"Materials Science" sounds niche, and even a little peculiar, but no manufacturer is a stranger to the implications of material science principles for building reliable products.
Materials react with one another. Material properties impact how these interactions occur. Accurately anticipating, and planning for, material interactions is an important component of each materials scientist's job. Savvy, proactive materials management provides manufacturers with a competitive edge because of increased production efficiencies, minimized waste, and improved performance in the field.
Surface properties can be intentionally, or unintentionally, manipulated to affect performance outcomes and controlling these surface property variables within the manufacturing process are what's key for predicting reliability.
What is Surface Quality?
Surface properties extend beyond macroscopic characteristics like roughness, sheen, or color. The nanoscale chemical and physical properties that are controlled by the molecular structure of a surface are the surface properties that are important for adhesion. This surface structure is always different from the bulk structure of a material. And it is this structure that determines the way materials interact to determine a host of critical properties like bonding, coating, and painting.
Surfaces can be sturdy and inert or ever-evolving as they move through manufacturing environments and processes. When Materials Scientists want to create a new surface, they will seek to alter the surface through a variety of methods including cleaning, activating, abrading or using other methods to manipulate a surface. Materials Scientists need to validate that their surface preparation methodologies create the precise surface that they intended.
Surface quality refers to achieving both chemical composition and chemical structure uniformity over the entire surface of a part. Surface energy measurement is the most convenient, accurate and quantitative way to test surface quality and liquid contact angle measurement is the most reliable and useful way to measure surface energy. The contact angle is a measure of the shape established by a drop of liquid on a surface and is directly related to how strongly the liquid is being attracted to the surface. It is exquisitely sensitive to very small changes in the composition and structure of a surface. Contact angle measurements allow manufacturers to quickly learn a great deal about the quality of their surfaces.
To learn more about how to create surfaces that result in successful adhesion, download our free eBook: The Manufacturer's Roadmap to Eliminate Adhesion Issues in Production
The contact angle (also referred to as a wetting angle) is the angle formed between the surface of a liquid drop and the surface it's placed on. If the liquid drop spreads out on the surface to form a low dome, the angle is small. If the liquid "beads up", it forms a large contact angle. The extent to which the drop spreads out is directly related to how strongly attracted the liquid molecules are to molecules on the surface of the material. The attractive force between the liquid molecules in the surface of the drop is called surface tension and the attractive force between the molecules on the material surface is called surface energy. If the surface energy is high compared to the surface tension, the liquid molecules are strongly attracted to the surface and the drop spreads out to form a low contact angle. When the surface energy is low compared to the surface tension, the liquid molecules are more attracted to themselves and the liquid beads up, establishing a high contact angle.
The contact angle of a water droplet is impacted by the amount of surface energy on a part that is being prepared for adhesion.
The contact angle of a water droplet is impacted by the amount of surface energy on a part that is being prepared for adhesion.
The surface energy of materials is an astoundingly accurate predictor of how a coating, adhesive, paint, ink or sealant will react with a surface. Through contact angle measurement, manufacturers can know the tolerances they need to hit during their cleaning processes to achieve the appropriate levels of surface energy and, therefore, successful adhesion.
The Evolution of Goniometry
The relationship between the shape of a liquid drop on a solid surface, the surface energy of the solid and surface tension of the water droplet dates back at least to Thomas Young in the early 19th century. Contact angle goniometry (literally "angle measurement") as a means to characterize liquid-solid interactions took a major step forward in the 1960's with the development of the NRL (Naval Research Laboratory) goniometer by William Zisman. The NRL-type goniometer is a precision instrument well suited for laboratory investigations. Most laboratory-based contact angle goniometers are based on this enduring design.
At BTG Labs, we use NRL-type goniometers extensively in R&D and product development. For example, we used a goniometer in our work on the Composites Affordability Initiative (CAI) program in the late 1990's. We were brought into the CAI project to provide materials science expertise to help establish the relationships between surface treatment of composite materials, surface energetics, and adhesive bond performance. Although at the close of the CAI program, the measurements we helped establish provided the process control insights we aimed for, it was also clear that a standard NRL-style goniometer was not suitable for making these measurements in a manufacturing environment. We recognized the need for a device that was portable and could be used directly on real parts in a high-rate, high volume manufacturing environment.
The U.S. Air Force agreed with these conclusions, and funded BTG Labs to develop an alternative approach. Initial results were so encouraging that we were able to garner additional development support from the Department of Defense (DoD) and the State of Ohio. The result was the state-of-the-art contact angle measurement technology embodied in the Surface Analyst™ product line. Our idea took hold. The Surface Analyst is used to validate cleanliness levels after surface cleaning has been performed. The Surface Analyst product line has been adopted for widespread use in all industries that rely on effective service adhesion for ongoing product performance. Manufacturers understand that the Surface Analyst line of products will help them achieve consistent quality and performance on all of their bonded surfaces.
Surface Analyst Technology vs Benchtop Goniometry
The fundamental differences between an NRL-style goniometer and the Surface Analyst allow for a more compact and convenient device that can either be easily handheld or deployed in fully automated instruments for process measurement and control. These differences significantly improve the speed and accuracy of the measurement as well as the flexibility of the types of surfaces that can be measured. The primary areas of differences are:
- the method of liquid deposition
- the method of contact angle calculation once the liquid drop is deposited
These distinctions provided the flexibility to create a convenient handheld device that is ideal for use directly on a production line as well as for process development in the laboratory.
Depositing a Drop of Liquid on the Surface
A standard goniometer creates a liquid drop by pumping liquid through a syringe needle or a piece of tubing, then transferring the fully-formed drop to the surface by gently touching the drop to the surface; adhesion between the liquid and the surface causes the drop to detach from the tubing or needle. The Surface Analyst family of instruments use a patented process called Ballistic Deposition: the liquid drop is literally printed onto the surface using inkjet technology. Among other advantages, Ballistic Deposition allows liquid drops to be rapidly printed on surfaces regardless of orientation or curvature, making it trivial to interrogate surfaces with complex molded, machined, or stamped geometries.
The kinetic energy imparted to the growing drop during Ballistic Deposition confers other major advantages, particularly for measuring real-world surfaces that have microroughness, dust particles, and chemical heterogeneity. The impact of the drop printing process causes the perimeter of the drop to advance and recede multiple times over the few hundred milliseconds required to deposit the liquid. This high-frequency vibration of the drop perimeter prevents the growing drop from being pinned by heterogeneities and as a result, it establishes a true equilibrium contact angle that accurately reflects the chemical composition of the surface.
Ballistic deposition, which occurs when a water droplet is printed on a surface using inkjet technology, provides a contact angle measurement that is far more sensitive to subtle changes in surface chemistry than the static drops used by NRL-type goniometers.
A further advantage is that because the amount of kinetic energy imparted to the drop is controllable, Ballistic Deposition can be tuned through a wide spectrum between advancing or receding contact angles. (We will get more into the characteristics of advancing and receding angles in a later blog article.) Suffice it to say that tuning in a certain amount of receding angle character into the Ballistic Deposition parameters results in a measurement that is actually more sensitive to the subtle changes in surface chemistry that control adhesion than the static drops used by NRL-type goniometers.
Measuring Contact Angle of the Drop
Another important differentiating characteristic of the Surface Analyst technology is the use of a top-down image of the drop to calculate the contact angle. This is in contrast to the profile view used by NRL-type goniometers. The top-down approach simplifies the optics by removing the need for a smooth, planar surface with an unrestricted side view or reflex mirrors to acquire an image. This feature allows the Universal Inspection Head on the Surface Analyst to be made small enough to obtain measurements from hard to reach locations on complex parts, such as recesses and grooves. Highly textured surfaces such as grit blasted metals or fabric-imprinted composite materials, where the texture obscures the true drop edge, are trivial to measure. This method also allows measurement in the bottom of a recess or sealing groove or on a populated circuit board.
While liquid drops placed on laboratory-prepared surfaces are frequently very round, real life surfaces encountered in manufacturing are frequently not round: there is a range of contact angles around the perimeter of the drop because of small variations in chemistry and texture. A side-view goniometer picks up only 2 of these angles to calculate the overall drop profile. A top-down approach returns a contact angle that is more representative of the true average contact angle as it is the average of up to 100 contact angles around the perimeter of the drop.
Repeatability and Accuracy are Vital for Goniometers Used for Process Control
We can see a great example of the sensitivity of the Ballistic Deposition process to subtle changes in surface chemistry when we directly compare contact angle measurements from a Surface Analyst with those from two NRL-type goniometers: a benchtop unit and a small portable unit tethered to a laptop computer. This comparison was performed by the Fraunhofer Institute in Germany to evaluate the effects of atmospheric plasma treatment on aluminum.
This Figure shows that the contact angle measured by all three instruments decreased as the residence time of the plasma treatment increased due to an increase in surface energy. However, drops deposited via Ballistic Deposition and measured via the top-down approach showed a much smoother, more consistent, and stronger response to treatment level. The advantages of the Surface Analyst approach for process development and process control are obvious.
The Surface Analyst is a goniometer in the sense that it measures an angle. However, it differs in fundamental ways from instruments that are based on traditional benchtop contact angle goniometers in the way it uses Ballistic Deposition to establish the contact angle, combined with a top-down imaging approach. It allows objective and extremely sensitive and repeatable measurements of surface chemistry that are especially convenient to use for manufacturing process control. An additional benefit is that it is easily deployed in fully automated systems for hands-off process control with a data stream that can be readily incorporated into an overall control system.
To learn more about innovative approaches to process control and how to create surfaces that result in successful adhesion, download our free eBook: The Manufacturer's Roadmap to Eliminate Adhesion Issues in Production.