Step 1: What Is It......?
A contact angle instrument is a piece of equipment used to determine certain specific properties of liquids and solid materials as well as interactions between the two. These properties include “cohesive forces, adhesive behavior, wetting
properties, and morphological properties” . The “contact angle”, measured with this type of instrument, refers to the angle
created by the surface of a drop of liquid and a flat surface of a known material at the point of contact between the two. The angle is determined by the shape that the drop takes when placed onto the surface. This shape is produced by the interaction between the
properties of the liquid and the solid surface, which are determined by the relative surface tensions of the two materials . To be more specific, the cohesive behavior of the liquid serves to increase the contact angle by attempting to keep the liquid together in the drop shape, while the adhesion interaction between the two materials attempts to decrease the angle by trying to spreading the liquid across the surface of the solid material . It is important to note that the angle is always measured through the liquid , as seen in
The instrument is used to create the liquid drop, and placing it onto the solid, and then taking a picture of it. Then there is generally a piece of software used to help measure the actual contact angle from the image. So, any contact angle instrument consists of at least four components: a dispensing system, the stage, the viewing system, and the measurement system.
The dispensing system generally consists of a micro syringe to hold and dispense the liquid, and can be operated manually or by a motorized system.
The stage is that part of the instrument that hold the solid surface. It must be flat and level - usually built with the ability to adjust the tilt to a level position - so that the contact angle can be accurately measured (a tilted surface will change the observed contact angle; increased on the "down hill" side and decreased on the "up hill" side). The stage is generally able to move up and down a few inches to allow the liquid to be transferred from the syringe needle to the solid surface, and sometimes able to move laterally as well.
The viewing system generally consists of a microscope and/or camera to magnify and capture the image of the drop sitting on the solid, and a light source to illuminate the samples and enhance visibility of the outline of the drop.
The measurement system is the part that actually measures the contact angle from the image created with the viewing system, and is not a physical part of the instrument. This is usually done using a software program setup to trace the drop profile and calculate the angle at the contact point. With the correct calibrations these programs can provide additional information like drop volume, and contact area.
Step 2: Design
The instrument that I created was designed to solve several problems that were found to be causing unreliable results in contact angle measurements made with the previous equipment.
The major changes included the addition of a motorized automatic dispensing system to improve the instruments ability to repeatably produce drops of a precise specified volume, a software user interface to control the drop dispensing, improved stage mechanisms, and the consolidation of all of the components onto a common base with adjustable feet to allow leveling of the stage even on an un even surface. The design incorporates the microsyringe and microscope/camera from the original instrument, and everything else was purchased or machined for the new instrument.
The design was modeled in CATIA (V5R21)
Step 3: Dispensing System
The mechanical pieces of this system include the microsyringe, 4 parts machined from aluminum (three on a CNC mill, and one manually turned on a lathe), four 5/16" X 6" steel rods, and four linear bearings (8mm ID, 16mm OD).
The four machined parts are the motor holder, the syringe holder, the motor-syringe adapter piece, and the top piece. The motor holder was machined on a Haas CNC mill with a relatively simple G&M code created with CATIA. The four linear bearings where then fit into the four large holes in the motor holder, and held into place with two c-clips each.
The syringe holder was also machined (mostly) on the CNC mill and consists of four holes to hold the steel rods, and a counter bored hole which holds the syringe. Also, there is a set screw in the front to hold the syringe in place.
The top piece is the third (and simplest) CNCd part. Its purpose is to keep the steel rods in place and correctly aligned so that the motor holder can move up and down the rods like it is supposed to. There are set screws that secure the rods when tightened.
The turned piece is a round aluminum piece that is used to transfer the motion of the motor shaft to the plunger of the syringe. This motor-syringe adapter was machined on a manual lathe, and consists of one hole for the motor shaft and a larger hole on the other side to fit over the top of the syringe plunger. Both ends have a set screw to fix the adapter onto the motor shaft and syringe plunger.
Step 4: Electronics
The motor has steps of 1.8 degrees (200 steps per rotation), and it is mounted to the motor holder with four small screws and its wires are connected directly to the Arduino motor shield. The motor shield snaps/plugs right onto the top of the Arduino Uno board, which is then connected to a computer through a USB cable.
Step 5: Dispensing Software
The program was made using LabVIEW visual programming language, which I taught my self for this project. The LabVIEW programming language is fairly straightforward to learn and it has a great built in interface with Arduino which made it very easy to program it to control the Arduino boards that the instrument uses. Also, I was able to get a free trial version, which has since expired, that lasted throughout the project and made it possible to work on the programming from anywhere, on my laptop.
The interface itself has several pages that can be navigated to through a menu on the upper left hand corner. The first page is a welcome message that introduces the interface and provides a link that opens a PDF of the users manual that I wrote for the instrument. The next page contains the drop dispensing controls. This is where the user specifies the volume (in micro liters) of the drop to be created, and then creates the drop by clicking the "Dispense" button. Inside the program, what is happening is the input volume get divided by 5 (the syringe produces 5 microliters per turn), and multiplied by 200 (the stepper motor has 200 steps per turn), which converts the volume into the number of steps necessary to create the specified volume, then that is sent to the arduino through the LabiVIEW interface for Arduino (LVIFA) and turns the motor. The other two pages contain links to open the image capture software and the angle measurement software.
Step 6: Stage and Stage Mechanisms
The large rack and pinion, which provides the lateral motion, was from a precision camera mount and has about four inches of travel and is sturdy and already has usable mounting holes. This piece mounts to a manually milled aluminum block which sets the stage assembly to the proper height that allows the liquid drop to be in view of the microscope when it sits on the solid.
There is another milled block which mounts to the top of the large rack and pinion, and which the smaller rack/pinion mounts to. This smaller mechanism is actually originally from a microscope stage assembly, and provides the vertical motion for this instrument's stage. The stage mounts directly to this mechanism so it will move when either of the mechanisms are moved. Both directions of the stage motion are manually operated through the knobs on each mechanism.
The actual stage piece is a manually milled aluminum piece, with two holes drilled and hand tapped, used to mount it to the vertical motion mechanism.
Step 7: Viewing/Image Capture System
The camera is connected to the computer through a USB cable, and can be controlled with the image capture software (in this case QCapture Pro).
Step 8: Base
The base was made from a wooden shelf, cut to the right size with a several holes drilled (some tapped), and a rectangular pocket, cut out using a dremel as a router (for mounting the microscope). The dispensing system is mounted to the basing using two pieces of aluminum angle bracket and two steel rods. Also included on the base are the adjustable feet which are used to level the instrument.
Step 9: Assembly
Step 10: Using the Intrument
The first thing to so once the instrument is fully assembled is to connect everything to the computer (camera and Arduino board), and then collect and prepare the materials to be tested. For the liquids this includes removing the syringe from the instrument (requires removal of motor/holder and loosening of the syringe holder set screw), filling it with the desired liquid (being sure to rinse out any leftover liquid that might still be left in the syringe), and then securing it back in place. For the solids, this means cleaning the surfaces thoroughly - using a clean lab tissue with water or isopropanol, depending on what material the surface is - and ensuring that the surface is smooth and flat. Next, place using a bullseye level placed on the stage, level the instrument using the adjustable feet on the base, and then place the first solid to be tested on the stage.
Now, using the drop dispensing software, dispense a drop of liquid onto the solid (this drop will be used to focus/adjust the image, and should not be used for contact angle data). Then, using the image capture software, position and focus the microscope as necessary to obtain the best image of the drop that you can. The best image is one that makes the outline of the drop shape easy to see (and easy to trace later). It will take some trial and error to get the best combination of microscope angle/focus, image adjustments (brightness, contrast, etc.), and lighting (the light source is the only physical part of the set up that is not mounted to the main base, so that it can be moved around as necessary). The properties of the materials used will also change what setup will produce the best image (material color, reflective properties, etc.)
Once a satisfactory image is achieved, clean the drop off of the surface (with the same cleaning method used before), and place the solid back onto the stage. Now, to begin creating drops to be measured, specify the drop volume in the drop dispensing program and click dispense. Using the vertical motion mechanism on the stage assembly, bring the solid surface up into contact with the drop, and then move it back down (at least far enough so that the needle of the syringe is not in contact with the drop, and the drop is completely within the view of the microscope/camera. Make any necessary adjustments to the microscope/image, and then take a snapshot of the drop in the image capture software. Save this image to the computer, to be measured later in the angle measurement software. Repeat the previous steps to create and save all of the images that you will need (with my instrument, I recommend at least ten images of each liquid/solid combination that is being tested).
Once you have all of the images needed, it is time to open the angle measurement software (I am using ImageJ, with a "drop analysis" plugin). Open the first image to be measured and convert it to a black and white image and crop it around the drop is necessary/desired. Now, launch the drop analysis plugin (Plugins > drop_analysis > Drop analysis - LB-ADSA) and a new window will pop up with a green curve shaped like a drop. Adjust the "Drop Properties" in the new window until the green curve matches the outline of the drop in the image, and record the Contact Angle that is displayed. Repeat these steps to measure the angle in each image.
This video shows the instrument creating a drop and taking a picture of it in the image capture program... (the talking in the background is not related to the video, just other people working in the lab)...
Step 11: References
1. Kabza, Konrad G., Jason E. Gestwicki, and Jessica L. McGrath. "Contact Angle Goniometry as a Tool for Surface Tension Measurements of Solids, Using Zisman Plot Method. A Physical Chemistry Experiment." Journal of Chemical Education 77.1 (2000): 63. Print.
2. Pirie, Brian J. S., and David W. Gregory. "The Measurement of Wettability." Journal of Chemical Education 50.10 (1973): 682. Print.
3. Hansen, Finn K. "The Measurement of Surface Energy of Polymers by Means of Contact Angles of Liquids on Solid Surfaces: A Short Overview of Frequently Used Methods." University of Oslo, 2004. Web. 16 Nov. 2012. <http://folk.uio.no/fhansen/surface_energy.pdf>.
4. Rosen, Milton J. "Wetting and Its Modification by Surfactants." Surfactants and Interfacial Phenomena. 3rd ed. New York: Wiley, 1978. N. pag. Print.
5. Kabza, Konrad G., and Kevin Cochran. "From Polarimeter to Contact Angle Goniometer - Inexpensive Conversion of Laboratory Equipment." Journal of Chemical Education 74.3 (1997): 322. Print.
6. "Ramé-hart Contact Angle." Ramé-hart Contact Angle. Ramé-Hart Instruments Co., 2012. Web. 8 Nov. 2012. <http://www.ramehart.com/contactangle.htm>