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Introduction to the Instrument Tilt Stage

This document will help you get started with your new instrument tilt stage.

The instrument tilt stage can tilt FTA188, FTA200 and FTA1000 systems through 90 degrees for advancing and receding contact angle measurements. The FTA2000 has an internal, built-in tilt stage.

It is not necessary to home the tilt stage or level the tilt stage before using it. These and other setup topics are therefore discussed later in this document. However, it is necessary to make sure the instrument is securely fastened to the tilt cradle before operation and to also make sure the sample stage is secure so that it will not slide off.

Tilt Stage mounted with an FTA188

FTA188 being tilted by the stage.

For additional information on the stage and tilting plate experiments, see Tilting Plate Example and Tilting Plate Readout.

Tilt Angle Readout (including FTA2000 tilt stage)

The current instrument tilt angle is always shown on the Stage | Tilt tab. When the tilt stage is moving, the tilt angle is written into the SnapShot or Movie. Beginning with Build 223, the angle is also written into the SnapShot or Movie if it is greater than 1 degree. Angles less than 1 degree are assumed to be static and level and therefore not part of a tilting plate experiment.

Note that the actual tilt angle plays no role in the final advancing and receding contact angle calculations. It is simply a mechanism to distort the drop through gravity. The contact angles are measured independently of the tilt angle's value.

Connecting the Stage to the Computer

The stage is driven by a controller box connected to the computer through an RS-232 port. The controller box is powered by an external 12V universal table-top power supply that accepts any voltage from 100 to 240VAC. There is no on/off switch and the controller can be left powered. The stage will not move when power is removed because of the pitch of the leadscrew.

The following connections must be made:

  • D25S cable from the tilt stage plugs into D25P connector on the controller.

  • RS-232 D9P cable from computer serial I/O plugs into D9S connector on the controller.

  • Power supply circular jack plugs into mating connector on the controller.

You must check the Instrument Tilt Stage option in the Hardware portion of the LogOn screen for the software. Your choice will be remembered, so it only need be done once. You must select an available RS-232 port for the serial line on the same screen. The tilt stage is controlled from the Stage | Tilt tab in the Fta32 software. Make sure the tilt stage controller box is powered before opening the Fta32 software. The box is initialized when you open the Fta32 Video window. If you seem to loose control of the stage, close Video and re-open it.

There may be additional connectors on the controller box if other accessories are driven by it.

Advancing and Receding Contact Angles

The Move command is the command used in ordinary operation. You specify the tilt increment, e.g., 45 degrees, and the tilt rate, e.g., 1 degree per second, on the Stage | Tilt tab. If you want the stage to return to the zero position after a tilt movement, you specify a negative increment. The Stop command can be used as an emergency stop of tilt (but not of Home) movements.

Do not operate the stage without making sure the instrument and sample stage are securely fastened to the tilt stage and all cables are placed so they will not bind.

Your general protocol will be

  • Setup to Capture a Movie. The Movie must be as long as the tilt operation will take. An option is to Trigger when the drop starts to move, or roll, in which case you will want to use the Video Trigger and you will want some number of images pre-trigger.

  • Deposit the sessile drop.

  • Start the Movie Run.

  • Start the tilt Move.

  • After the Movie is captured, analyze the drop shape just prior to actual movement.

The secret to these measurements is to understand that you want the drop shape just prior to actual movement. Furthermore, the advancing side may move slightly before the receding side does, as the drop shape, when viewed from above, is no longer circular.

The actual angle of the tilt stage is irrelevant to the measurement. You just have to be sure the drop is ready to move, and the best way to ensure that is to have it actually move and then go back a frame or two.

Gravity is, of course, the force that makes the drop move. It is opposed by the adhesion of the drop to the sample surface. It is best to use a large drop (you expect the drop to be non-spherical anyway). The benefit of this is that the gravity force accumulates faster than the adhesion force as the volume is increased, so a large drop will roll at a lower angle. This has two benefits:

  • The drop will evaporate less during the tilt motion.

  • It will roll in a "straighter" line if the tilt angle is not extreme, so it will remain in focus.

Gravity force is a function of the cosine of the tilt angle, so an angle of 45 degrees has 70.7% of the maximum downward force available. A tilt angle of 30 degrees provides 50%. You should attempt to achieve rolling by 45 degrees of tilt. The full 90 degree range is provided only as a matter of principle and to salvage experiments with marginally small drops.

Tilting is done slowly so as to minimize vibration. Values of 0.5 and 1 degree per second are typical.

The upper limit of the standard motor is just over 1 degree per second. A doubling of speed can be obtained by substituting a 1.8 degree per step motor rather than the 0.9 degree per step motor used. (These constants refer to the motor shaft, not the tilt table itself, which is geared down greatly.)

Software Setup Constants

Motor parameters are setup on the Stage | Tilt tab. The most important of these is the Gear constant which sets the number of controller steps per degree tilt. This value should be considered the calibration constant for the stage: the stage will move the desired distance and at the desired rate only if this number is correct. The standard motor is an Oriental Vexta PX245M-03AA, a 0.9 degree per step motor, and the value for it is 8000.

The remaining setup constants are straightforward. The maximum recommended rate is 1 degree per second. The Home Offset value is the distance the stage should move after finding the home sensor flag in order that the final position truly be zero degrees, or level. This number varies from stage to stage, but is typically 1 or 2 degrees. The low and high range limits are so you can set software limits on stage motion.

Leveling and Homing the Stage

It is not necessary to level the tilt stage before using it. This is discussed above in the measurement protocol section. However a homing mechanism is available to always start at the same "zero" position.

You can bring the tilt cradle to a level position by using the Home command. You will need to use a bubble level in order to precisely set the zero tilt position. The stage will move down to a nominally level position, then move back and forth to get the best Home position.

After finding Home, the stage will do a movement to the Home Offset position, if a non-zero position has been specified in the Driver Setup frame on the tab. You must adjust this number so the position the stage comes to is truly zero, or level. If you miss the first time, use ordinary Move commands to find the level position as indicated by the bubble level. Change the Home Offset value and then repeat this procedure as a check. Remember your Range limits may preclude a motion when using the Move command, so you may have to change them also.

Once you have leveled and homed the mechanism, the software should keep track of the tilt cradle's position, even when the software is closed and reopened or the instrument depowered, so you should not have to repeat the Home operation often.

Belt Tension

The drive belt is hidden under a protective cover. This connects the motor to the lead screw. The belt should not need service or tension adjustment in ordinary use. However, if the belt is replaced or re-tensioned, it should not be too tight, else this will bind the motor bearings and decrease how fast the stage can run. A good rule of thumb is that you should be able to squeeze the two belts together about half way with moderate finger pressure. It is OK if you see some slack in the return side of the belt when the motor runs.