process color images)

Plugins/drop-analysis/LB-ADSA
    Click on slider bar to adjust by 1 using arrow keys
        y0 (vertical top)
        x0 (horizontal center)
    c = water (do not adjust)
    Optimize b and h ONLY (uncheck default x and y) by gradient energy
        b (top curvature)
        h (bottom extension)
    Manually adjust h so the two circles meet at the surface, then possibly optimize gradient energy again
    Record the contact angle

Chemicals

Zinc strips, for example from Home Science Tools

To make a corral, punch a 1/4" hole in a zinc strip. Prepare a superhydrophobic coating as for the samples.

Equipment

Celestron 5 MP Handheld Digital Microscope Pro (Amazon)
SHOOT 4 Way Macro Focusing Rail Slider with 1/4" Screw Hole for Close-up Shooting (Amazon)
Attach slider to 8" tall wooden T-stand with 1/4" bolts
Attach microscope to rail slider with cable ties
Attach T-stand to benchtop with a C-clamp
Project box, approximately 3.5" x 4" x 6" with a stage that extends
4 µL syringe
Bubble level (Menards)

An alternative system with a computer controlled motorized automatic dispensing system is described at http://www.instructables.com/id/Contact-Angle-Instrument/

This page created by George Lisensky, Beloit College. Last modified July 9, 2019.