Controlling Liquid Droplets with Light-Sensitive Surfactants

Transporting liquid Droplets quickly and precisely on solid surfaces is important for various applications like medical devices, energy systems, and cooling mechanisms. This work showcases a method that uses Light to control how droplets move using special Surfactants. These surfactants can change shape when exposed to light, leading to changes in how the droplets behave.

#Current Methods of Droplet Control

Traditionally, researchers have developed various surfaces to passively regulate droplet movement. They have created surfaces with uneven structures or specific chemical properties called wettability. While this has allowed some control over droplets, these methods have limitations. They often need complex setups or powerful external forces, such as electricity, heat, or magnets.

#A New Approach with Light and Surfactants

In this study, we introduce a more straightforward way to control droplets using Liquid-infused Surfaces (LIS) and light-sensitive surfactants. When these surfactants are exposed to certain wavelengths of light, they undergo a reversible transformation that alters the Surface Tension in the liquid. This change creates a flow that makes droplets move in specific directions.

We have synthesized two types of surfactants that respond well to light. Using these surfactants, we show that droplets can move quickly in straight lines or follow complex paths on various surfaces. We also used tiny particles to visualize the liquid flow patterns inside the droplets, confirming how their movements depend on the droplet size and the light intensity.

#Importance of Manipulating Droplets

Controlling droplets is vital for many processes, including water desalination, cooling systems, liquid transport systems, and biological tests. For example, in cooling systems, it's crucial to remove droplets rapidly from surfaces to enhance heat transfer. Existing methods for moving droplets often rely on complex designs that limit real-time control and adaptability.

#Advantages of Light-Activated Surfactants

Light-based methods have significant advantages. Light allows precise spatial control and can be easily adjusted. There is no need for complex structures like microelectrodes. Previous studies have shown that certain photo-sensitive surfaces, like titanium dioxide (TiO2), can effectively change the behavior of water. However, direct control of droplet movement using these surfaces has not been achieved.

Optical tweezers are often used to manipulate small solid particles, but they have not been effective for liquid droplets. Recently, researchers have begun using photo-responsive surfactants to influence multi-phase fluid systems. These surfactants change their shape when exposed to light, allowing for changes in surface tension. This results in the generation of flow that can move droplets.

#Experimenting with Light-Sensitive Surfactants

In the current work, we tested the effectiveness of two synthesized surfactants, SP-DA-PEG and MCH-para. These surfactants induce significant changes in surface tension when exposed to light. When water droplets containing these surfactants are illuminated, they create flow that can transport droplets quickly across surfaces.

#Movement on Liquid-Infused Surfaces

We began by observing how droplets moved on liquid-infused surfaces using both SP-DA-PEG and MCH-para. The liquid-infused surface we used consisted of a special porous material saturated with a lubricant oil, Krytox. This setup helps droplets move more easily and provides low resistance.

When we placed droplets containing SP-DA-PEG and MCH-para on the liquid-infused surfaces, we noticed interesting patterns. For SP-DA-PEG, when light was focused on one side of the droplet, it created a higher tension on the illuminated side, resulting in the droplet moving away from the light source. The movement was confirmed through time-lapse images which showed clear trajectories.

Conversely, droplets containing MCH-para moved toward the light when illuminated. This behavior is due to the decrease in tension created by the surfactant on the illuminated side. By adjusting the light intensity, we could control the direction and speed of the droplets.

#Internal Flow Patterns

To gain a deeper understanding of the internal flow patterns within the droplets, we used small tracer particles. These particles helped visualize how fluid moved inside the droplets. Tracking their motion confirmed our hypothesis about the different flow directions depending on the surfactants used.

#Combining Light and Droplet Dynamics

Using the surfactants, we could also merge droplets together. When droplets containing different surfactants came into contact, the resulting droplet would continue to move under the influence of light, demonstrating the potential for complex fluid interactions mediated by light.

#Movement on Other Liquid Surfaces

The versatility of our method extends beyond liquid-infused surfaces. We tested the movement of droplets on other immiscible liquids, like Krytox. Here, we achieved fast movement and more complex pathways. The droplets moved away from the light source if they contained SP-DA-PEG and toward it if they contained MCH-para, which allowed us to design intricate patterns.

#Investigating Droplet Velocity

We also examined how the size of droplets influenced their movement speed. We found that the maximum velocity occurred for droplets around 20 microliters in size, resulting in movement that was more than twice as fast compared to previous studies. As droplet size increased past this threshold, their velocity began to decrease.

#Light and Temperature Effects

One area of exploration in our study was the possibility of light-induced heating affecting droplet motion. We measured the surface tension changes resulting from heating versus those caused by the photo-responsive surfactants. Our results indicated that the primary factor driving droplet motion was still the surfactant’s response to light, even though slight heating occurred.

#Moving Liquids in Microchannels

Beyond moving droplets on surfaces, we also investigated how to drive liquid movement within solid-wall microchannels using our light-responsive surfactants. We demonstrated that by illuminating one side of a liquid column in a microchannel, we could create a net force that moved the liquid in the opposite direction.

#Implications for Future Applications

The technique described in this study offers a novel platform for manipulating liquids without physical contact or invasive methods. This method is not only simple but also requires less energy compared to traditional methods.

By utilizing light, we can potentially achieve microfluidic manipulations at very small scales, down to one micron in size. With further advancements in surfactants that respond even faster to light, we could enhance the applicability of this method in various industries, including energy generation, water treatment, and biomedical applications.

#Conclusion

This research opens new avenues for the dynamic control of liquid droplets using light-sensitive surfactants. By achieving rapid and programmable transportation of droplets, we can improve various processes that depend on the precise manipulation of liquids. The applications range from microfluidics to thermal management systems, providing a powerful tool for future scientific and industrial advancements. We look forward to exploring the many ways this revolutionary method can be applied in real-world scenarios.