The Dynamics of Water Drops on Oil Surfaces

When a water drop hits a surface covered with a layer of thick oil, interesting things happen. This study looks at how these water drops behave when they land on surfaces that have a thin coating of silicone oil. Understanding these behaviors is important because they can help us in different fields like printing, cooling systems, and even medicine delivery.

#The Behavior of Water Drops on Oil

When a water drop falls onto an oil-coated surface, it goes through several key stages. First, the drop spreads quickly, reaching its largest size in just a few milliseconds. After this fast spreading, the drop starts to pull back, creating small oscillations, while maintaining a steady angle at its edges. Finally, the drop spreads out gradually on the solid surface beneath the oil as the oil layer gets disrupted.

The maximum size that the drop can reach is influenced by the drop's speed and the oil's properties, but interestingly, it doesn't change much based on how thick the oil layer is. When we add small fluorescent particles to the oil, we can see that during the initial spreading, these particles do not move out of their places. This shows that the oil layer stays intact until the drop starts to pull back. Once the drop retracts, it disrupts the oil layer, allowing the water to spread over the solid surface below.

#Factors Affecting Drop Impact Dynamics

The way a liquid drop behaves after it hits a surface depends on a few things, including the drop's size, how fast it falls, the liquid's properties, and the surface it's hitting. For instance, a drop may spread out, bounce back, splash, or break apart based on these factors. When the drop lands, its movement is influenced by the inertia, surface tension, and the surface's properties like how wet or rough it is.

The interaction between the water drop and the oil layer is affected by the speed and viscosity of the oil. Studies have shown that the thickness of the oil layer plays a role in how the drop behaves, but sometimes this impact is less than expected, especially with highly viscous oils.

#Experimental Setup

To study this phenomenon, we coated glass slides with different thicknesses of silicone oil. We then dropped water drops from a set height and observed their behavior using a high-speed camera. This setup allowed us to record and analyze the changes in size and shape of the water drops after they landed.

We used different oil Viscosities to see how they affected the drop’s behavior. After numerous tests, we were able to determine how quickly the water drops spread, how they retracted, and how the oil was affected underneath.

#Key Observations

#Rapid Spreading Phase

Upon landing, the water drop quickly spreads out. It reaches its maximum size in about 4 milliseconds. The time it takes to reach this maximum size decreases as the drop falls from a greater height, meaning faster drops spread out quicker.

#Retraction and Damping Oscillations

After reaching maximum spread, drops retract. During this retraction, the edges of the drop pull the oil layer inward. This process helps to dissipate the energy from the initial impact. Following this retraction, the drop experiences damping oscillations, a sort of bouncing motion that gradually lessens over time.

#Penetration into the Oil Layer

After retracting, a crucial stage begins where the water drop starts to penetrate into the oil layer. This process is not immediate; there is a delay before the water breaks through the oil. The time it takes for this penetration to start increases with the thickness of the oil layer and the drop’s impact velocity. This means a thicker oil layer or a faster drop may slow down how quickly the drop interacts with the surface beneath.

#Dewetting Process

As the water drop penetrates the oil layer, a dewetting process starts. This is where the oil layer is disrupted, allowing the water to spread onto the solid surface below. This process occurs only a fraction of a second after the drop impacts the oil, indicating a complex interaction happening between the water and oil.

Once dewetting occurs, a small oil droplet forms under the water drop. The strength of the forces at play between the water and oil primarily drives this droplet formation.

#Influence of Oil Viscosity

The viscosity, or thickness, of the oil layer significantly shapes the behavior of the water drop. When the oil is thicker, the dynamics change. For example, a thicker oil layer can slow down how quickly the water spreads beneath it. In contrast, with thinner oils, the water drop can cause more noticeable movement in the oil layer.

#Effects of Temperature and Air

Temperature differences between the water and the oil surface can also impact how the drop behaves. If the water is significantly warmer than the oil or vice versa, it can lead to unique behaviors when the drop impacts the surface. Additionally, air trapped between the water drop and the oil can change how the drop interacts with the oil layer, potentially delaying its penetration into the oil.

#Conclusion

This study uncovers fascinating behaviors of water drops when they hit surfaces coated with thick oil. We see that the maximum spread of the water drop is mainly influenced by its speed and less affected by the oil's viscosity.

As the drop interacts with the oil layer, it goes through phases of rapid spreading, retraction, and eventually penetrates into the oil, causing a dewetting process. Furthermore, the thickness of the oil layer significantly alters the dynamics of spreading and penetration.

Our findings open the door for further investigation into the interactions of drops on different surfaces, which could lead to advancements in various practical applications, from manufacturing to environmental science.