Measuring the equilibrium properties of a drop


To make any measurement with this instrument the computer and the control unit must be turned on. Start the software by clicking on the WDrop icon, and then load your configuration file (for new users copy the one in Data/Rob into your directory and then use the copy). Install a syringe, needle and the sample stage or cuvette you wish to use. Move the needle until it is visible on the monitor then follow the 10 steps below (please note most commands can be obtained in various ways only one obvious method is listed below)
Then make the measurement as described in the last section.


Instrument Setup

1. Focus

This enables the best optical resolution (both spatially and in contrast) of the drop to be obtained and hence the most precise measurements possible. In the pull down Workshop menu select Focus (the Focus window will appear), on the left the image is displayed with a short white line (the length of which can be changed using the yellow arrow buttons) overlaid and on the right a graph of the intensity of the pixels across that line. Move the line to the edge of the needle or drop (using the red arrows or the mouse). Then adjust the gain and offset (using the sliders and the number buttons) until the curve goes from about 10 to 200 on the grey level axis. Now adjust the camera position by loosening the front of the two locking screws (visible under the camera once the dust cover is removed) and sliding it towards and away from the needle to sharpen the image (the adjustment should result in the gradient of the graph increasing). Once the maximum gradient is achieved the camera can be locked in position. Once you have completed this click ok then the confirm box will appear click yes to save the new parameters.


2. Adjusting the optical threshold

This is the value at which the software distinguishes between the drop and the background it should be set at the steepest part of the curve found in the previous section (normally around 100). In the pull down Workshop menu select Histogram (the Histogram window will appear), on the left the image is displayed and on the right a histogram plot of the number of pixels at a particular intensity. Click go and the histogram will appear select a threshold (that falls between the peak at high grey level and the peak at low grey level) and then click on the light bulb, the drop should appear black on a white background. If the outline is not smooth and does not represent the drop properly adjust the threshold until it does (if you cannot achieve a sensible out line it will be necessary to adjust the camera parameters in the focus window. Once you have completed this click ok then the confirm box will appear click yes to save the new threshold.


3. Adjusting the orientation of the camera

Generally this does not need adjusting however if you suspect the vertical axis of the camera is not vertical it can be adjusted using a plumb line installed in the syringe holder (wait until the line is stationary). In the pull down Workshop menu select Vertical Setup (the Vertical Setup window will appear), on the left the image is displayed (with a cross hair overlaid) and on the right are two dials. If the camera needs adjusting (i.e. the plumb line is not parallel to the vertical line of the cross hair) the locking screws adjacent to the camera les need to be loosened and the camera rotated until the plumb line is parallel to the vertical line of the cross hair. Once this course adjustment is completed, with the mouse click and drag to draw a box containing the line and then click go the dials will start to read the angle. Fine adjust the camera until the error is less than 0.1°, then lock the camera. Once you have completed this click ok.


4. Checking the Optical Calibration

Generally this does not need adjusting unless instrument has been changed dramatically. It is required to check the magnification of the system by using a calibrated ball (mounted on a slide attached to a needle). In the pull down Workshop menu select Optical Calibration (the Optical Calibration window will appear), on the left the image of the ball is displayed and on the right are some data entries record the initial values of x and y parameters (make sure the ball diameter corresponds to the ball you are using). Move the cross hair until it lies inside the ball, click on the light bulb and the ball should appear black on a white background if it does not repeat the focal and threshold adjustment. Once the optical parameters are correctly set click on the go button x and y magnification parameters are calculated (if they are dramatically different (more than 1%) from the initial values contact the SAF if not click ok then the confirmation box will appear click yes to save the new magnification parameters.


5. Adjusting the orientation of the needle

Once the camera is correctly orientated it can be used to vertically align the needle. In the pull down Workshop menu select Vertical Setup (the Vertical Setup window will appear), on the left the image is displayed (with a cross hair overlaid) and on the right are two dials. Adjust the holder syringe tilt (using the knob on the back of the holder) until the needle is approximately parallel to the vertical line of the cross hair. Once this rough adjustment is completed, with the mouse click and drag to draw a box containing the needle and then click go the dials will start to read the angle. Fine adjust the needle until the error is less than 0.5°. Once you have completed this click ok.


6. Adjusting the drop frontier

This is the region within which the software expects to find a drop one edge will be the needle or the surface onto which the drop is placed. Adjustment can be obtained in the majority of the windows or where Frontier is displayed for instance in the measurement setup window obtained from the experiment drop down menu. The horizontal line should be placed at the edge of the needle or on the surface and the vertical lines should be put as close to the edges of the image as possible (but not onto other solid objects!). The horizontal frontier can be split using the hand pointing buttons, for a rising drop/sessile down (see Selecting the measurement configuration) the upper line defines the limit of data used in calculation and the lower line gives the limit to which the calculation is extended to obtain contact angles. For a pendant drop/sessile up the inverse is the case.


7. Calibration of the Syringe

For precise drop volumes it is necessary to calibrate the volume expelled from the syringe by one rotation of the motor. In the pull down Workshop menu select Volumetric Calibration (the Volumetric Calibration window will appear), on the left the image is displayed (with a frontier overlaid) and on the right at the top is a graph and below it a data entry. The frontier must be placed at the tip of the needle (at the bottom of the screen for a rising drop and the top for a pendant drop) see the previous section. Select the correct drop configuration and syringe number, and form a small drop with the manual motor commands. Click go the drops volume should increase linearly producing a straight line on the graph at the end of the calibration or when you press stop a value should appear in the microlitre per round box. Click OK and the confirmation window will appear at click ok to accept the calibration.


8. Entering the material properties

To enable the software to calculate the interfacial tension the material densities need to be entered. Open the Measurement Board by clicking on measurement setup in the experiment dropdown menu. In the top left corner enter the parameters for the drop density, r (using the polynomial expression: r = a + b(T-25°C) + c(T-25°C)2 + d(T-25°C)3) where a is the density at 25°C and b, c and d are parameters defining the density's temperature dependence. Adjacent to the drop density the bulk density parameters must also be entered for a similar expression.
For water:

a = 0.99704 Kg/dm3
b = -2.4748 x 10-4 Kg/dm3 °C
c = -4.7006 x 10-6 Kg/dm3 °C2
d = 1.5324 x 10-8 Kg/dm3 °C3


For air:

a = 0.001170 Kg/dm3
b = c = d = 0


For dilute solutions the water values can be used for all other materials the parameters for that material must be entered.


9. Selecting the measurement configuration

Open the measurement board as described above, and below the density parameters, select the drop status from the menu. A rising drop is formed when a u shaped needle is used, a pendant drop is formed from a straight needle, a sessile down is a drop deposited on the upper surface of a sample and sessile up is a drop deposited on the lower surface of a sample.


10. Selecting the calculation precision

This enables you to select the precision of the calculation (and thus the time taken for the measurement). Normal corresponds to the fastest and least precise calculation, high precise is the most precise and hence slowest. Precise is a compromise between the other two. It is possible for the calculation to be completed independently on both the left and the right sides of the drop, useful when measuring contact angles by selecting L/R precise or L/R High Precise modes. For almost all measurements the High Precise mode is recommended.


Making the Measurement

Manually form a drop using the control box or manual motor control, then from the Measurement board or in the Experiment pull down menu select One image analysis, the one view board will appear. At the top left is the image with the frontiers superimposed on it along with some of the other image parameters (which can be adjusted). As well as the light bulbs there is a zoom button to enable you to view a magnified part of the image. Select standard error display and the click go. On the top right the results will appear and on the bottom right an error graph is shown. If the error graph shows a random scatter around 0 the drop is a good Lapalcian drop if not the instrument is not set up well or the needle was vibrating when the image was taken (try a second image, if it still does not fit well repeat the Setup).