ramé-hart Newsletter

                   

September 2024
 

The contact line refers to the boundary where the liquid, solid, and gas phases meet on a surface. It is the line along which the liquid interfaces with both the solid and the surrounding gas (typically air). This line is critical in determining the contact angle, which is the angle formed between the liquid and solid at this boundary. The contact line is also often referred to as the three-phase line as it represents the intersection where all three phases – solid, liquid, and gas – come together.

Contact line dynamics refers to the movement and behavior of the contact line. The dynamics of this line are crucial in understanding how liquids spread or retract on a surface. The behavior of the contact line is influenced by several factors, including surface roughness, chemical heterogeneity, and the forces acting on the liquid (e.g., gravity, capillary forces, and external stresses).

Superhydrophobic Surfaces

Superhydrophobic surfaces are engineered to have very high water contact angles, generally over 150º, causing water droplets to bead up and roll off easily. However, under certain conditions the surface may undergo a wetting transition where it loses its superhydrophobic properties, and water begins to spread out more easily.

Superhydrophobic surfaces can experience a wetting transition from the Cassie-Baxter state to the Wenzel state (aka Cassie-to_Wenzel transition). In the Wenzel state, the liquid fully penetrates the surface roughness, resulting in a significantly lower contact angle and increased wetting. This transition undermines the superhydrophobic properties and leads to a surface becoming more hydrophilic (less water-repellent). There are a number of triggers that can cause this behavior.

One example is external pressure on a droplet which can force the liquid to penetrate the surface roughness, transitioning the surface to the Wenzel state. High-pressure environments, such as deep underwater conditions, can also cause this transition. If a droplet is large enough, its weight might create enough pressure to drive the liquid into the roughness, especially on less robust superhydrophobic surfaces.

Likewise, dust, oil, or other small particles can also fill in the surface asperities. This reduces the trapped air pockets that are crucial for maintaining the Cassie-Baxter state, leading to a shift toward the Wenzel state. Additionally, exposure to chemicals that interact with the surface, reducing its hydrophobic properties, can also cause a wetting transition.

When water vapor condenses on a superhydrophobic surface, tiny droplets can form and coalesce, leading to the filling of surface asperities. This can cause a transition to the Wenzel state, especially in humid environments or during a temperature drop that promotes condensation.

Scratches, wear, or any form of surface damage can alter the roughness pattern, compromising the air pockets necessary for the Cassie-Baxter state and leading to a wetting transition. Over time, chemical exposure (e.g., UV radiation or harsh chemicals) can degrade the surface's hydrophobic coating, making it less effective at repelling water.

Lastly, the presence of surfactants in the liquid can lower the surface tension, enabling the liquid to penetrate the surface roughness more easily. Surfactants reduce the energy barrier that typically maintains the Cassie-Baxter state, resulting in a wetting transition. Liquids with naturally low surface tension (like certain oils) are more likely to wet a superhydrophobic surface, driving the transition to the Wenzel state.

There are a number of actions that can prevent the Cassie-to-Wenzel transition. Regular cleaning and maintenance of superhydrophobic surfaces to remove contaminants and avoid surface wear is one such action. It’s also beneficial to use more robust materials or coatings that resist wear and chemical degradation. Designing surfaces that minimize condensation or using hydrophobic coatings that are resistant to water vapor can also improve the robustness of nano-textured surfaces.

In conclusion, a superhydrophobic surface can undergo a wetting transition due to various factors like pressure, surface contamination, condensation, surface damage, and the presence of high-energy liquids. Understanding these causes can help in designing more resilient superhydrophobic surfaces that maintain their water-repellent properties over time.

ramé-hart Quartz Cell (p/n 100-07-50)

Used alone, the Quartz Cell is ideal for measuring interfacial tension at ambient conditions. If the liquid drop phase is more dense than the external phase liquid, then use a standard straight needle to produce a pendant (or hanging) drop. On the other hand, if the drop phase is less dense than the external phase, use an inverted needle to produce an inverted pendant drop.¹

The Quartz Cell has also been designed to be used inside the ramé-hart Environmental Chamber (p/n 100-07), Heated Environmental Cell (p/n 100-33), Advanced Chamber (p/n 100-26), and Hot Plate (p/n 100-33-HP). In these chambers, the Quartz Cell and its contents can be heated up to as high as 300°C. The Quartz Cell can also be added to the Peltier Environmental Chamber (p/n 100-30) for working in temperatures as high as 150°C or as low as -50°C.

The ramé-hart Environmental Fixture (p/n 100-14) includes a Quartz Cell and adds an adjustable specimen stage making it ideal for captive bubble and other similar studies. If you need to do captive bubble at elevated temperatures, then go with the Environmental Chamber (p/n 100-07) and add the optional Chamber Cover with Stage (p/n 100-09).

In short, the ramé-hart Quartz Cell with its superior optical qualities is suitable for measuring interfacial tension - including both pendant and inverted pendant drop techniques, and can be used in various ramé-hart environmental chambers to conduct experiments at temperatures ranging from -50°C to 300°C, including specialized setups for captive bubble studies.

¹ This video shows how to measure interfacial tension using an instrument equipped with DROPimage Advanced, the ramé-hart Quartz Cell, and an inverted needle.

Carl Clegg
Director of Sales
Phone 973-448-0305
www.ramehart.com
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