Rame-Hart Goniometer

Table of Contents

Author: Audrey Hammack
06/25/2018

Overview

Introduction

This manual describes the use of the Rame-Hart goniometer. The instrument is located in the lithography bay (Bay 2) of the Cleanroom in NSERL at UTD. It is used to measure static surface contact angle of a water drop on a solid surface.

The measure of the contact angle indicates if the surface chemistry of a substrate is hydrophilic (“water-loving”) or hydrophobic (“water-fearing”). The magnitude of the contact angle depends the magnitude of attractive forces in between water molecules in the water drop relative to the magnitude of attractive forces in between water molecules and the chemical groups that make up the surface chemistry of the sample.  If a sample surface is hydrophilic, the attractive interactions between the water molecules and the chemical groups on the surface are greater than the attractive forces between the water molecules. This causes the water drop to spread out on the surface, and the contact angle is narrow. Conversely, if the surface is hydrophobic, interactions between the surface and the water molecules are less attractive than the forces between water molecules. As a result, the water drop does not spread, and the contact angle is very wide. This information is very useful to users that are attempting to optimize adhesion between layers of dissimilar materials.

Description

This section describes and indicates the components of the goniometer as they pertain to performing contact angle measurements.

The goniometer consists of a sample stage, where a substrate is placed for measurement. The stage is mounted to positioners that level the stage and translate the location in the x- and z-directions. A microsyringe assembly is located above the stage. The syringe dispenses a 2 µL drop of deionized water on the sample surface. The syringe assembly is mounted to the tool, and the location of the syringe can be adjusted in the z-direction.

Figure 1. Images of the goniometer, including (A), an image of the backlight illumination, the microsyringe and assembly, and the sample stage, and (B) an image of the knobs that manipulate the position of the stage

The sample surface and water drops are viewed through a specialized inverting telescope. An external illumination source provides backlighting to the stage in order to view the sample and water drops. The eyepiece has two reticles that are utilized in making the manual contact angle measurement. The reticle that is closest to the viewer is a fixed horizontal reference. The other reticle is graduated in 1° markings, and can be rotated. The position of this reticle, relative to the reference, makes contact angle measurements.

Figure 2. Images of the telescope and reticles. Figure 2(A), shows the telescope and the locations of the 1st and 2nd reticles. Figure 2(B), shows schematics of the reticles, including the types of markings and graduations. Figure 2(C) shows an image of the 2 reticles overlaid, as seen through the telescope eyepiece

Operation Instructions

Setup

This instrument is not on the Cleanroom Scheduler, and does not have a log book. The staff asks that you kindly share this resource with other users and coordinate usage schedules. The staff also asks that you please leave the tool as you found it, and please report any operational issues or problems.

1: Level the instrument using the knobs on the front left and front right of the tool. Monitor the tilt of the instrument by observing the location of the bubble in the level on the back of the instrument. Adjust both knobs until the level bubble is centered in the level. Be aware of parallax error that may result if you cannot look directly over the level. Use a step stool or ladder in order to look directly over the level.

Figure 3(A) shows the screws that are turned in order to adjust the tilt of the instrument. In Figure 3(B), the level on the back end of the instrument indicates tilt.

2: Turn on backlight illumination, by turning intensity switch

Figure 4. The source for backlight illumination is on the shelf above the instrument. The source is clicked on and off by turning the light intensity knob.

3: Place sample on stage.

4:Looking through the telescope, bring the sample surface into focus, using the droplet focus knob (Figure 3A).

5:Using the two knobs indicated in Figure 5, set the stage level with the round level from rear mainframe on top of sample stage, and use these two knobs to adjust stage to an accurate level. Looking through the telescope, adjust the horizontal reference line until it is parallel to the edge of the sample.

Figure 5. Image of the stage leveling knobs. Only adjust the knobs indicated by the red arrows.

6: Move the syringe assembly until the tip of the syringe is visible in the eyepiece. This may be a simple adjustment that only requires you to raise/lower the tip, or the whole assembly may need to be raised. The syringe tip should be ½ to ¾ of the way above the horizontal reference line. Look at the assembly to verify that the arm is parallel to the ground. Tighten any knobs that were loosened to make this adjustment.

Figure 6. Images of adjustment knobs on the microsyringe assembly.
Figure 7. Images as viewed through the telescope. Figure 7(A) shows the inverted image of the sample surface. The surface is in-line and parallel to the horizontal reference line. In Figure 7(B), the tip of the microsyringe is lowered ¾ of the way above the horizontal reference.

Contact Angle Measurement

1: Twist syringe until liquid builds a drop on the syringe tip. Line up the graduations on the syringe with the vertical reference mark (Figure 8A). Using the corner of a lint-free towel, absorb the water drop from the syringe tip. You are prepared for the next time that you will twist the syringe knob to dispense exactly a 2 µL drop of water

Figure 8. Images of the microsyringe, including (A), the 1 µL graduations, and (B), the procedure for using a lint-free wipe to absorb the initial drop from the syringe tip.

2: Twist the syringe knob exactly one notch. A 2 µL drop of water will hang suspended from the syringe tip.

3: Move the syringe tip down until the drop touches the sample surface, and quickly move the tip back up.

4: Look through the telescope and adjust focus until the image of the inverted water drop and the reflection is sharp. The reflection of the drop appears above the horizontal reference line.

5: Use the stage x-translation (Figure 1) to move the stage to the correct location. Line up the vertex created by the water drop +reflection with the intersection of the horizontal reference line and the straight line that bisects the measurement reticle

6: Carefully rotate the measurement reticle until the bisecting line appears to lie tangent to the edge of the water drop. Read the angle measurement through the telescope using the graduated lines on the measurement reticle.

Figure 9. An image of a water drop on the sample surface as viewed from the telescope. The drop and its inverted image form a vertex that is lined up with the intersection of the horizontal reference and the bisecting line. The 2nd reticle is turned, such that the bisecting line is tangent to the surface of the water drop. The contact angle is measured to be 16.5°

7: When a measurement is complete, a corner of a lint-free towel can be used to absorb the water drop from the sample surface.

8: The process of dispensing a fresh drop of water and making the contact angle measurement should be completed as quickly as possible, in order to minimize evaporation of the water drop. As water evaporates from the drop, the location of the drop edge does not change, but the amount of liquid decreases. As water evaporates, the angle measurement decreases significantly in a short amount of time. It may be very useful to spend some time practicing a quick transition from water dropping to making angle measurements in order to prevent measurement artifacts.

9: It is also best to take several measurements (5 or more water drops) from the left and the right side of the drops. This allows a user to report an average contact angle± standard error of the mean. These statistics compensate for variations in angle due to any water evaporation.

Clean Up

1: Remove sample from stage, and wipe up any drops of water that may have fallen on stage

2: Turn off backlight illumination by turning intensity knob off until it clicks

3: Verify that there is water in the syringe for the next user