Contact Angle meter/Gonimeter: Theory

Contact Angle: Theory

The following is a short introduction to the concepts involved in the measurement of contact angles. Included is an introduction to the techniques involved and some practical advise. For those interested in further information a list of references appears at the end of this chapter.

What is contact angle?
Contact angle ,q, is a quantitative measure of the wetting of a solid by a liquid. It is defined geometrically as the angle formed by a liquid at the three phase boundary where a liquid, gas and solid intersect.

It can be seen from this figure that low values of q indicate that the liquid spreads, or wets well , while high values indicate poor wetting. If the angle

q is less than 90 the liquid is said to wet the solid. If it is greater than 90 it

is said to be non-wetting. A zero contact angle represents complete wetting.

Hysteresis: For any given solid / liquid interaction there exists a range of contact angles which may be found. The measurement of a single static contact angle to characterize the interaction is no longer thought to be adequate. The value of static contact angles are found to depend on the

recent history of the interaction. When the drop has recently expanded the angle is said to represent the ‘advanced’ contact angle. When the drop has recently contracted the angle is said to represent the ‘receded’ contact angle. These angles fall within a range with advanced angles approaching a maximum value and receded angles approaching a minimum value.

The difference between the maximum(advanced) and minimum(receded) contact angle values is called the contact angle Hysteresis. A great deal of research has gone into analysis of the significance of hysteresis. It has been used to help characterize surface heterogeneity, roughness and mobility. You are recommended to the papers listed in the reference of this section for details on experiments regarding hysteresis.

Contact angle can also be considered in terms of the thermodynamics of the materials involved. This analysis involves the interfacial free energies between the three phases and is given by:

g_lv cos q = g_sv - g_sl

where g_lv ,g_sv and g_sl refer to the interfacial energies of the liquid/vapor, solid/vapor and solid/liquid interfaces.

Coatings: Coatings may be applied to any solid for testing contact angles. Microscope slides and cover slips make ideal supports for such an application. Ideally the coating should be applied to all sides. If you reduce the size of your sample by cutting after coating you will expose part of the support which is not coated. This will introduce an error in your data especially if the cut side is rough. Your data may be consistent enough to compare between similarly treated samples but the absolute values will not be accurate.

Tensiometer Kulim Kht

Installation in Malaysian Top University complete system

Potentiostat LPR technique

The LPR technique is the most frequently used being both quick and easy. A small sweep from typically -10 mV to +10 mV at 10 mV/min around the rest potential is performed. The resulting current/voltage plot usually exhibits a straight line the inverse slope of which is proportional to the corrosion rate. The Gill AC Gill 8 and 12 the Field Machine the Pocket Machine the LPR meter and the Bubble Test software all use this method.

Kulim Kht Contact Angle/Goniometer

Captive bubble method in contact lens studies

Contact lenses are widely used and hydrogel contact lenses are the most

prescribed type of contact lenses around the world. Hydrogel lenses are made from

hydrophilic polymers, such as polyhydroxyethyl. Contact lenses should have certain

characteristics to avoid discomfort during wear. These include an adequate oxygen

permeability, lens motion and surface wettability. A fairly recently discovered

material for contact lens manufacturing is the silicone hydrogel, which is suitable for

long term use as it has improved oxygen delivery on the ocular surface. However,

this material has been reported to have reduced wettability compared to the other

materials used in hydrogel contact lenses.

The wettability of a lens is measured on the abilities of the lens to support a

continuous and rupture-resistant tear film on its surface. A lens with a limited

wettability is uncomfortable and has a reduced visual performance. Therefore it has

been a major investment for the contact lens industry to develop a soft lens surface

that is highly wettable in the ocular environment.

How to study the wettability of contact lenses?

Contact angle measurement is the most common method used to characterize

wettability of solid surfaces. There are a variety of contact angle techniques that can

be used for this purpose: the sessile drop method and the captive bubble method.

Sessile drop and captive bubble methods can also be used simultaneously to

measure the different contact angles of a lens. Sessile drop method can be used to

examine the advancing contact angle and captive bubble method to obtain the

receding contact angle. Both of these contact angles can also be obtained only by

captive bubble method. Captive bubble method is very useful as it is possible to

maintain fully hydrated conditions on the lens surface during the experiment. This is

essential as the drying of the lens causes the setup to be far from the ideal

conditions. Figure 1a illustrates the sessile drop method, where a drop of water is

replaced on a glass slide. The contact angle obtained with this method was 140°. In

Figure 1b the contact angle of an air bubble in water is obtained with the captive

bubble method and it is calculated to be 40°. Contact angle hysteresis is the

difference between the advancing and receding contact angles. It can be used to

characterize the surface roughness, chemical heterogeneity, re-arrangement of

molecules at the surface, solid swelling and mobility. In highly water wetting lenses

the contact angle hysteresis is eliminated and both the advancing and receding

contact angles approach zero.

Captive bubble technique

In the captive bubble method the contact angle is measured by placing a bubble of

air onto the lens surface with a capillary while the lens is immersed in a liquid.

Preferably the lens should be soaked in liquid for a couple days before the

measurement takes place in order to get the lens fully wet. The conditions in this

experiment should mimic the in vivo conditions as good as possible, meaning that

the liquid used in the contact angle experiments should be close to the real nature

of a tear film in the eye. The tear film is composed of surface active components

such as mucin and lysozyme.

Receding and advancing contact angles are both important measures of contact

lens wettability. A receding angle forms as the water phase moves away from the

capillary. This corresponds to the situation when the eyelid is open and the tear film

starts rupturing on the lens. The advancing contact angle develops as the water

phase moves towards the capillary. This is the case when the eyelid closes and the

tear film on the lens starts to recover.

A solution for finding improved wettability?

With captive bubble method contact angles of lenses can be measured in different

solutions to obtain the most ideal solution that wets the lens best. Some have

reported that significant change can’t be seen in whether the solution is pure water

or a liquid containing surfactants (like the contact lens solutions on the market).

Also studies of different coatings to improve the lens wettability have been made.

Protein coatings have found to be successful in improving the wettability of contact

lenses. Protein coatings resist the tear protein adsorption and deposition into the

lens.

Biolin Q-Sense Kulim Kht QCM-D

QCM-D Technology

By collecting both the dissipation and the resonance frequency of a quartz crystal,
QCM-D technology can be used to characterize the formation of thin films (nm) such as proteins, polymers and cells onto surfaces, in liquid. This is QCM-D, Quartz Crystal Microbalance with Dissipation monitoring, Q-Sense's proprietary sensor technology.

In liquid, an adsorbed film may consist of a considerably high amount of water, which is sensed as a mass uptake by all QCMs. By measuring several frequencies and the dissipation it becomes possible to determine whether the adsorbed film is rigid or water-rich (soft) which is not possible by looking only at the frequency response. The amount of water in an adsorbed film can be as high as 95% depending on the kind of molecule and the type of surface you are studying. Picture elongated molecules - if they would adsorb flat on the surface, little water will be coupled to the molecules. However, if they adsorb standing up at the surface, lots of water will be coupled. With QCM-D the kinetics of both structural changes and mass changes are obtained simultaneously.

Would you like to ask specific questions or gain more information about the QCM-D technique? You are welcome to contact us at any time.

The QCM-D Principle
Unlike all other QCMs, QCM-D monitors the response of the freely oscillating crystal, which is faster and more accurate than the usual frequency sweep principle.

ASTM D971 standard Tensiometer Kulim Kht .

Performing an interfacial tension measurement

between water / oil according to ASTM D971 standard.

1. Check the instructions in section 5.1 and make necessary preparations.

2. Set oil density to correct value. You may also measure the density of the

oil (see section 5.2).

3. Make sure that the correction method of your preference is selected. You

can change the active ring correction method from Adjust parameters

menu. To be strictly compliant with ASTM D971 standard, choose

Zuidema-Waters. However, Huh-Mason is a more modern method that is

generally regarded more accurate.

4. Pour distilled water at a temperature of 25 ± 1°C into cleaned sample

container to a depth of 10 to 15 mm. Place it in to the middle of the

tensiometer platform. Make sure no foam is present at the water surface.

If foam is visible, discard the water. Foam indicates that the water is

contaminated or that the vessel is not clean.

5. Mount the ring in the tensiometer hook. If the oil to be measured is denser

than water, the measurement must be performed in Push Mode1, and the

supplied weight therefor needs to be attached to the ring before mounting

the ring.

6. Select "Measure oil interface" from the menu selection and press the OK

button. Sigma will now measure automatically the surface tension of the

water. Wait until the measurement has completed. If the measurement

was successful, the ring will automatically descend back into the water,

except in the case of Push Mode measurement with dense oil, however,

the ring will remain about 5 mm above the water.

7. When the ring has stopped moving, carefully layer the oil on the surface of

the water, until an oil layer of at least 10 mm is reached2. Do not move

the sample container from tensiometer platform, nor move the ring.

8. Press the OK button right after the oil pouring is finished. The tensiometer

will then wait for 29 seconds (as the standard requires a wait between 29

and 31 seconds), until it starts the measurement of the interfacial tension

between water and oil. Wait until the measurement has completed.

9. Interfacial tension value is shown on the display.

As the ring is pulled through a surface or an interface, a curve is plotted,

showing the tension value calculated from the balance reading at each

instant. The highest value encountered will be displayed as the result of the

measurement.

1 If the oil is denser than water, it forms a layer below water and interfacial tension must

measured by pulling the ring downwards from water to oil; hence the name Push Mode. The

case of Push Mode measurement is not explicitly addressed in ASTM D971 standard.

2 Layering the oil should not take more time than 30s since completing the entire interfacial

tension measurement have to be done within 60s ±10s

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