Several surface tension techniques of Nanoscolo


Why measuring surface tensions or contact angles? Because these techniques can provide a lot of information about the thin layer between 2 phases: liquid and air, solid and air, liquid - liquid or solid - liquid. The surface has other properties than the bulk and adhesion and wetting are affected. With scanning probe microscopy this can be studied in detail, but our larger scale techniques are faster and can study much larger surfaces. Interfaces are dynamic and with bubble techniques we can study this down to milli-second time scales. With wetting tests we can probe large surfaces in short time. Often those wetting phenomena provide the ideas, locations on samples, conditions, that can be studied later in great detail with the other physical surface techniques like our scanning probes.

It is a huge advantage that our physical techniques can be used at controlled ambient conditions, unless electron microscopy or XPS / ESCA that need high vacuum that changes the conditions at the interface.


The picture shows the KSV Sigma 70 force tensiometer, the CAM 200 optical tensiometer and contact angle meter, and the SITA maximum bubble pressure technique. All equipment is controlled by one computer and the Attension software of Biolin Scientific enabling to concentrate the techniques on 150 cm table space. Additional tools for captive bubbles, tilting plates, fibers, Washburn wetting of porous systems and powders, sedimentation balance and density measurements of solids and liquids, and more are available. Related techniques like our KSV NIMA Langmuir-Blodget trough and coater will be discussed in the coating part.

More information about the techniques can be found on Wikipedia and by several excellent suppliers like Biolin, Kruss, Data Physics and Kibron. Also the explanation of different techniques in the books of J. Lyklema, Fundamentals of Interface and Colloid Science, especially volume III: Liquid - Fluid interfaces is helpful to better understand the techniques. Below we provide more Nanoscolo specific extra information and some information about how all of this can help you solving problems or making better processes and products. 

Why measure liquids in contact with solid surfaces? 1) To measure properties of the liquid, mainly the surface tension. This is an important parameter related to the spreading and wetting of the liquid. The surface is needed to support a drop or to feel the tension. Next to coating also in foams, emulsions and fouling research these techniques are extensively used.  2) To measure surface properties of the solid surface. Example of such test is the water contact angle measurement on a plate. Indirect this tells about adhesion of inks, paint or adhesives but to actually solve problems or understand phenomena it should be considered part of a broader research. Just the contact angle number alone is not sufficient because drops affect the surface and the surface can affect drops especially if it is modified by plasma or corona treatment. Surface roughness also affects the contact angle.  A contact angle can be small because a surface has a high surface energy, because it has surface active components that reduce the surface tension of the drop, or because the surface is rough.

Working principle of the force tensiometer

The principle is simple; feel the force during the displacement of the sample to a probe. Result is a force-distance relation of the probe approaching, contacting and inserting, and subsequently retracting from the sample. Depending on the length-scale of the experiment and sample properties there are different techniques. Note that the principle equals that of the AFM - scanning probe microscope of the previous chapter. The difference is the force- and length scale. And the time-scale. Due the the much longer displacement distance the scanning in the horizontal directions to obtain an image would take too much time.

The typical probe of the force tensiometer measures forces in the order of 10 nN - 1 N and the displacement is from µm to several cm. Typical probes like a Wilhelmy plate, a Du Nouy ring, or a sedimentation probe have a size of about 2 cm but with the sensitive ultra-balance that we use also fiber probes from 4 µm diameter are used. We explore the domain between AFM and force tensiometers. High surface energy probes, like the platinum Wilhelmy plate are completely wetted by most liquids and the product of the perimeter and surface tension is then the measured force when the plate is just at the surface level in contact with the liquid.

It is also possible to inverse probe and sample by filling the beaker with a calibration fluid like water and attach the sample, e.g. a fiber or a polymer plate, to the balance. This is an alternative way to measure dynamic contact angles with the advantage that the complete sample is scanned avoiding the need to measure many drop contact angles to obtain a statistical trustworthy average.

A tensiometer refers to the surface- or interfacial tension but, like with the AFM, many other physical properties are felt. Density (buoyancy), viscous drag and elastic properties (rheology) can all be studied with appropriate probes. At Nanoscolo we develop such techniques and those are the only proprietary information that we will not share yet with our customers. 


Working principle of the optical tensiometer and contact angle meter


Again the principle is simple. This technique makes pictures or (high-speed) movies of drops and wetting phenomena. The camera is calibrated to have an accurate value for the size of the pixels, typically in the order of 10 µm but we can go to less than 1 µm by changing the optics and measure picoliter drops. By assuming an axisymmetric geometry we have the full information about volumes and surface curvatures and via the density of the liquid of the drops also the gravitational force. Enough to use Young-Laplace to calculate the surface tension and obtain more accurate values of the contact angle of a drop with a substrate. Again, both the liquid (surface tension measurement) and the substrate (contact angle, surface energy) can be samples.

The axisymmetric assumption can be wrong in sessile drop measurements. Many surfaces are not isotropic in composition or surface topology / roughness. Very small contact angles are difficult to measure. Calibration liquids can affect the sample e.g. by water absorption. Worse for Owens Wendt surface characterization with several calibration liquids, e.g. water, di-iodomethane, formamide and glycol that are standard in the dispensers of the picture. It is important that these liquids do not affect the substrate and vice versa. Contact angles on thin fibers can not be simply direct measured by this method and also the meniscus on vertical plates in a liquid is too much affected by edge effects that are usually not shown in the textbooks. Hysteresis and advancing versus receding contact angles and dynamic contact angles that are affected by viscous drag. There are so many complications and specific conditions that make this type of measurements operator and method dependent and thereby not so useful unless all details are known and there effects understood. Some detail tips and examples;

Always look also to the drop from the top to see whether the contact line and projection of the drop is a circle. If this is not the case then check the direction of the deformation of drops and whether all drops have this direction and measure drops in more directions. Also align this with other tests like the peel direction in subsequent mechanical tests for adhesion. A practical method is to measure the baseline length of the drops. Axisymmetric small drops have the shape of a sphere cap and it is then not so difficult to calculate the relation between the length of the baseline and the contact angle for a given drop volume. The drop volume can be adjusted rather precise and contact angle - baseline length values are then supposed to follow the calculated relation as shown below for 3 µl drops.  This can also be a more accurate way to obtain the contact angle if its value is very small. Measure the length of the baseline and calculate or use the graphs. Deviation from the line can be because of asymmetric drops, absorption of drops in the substrate, evaporation, or sometimes the software has difficulty to find an accurate base-line. Measure also large drops. Those deviate from the sphere cap geometry because they are flattened by gravity. This can be used by Young Laplace theory to calculate and verify whether the surface tension of the drop has the correct value. It may be that surface active material is released from the substrate and in such case the contact angle may be low by this effect and not by a high surface energy of the substrate. Make the substrate dry with a tissue after measuring and look for marks, staining by the test liquids. These are indications for solvent interactions that change the surface properties. Treatments like cleaning can have a large effect and if contact angle measurements are done to better understand adhesion then the surface should be prepared like done when the adhesive is applied. Because we measured already contact angles before the computer time area using photographic pictures we still report values after 30 seconds contact. This was the typical time needed to take the picture and like this we could compare new measurements with old data. In current measurements the evolution of the drop in time can be accurately measured and most of the time this has less to do with dynamic wetting as found in literature but more with surface adaptation, absorption and evaporation.


Adaptive surfaces and the difference between humid and wet


Instead of measuring the baseline of drops with a very small contact angle it is also possible to measure a captive bubble under the surface immersed in water. The same fluids are in contact and should result in the same contact angle at the three phase contact line on the substrate. The bubble is then easier to measure because the large contact angle side is in the bubble (theory picture below). The contact angle of the bubble is 180° minus the contact angle of the drop.

However, in most materials the surface adapts to the environment and the environment in water (wet) is different from the environment in air. For example in polymeric materials the properties of the surface is depending on the preparation and climate. Especially at high humidity surfaces of polymers tend to be hydrophobic and therefor difficult to coat or adhere. This is because humidity can plasticize the polymer enhancing the mobility and enabling the more hydrophobic groups to go to the surface to reduce its energy. Under water it is energetically more favorable to have the hydrophilic groups in the water. The theoretical picture is therefore seldom found in practice and the question "what is the surface energy of polymer X?" should be answered with "depends". Nevertheless, values of various polymers are tabulated in literature. Probably these are pure homopolymers processed under inert conditions and even then the cooling rate and type of surface of the mold shaping the surface can have significant effects.  By (cryo)-microtomy the bulk can be exposed and a fresh coupe can be measured and usually reacts different than the surface. Important to realize that the production of plastics is much like cooking food. There are many additives that affect bulk and surface properties (glass fibers, flame retardants, impact modifiers, mold release agents, pigments, fillers, and more) and also the preparation, time - temperature, inert or exposed to air, shaping materials and blending affect bulk and surface. It is the combination of techniques that enables to learn about such complex materials in a short period of time.



Contact angles on fibers

On a curved surface like a fiber or inside a capillary the contact angle is more difficult to obtain by optical methods. Also in the case of complete wetting the film is not stable and will break-up into drops on the fiber or trains of water alternated by air in a capillary. The profile of the drop is again governed by Young-Laplace but the actual contact angle is difficult to perceive. There may be no actual contact line but a remaining surface film on the fiber as a result of a disjoining pressure and such phenomena are at sub-micron length scales. The calculated drop shape on a 20 µm fiber is show below. It is independent of the surface tension of the liquid and the contact angle can be calculated from the length and width of the drop in equilibrium. If drops are formed after dip-coating the fiber in a resin then contact angles larger than 60° make the geometry unstable and such drops will "jump" aside after the liquid flows that stabilize the drop are stopped and the unstable equilibrium profile is (almost) attained. Barrel to clamshell configuration. At large contact angles an almost spherical drop in lateral contact with the fiber is the result.


Force measurements seem more suited and easy to measure wetting of fibers, but the rich behavior of wetting resins is very interesting and provide information about several aspects of the rheology of a resin, its surface tension, density and properties of the fiber surface. An interesting book on this is of P.G. de Gennes, F. Brochard-Wyart and D. Quéré, Capillarity and Wetting Phenomena. 


Interfacial tension measurements


We utilize both fiber probes and the pendent drop method to measure interfacial tensions between fluids. Our flexibility extends to relocating the equipment to our customers' premises, as they may possess superior equipment and possess in-depth knowledge about their systems and how to manage them safe and effectively.