Friction is an inevitable consequence of moving parts sliding against each other. As two surfaces come into contact, microscopic irregularities on each surface catch and rub against each other, creating resistance. This not only generates heat, but also leads to the gradual wear and tear of the surfaces, ultimately causing mechanical components to degrade over time. In the worst cases, friction can cause machines to overheat, or even fail completely.

To prevent this from happening, lubricants such as oil or grease can help to reduce friction by forming a protective film between surfaces, allowing them to slide across each other more easily. But even with lubricants, friction can’t be eliminated completely. To further improve the efficiency of lubrication, one promising approach is to add textures such as dimples to the surfaces of mechanical components.

As lubricant moves through the narrow gap between two surfaces, it creates hydrodynamic pressure – essentially a thin cushion of fluid that lifts the surfaces apart, reducing direct contact between them. By creating tiny, strategically placed dimples on the surfaces of mechanical parts, it becomes easier for lubricants to flow and spread between the two surfaces.

These dimples work by trapping and redistributing the lubricant, helping to maintain the fluid film. In turn, they ensure that the lubricant stays between the surfaces – even under heavy loads or at high speeds. This not only reduces friction significantly, but also results in less wear and tear on the machine’s components.

However, while the basic principles of this approach are well understood, there is still a lot of room for improvement. In particular, friction reduction could be maximized by optimizing the shape, size, and distribution of dimples on both surfaces. However, since these factors are influenced by many variables, finding the ideal combination has proven to be incredibly complex.

In their research, Robert Tomkowski and colleagues took on the challenge of optimizing dimple textures using a technique called computational fluid dynamics, or CFD for short. This cutting-edge method allows researchers to simulate the behaviour of fluids such as lubricants by solving complex mathematical equations. With CFD, the team was able to predict how the lubricant film forms between two textured surfaces and how this affects the hydrodynamic pressure that reduces friction.

Through these simulations, the team could test different dimple shapes, sizes, and arrangements in a controlled virtual environment. This allowed them to study how these variables affect the formation of the lubricating film and the resulting hydrodynamic pressure between surfaces: providing insights that would be difficult to obtain through experiments alone.

To better understand how dimple textures influence friction, Tomkowski’s team focused on five key factors. Firstly, they tested five basic dimple shapes—circle, square, trapezoid, ellipse, and triangle—while keeping the size, depth, and distribution of the dimples identical between each experiment.

Secondly, they varied the depth of the dimples between 3 and 40 microns. This allowed the researchers to see how different depths influenced the ability of the lubricant to create a pressure cushion between surfaces. Next, the team varied the number of dimples on the surface, providing insights into how the concentration of dimples impacts overall friction reduction.

Fourth, they examined the steepness of the dimple walls. This steepness angle could influence how the lubricant flows into and out of the dimples, affecting the buildup of hydrodynamic pressure. Finally, the thickness of the lubricant film was varied between 10 and 30 microns to see how this influenced the effectiveness of the dimpled textures.

Through these simulations, the team made several important discoveries. First, they found that the impact of dimple textures increases steadily as the density of dimples goes up. Simply put, the more dimples there are, the more effectively they can distribute the lubricant and reduce friction.

Next, the researchers identified an ideal dimple depth that works best for a certain thickness of the lubricant film. In their simulations, a dimple depth of 10 microns was the sweet spot for a range of film thicknesses. However, as the lubricant film became thicker, the effect of the dimples became less pronounced.

The team also discovered that the angle of the dimple surface plays a significant role in reducing friction. The most dramatic effects were observed when the surface angle was smaller than 30 degrees, which helped maximize the buildup of hydrodynamic pressure.

By combining these insights, the researchers believe that it is possible to minimize friction in lubricated systems by carefully tuning the parameters of the dimpled surfaces. Yet while these findings are promising, there are still some practical challenges to implementing them in real-world applications.

The techniques currently available for creating small-scale textures on surfaces, such as laser texturing and micromachining, are limited in their precision. For example, while the simulations showed that square-shaped dimples gave the best performance in maximizing hydrodynamic pressure, these shapes are difficult to produce using existing methods.

In contrast, circular dimples are much easier to fabricate using available techniques, making them a more practical choice for industrial applications. However, as manufacturing technologies improve, it may become possible to produce more complex dimple shapes with greater precision.

Despite these challenges, the results of Tomkowski’s simulations provide valuable guidance for manufacturers of mechanical components. By accounting for these discoveries when designing micro-scale dimples, manufacturers could significantly reduce friction between lubricated surfaces. This would not only make mechanical systems more energy-efficient but also reduce the amount of wear and tear on components over time, extending their lifetimes. This research is progressing within a new project called Fun4MEL – Advanced Surface Functionalization for Machinery Enhanced Lifetime, which is co-funded by Vinnova, Sweden’s Innovation Agency.

As surface texturing techniques continue to improve, the potential for dimpled surfaces to minimize friction and wear will only increase. In the future, we could see widespread use of optimized dimple textures in a variety of applications—from automotive engines to wind turbines—helping machines run more smoothly and efficiently for longer.