• Insights

Published on November 30, 2021 by Jenny Maat

Paints have been used for thousands of years throughout human history. The earliest paints were mainly used as a form of expression and generally consisted of natural pigments and binders. Currently, paints have a much larger scope and functionality. Paints are not only used for aesthetics due to their optical properties, but also to protect everyday objects against aggressive environmental conditions, such as heat, UV-irradiation and moisture1. Furthermore, paints are not static layers anymore, but can respond and adapt to external stimuli by changing their functionality2.


Iridescent paints are a specific example of a very exotic type of paint that can change color depending on the viewing angle. The color-changing effect is caused by a regular array of sub-microscopic particles in the dried paint layer. For the effect to be optimal, however, the array must be as close to perfect as possible which is difficult to achieve in practice. RheoCube can help to understand where imperfections come from and find ways to prevent them.


Perception of paints: colors and light interaction 

In general, we perceive paints through our eyes and we refer to them by their color and brightness. But how do we actually perceive colors? Colors are the result of light interacting with objects. Light travels through space as a straight beam of “dancing” photons which is composed of multiple colors. Each color is represented by a wave with a distinct wavelength. When we talk about the solar spectrum, we refer to all wavelengths that are present in a beam of light coming straight from our Sun.

Therefore, when we look straight towards the sun, we perceive the full visible region of the solar spectrum composed of blue, green and red wavelengths. Combined, they turn into a very bright white color. In contrast, when we look at an arbitrary object, we only perceive that part of the light which is reflected from the object straight into our eyes. For instance, an object absorbing in the green region of the solar spectrum will reflect blue and red light. We then perceive this object as magenta. In general, paints are thin layers that can absorb different regions of the visible spectrum thanks to the pigments that they contain. All non-absorbed light is eventually reflected at the object surface (Figure 1).


If we look into how light is reflected, the surface of the object plays an important role on how bright we perceive the color. The angle at which the light is reflected back to us depends on the surface roughness. Very flat “ideal” surfaces, such as glass or metals, reflect a beam of light at the same angle of incidence leading to bright colors. This phenomenon is called specular reflection. In contrast, when the surface has certain roughness, light bounces in different directions, a phenomenon called scattering, leading to pale colors. Both phenomena are very important in the generation and perception of iridescent colors.


Iridescent colors: what are they and why are they interesting?Iridescent coloration arises from the interference of light with the nanostructure of the material itself. Light interacts with the structure in such a way that only a narrow range of the solar spectrum is reflected back to us, while the rest is transmitted and/or absorbed. Therefore, we do not perceive colors as a combination of wavelengths, but as a pure single color from a narrow range of wavelengths. Just like those seen in bugs and butterfly wings3,4 (Figure 2). A very particular effect of iridescent colors is that the reflected color depends on the viewing angle. When looking at the iridescent surface from an angle, the color shifts towards lower wavelengths (blue region of the solar spectrum). This effect is visible at the edge of the bug shell where the curvature is more pronounced. There we perceive a bluish coloration.


If we look into how light is reflected, the surface of the object plays an important role on how bright we perceive the color. The angle at which the light is reflected back to us depends on the surface roughness. Very flat “ideal” surfaces, such as glass or metals, reflect a beam of light at the same angle of incidence leading to bright colors. This phenomenon is called specular reflection. In contrast, when the surface has certain roughness, light bounces in different directions, a phenomenon called scattering, leading to pale colors. Both phenomena are very important in the generation and perception of iridescent colors.


Iridescent colors: what are they and why are they interesting?

Iridescent coloration arises from the interference of light with the nanostructure of the material itself. Light interacts with the structure in such a way that only a narrow range of the solar spectrum is reflected back to us, while the rest is transmitted and/or absorbed. Therefore, we do not perceive colors as a combination of wavelengths, but as a pure single color from a narrow range of wavelengths. Just like those seen in bugs and butterfly wings3,4 (Figure 2). A very particular effect of iridescent colors is that the reflected color depends on the viewing angle. When looking at the iridescent surface from an angle, the color shifts towards lower wavelengths (blue region of the solar spectrum). This effect is visible at the edge of the bug shell where the curvature is more pronounced. There we perceive a bluish coloration.


Colloidal paints: preparation and challenges

Colloidal paints exhibit very interesting optical properties arising from a perfect 3D crystalline structure. This means that they are very sensitive to defects originated during the drying process where the colloidal assembly occurs. Defects, such as voids or air cavities at the interface between particles and the binder lead to light scattering and therefore to milky and pale colors. Light scattering is one of the major challenges in colloidal paints as defects are very common during the application process. Over the years, many strategies have been developed to remove and/or prevent scattered light. The most common way to remove light scattering is the addition of light absorbers that help to “clean” the reflected light. Examples include black substrates, pigments (i.e. carbon or carbon black), metal nanoparticles or black-coated particles6


Despite all these strategies, the application process remains the most crucial step towards preventing defects. Applying the paints without interfering with the final colloidal assembly is of utmost importance. It becomes clear that formulating and applying iridescent paints is not an easy task. Formulating new paints demands a large set of experiments to achieve colloidal stability and specific viscosity trends. Furthermore, applying paints via different techniques demands an accurate study of the rheological properties. Unfortunately, experimental techniques, such as the use of rheometers, do not give a clear insight on the reason why a certain paint behaves differently than another. This results in multiple trial and error experiments to find out the best combination of ingredients.


What if we could visualize the stability of particles depending on their surface properties and components of the binders? What if we could look closer to the interactions between particles when the liquid paints are applied on a substrate? What if we could put all this together and visualize the evolution of the rheological properties, such as viscosity and elasticity? This is where RheoCube can help. 


How can RheoCube aid scientists in formulating colloidal paints?

RheoCube can help scientists to find out what causes unexpected rheological trends and why. RheoCube is essentially a virtual rheometer that can provide detailed insights on how the ingredients of a complex mixture interact with each other through powerful simulation methods and detailed visualization and data analysis tools. RheoCube can simulate complex mixtures containing different fluids and particles under different conditions. A common example is a particulate mixture under shear. While a lab rheometer would provide you with the evolution of bulk properties (i.e. viscosity, elasticity), RheoCube, in addition, gives you all the information related to the ingredient interactions during the experiment (i.e. hydrodynamic, viscous and pressure forces). 


In the case of colloidal paints, RheoCube can help to analyze the colloidal stability taking into account the particle’s surface roughness, physical chemical properties, and shape. Here, we show an example of two suspensions of spherical particles in water under a shear rate of 1000s-1 equiparable to brush-painting. One of the suspensions contains hydrophobic particles, while the other has hydrophilic particles. During the simulation, we observe the particles moving under the shear forces. We clearly see aggregation events in the case of hydrophobic particles, whereas hydrophilic particles tend to repel each other. This means that creating a regular 3D crystalline structure during drying is easier for the hydrophilic particles, since defects and irregularities due to particle aggregation are prevented. Using these “simple” simulations, the effect of different alteration to particle surface properties on their colloidal stability can be investigated. The results can aid in gaining more insight in possible sources of defect formation and possible means to prevent them. Eventually leading to easier application methods of iridescent paints.  


References:

  • L. Wu, J. Baghdachi, Functional Polymer Coatings: Principles, Methods, and Applications (Ed: L. B. J. Wu), Wiley, Hoboken, New Jersey, USA 2015.
  • J. E. Stumpel, D. J. Broer, A. P. H. J. Schenning. Stimuli-responsive photonic coatingsChem. Commun. 2014, 50, 15839.
  • V. L. Welch and J.-P. Vigneron. Beyond butterflies—the diversity of biological photonic crystals. Opt Quant Electron (2007) 39:295–303.
  • A. G. Dumanlia and T. Savin. Recent advances in the biomimicry of structural colours. Chem. Soc. Rev. 2016. DOI: 10.1039/c6cs00129g
  • P. Liu, L. Bai, J. Yang, H. Gu, Q. Zhong, Z. Xie and Z. Gu. Self-assembled colloidal arrays for structural color. Nanoscale Adv., 2019, 1, 1672–1685.
  • C-F. Lai and Y-C. Wang. Colloidal Photonic Crystals Containing Silver Nanoparticles with Tunable Structural Colors. Crystals 2016, 6, 61; doi:10.3390/cryst6050061