Introduction to the FTA136
This document will help you get started with your new FTA136.
The FTA136 uses Telecentric fixed magnification optics, but the magnification can be changed over some range by moving the lens closer for higher magnification or further for lower. The horizontal field of view of the lens is approximately 10mm. An accessory x2 lens is provided for a 5mm field of view.
A single cable connects to FTA136 to the computer's frame grabber. The connectors are arranged so they only plug in one way on the FTA136. Make sure you plug the computer end into the FTA supplied frame grabber and not some other function that might have a similar connector on the computer.
The FTA136 can be operated in two fundamentally different ways:
Top down view of a low contact angle sessile drop on a transparent sample. This is the most accurate way to measure contact angles less than 20 degrees. In this mode, the viewing path is straight down from the lens. A small flexible needle is guided over the center of field of view and dispenses liquid onto the sample. The top view method can be used for all contact angles, even above 90 degrees, but it loses accuracy above 90 degrees and requires knowledge or measurement of the dispensed volume (discussed below), so is not the preferred approach when side view measurements can be made. Below 20 degrees, though, there is no question top view is preferred.
Conventional side view of sessile or pendant drops. A 90 degree mirror turns the viewing path of the lens from vertical to horizontal.
The instrument is shipped in the top view configuration. The top view configuration uses a separate incandescent back light. It plugs into the original side view light box. The following figure shows a top view image. The contact angle of the water on the glass is 5.05 degrees.
One particularly nice feature of the FTA136 software is that the entire drop does not need to be in the image for an accurate measurement. This is because the regression algorithm can predict the drop diameter from only a small portion of the edge curvature. In addition, the asymmetry of the drop is reported. This tells you how uneven the surface is chemically. The straight yellow construction lines in the image show how the regression is working. The dashed yellow line is the best fit circle to the shape.
Syringe and Needle
A Gilmont micrometer type syringe is supplied. It has a 2 milliliter glass burette with a Teflon plunger. It is easily disassembled for cleaning. Standard Luer-hub needles attach to the burette.
When used in the top view mode, a long flexible fused silica needle is used. These have the trade name MicroFil. The 34GA MicroFil is the best choice. The user cuts these to length so the end falls in the image. The rack-and-pinion adjustment on the syringe will fine-tune the needle position to the center of the image.
If you want to do traditional side view work, needles are available in a wide variety of sizes and materials. A small selection of needles is included with the instrument, but you should ascertain what sizes are appropriate for your work and purchase them separately.
Dispense liquid by turning the topmost portion of the syringe barrel clockwise. You can retract liquid by turning counter-clockwise.
The syringe drops into the holder and is secured by a thumbscrew on the right-hand side. Use just enough pressure to prevent the syringe body from turning when you twist the barrel to dispense liquid. It is possible to set the thumbscrew so you can remove and replace the syringe without touching the screw, yet have enough friction to keep the syringe body from turning during dispense. The syringe rides on a rack-and-pinion Z axis. Adjust this up and down as required.
The syringe body is marked so you can tell how much volume the plunger has displaced during a move. The smallest graduations on the scale correspond to 2 microliters, but you can adjust the volume to a fraction of a microliter.
There are two spare syringe holders to the side in case you have multiple syringes.
Always keep your syringe clean. Washing with methanol, followed by a thorough dry, is sufficient for most test liquids. Try to prevent air bubbles when you fill the syringe. One way to clear bubbles is to turn the syringe upside down and dispense a small amount. This will push the bubbles up and out.
Microscope and Camera
The lens, or microscope, has two adjustments: focus and aperture.
Focus is adjusted by moving the bottom portion of the lens closer or further away from the drop. A chrome thumbscrew secures portion of the lens that moves to focus.
The aperture, or iris, is the ring with numbers on it like 2.8, 11, and 32. It also has a thumbscrew to secure it. The highest number is "C" for closed. The 2.8 end is the open aperture position which lets the most light in. Always focus with an open aperture.
When the FTA136 is used in the top view mode, the aperture should be open or almost open. You should not close the aperture to such a degree that the corners of the image become dark. The purpose of using a smaller, partially closed aperture, is to increase the depth of field. The side view optical setup allows you to close the aperture more before the image becomes dark.
The FTA software has a Focus tool which is enabled on the Analysis | Live Analysis tab of the Video form. This will show a profile of gray scales about the cursor: place the cursor over a sharp edge in the image. Adjust the focus for greatest sharpness. The way most people do this is to rock the focus back and forth to determine the best central setting.
Magnification can be calibrated by measuring an object of known size. A 3mm diameter sapphire ball mounted on a support base is provided for calibration. The calibration procedure is
Place the sapphire ball in the image and focus the image.
Take a SnapShot.
Measure the diameter. The easiest way is with a top view Contact Angle measurement, but you can use the Distance tool also.
On the Calibration tab, enter the known distance, 3mm, and the measured distance from the previous step, then click Apply. This will update the magnification calibration.
It is important to use the same focus setting for the magnification calibration as you use for the real measurement. Since glass comes in different thicknesses, and you want to focus on the top side of the sample, the sapphire ball can be screwed into its base in many different positions, each corresponding to a different thickness.
The main instrument and the illumination base are in two separate parts so large sheets of glass can be placed between them. The illumination base always goes under the glass sample. Small pieces can be laid on the illumination base directly.
Top View Contact Angle Measurement
Contact angles can be measured two different ways, keeping in mind the volume of the dispensed drop must be known:
Measure the average volume that falls off this class of dispense tips before beginning measurements. Typically you will dispense 10 drops into a small weigh boat and measure on a microbalance. The advantage of this approach (compared to the following one) is that you can use SnapShots. The disadvantages are that the drop must fall clear of the tip onto the sample and you have to trust the statistics of the earlier drop volume measurement. The average volume is entered in the text box on the Contact Angle tab.
Take a Movie of the drop on the tip, its detachment from the tip, and its spreading on the surface. Check the checkbox on the Contact Angle tab to Auto Measure Dispensed Volume. When you click the Contact Angle tool, a macro will inspect all movie images from the currrent one to the end. It assumes the initial drop is hanging on the tip and it measures that volume. When the drop detaches, the drop diameter will increase and that is used as a signal to switch to contact angle measurements from drop volume measurements. This method has the advantages that the actual volume is measured and the drop need not fall clear of the tip on to the sample (it may touch-off). The disadvantage is that you must take a Movie and sort through it, although the macro does most of the work.
For further information, see Top View Contact Angle Analysis.