Experiment 1 Super Hydrophobic surfaces on glass using flame

Goal

Evaluate the feasibility and efficacy of applying the superhydrophobic coating method to diverse materials like aluminium, wood, and paper, ensuring compatibility and functionality across substrates.

Assess the robustness and longevity of the superhydrophobic coating against mechanical stressors such as rubbing with kitchen paper, exploring adjustments to enhance its durability and resistance to abrasion.

Explore practical applications of superhydrophobic materials, particularly using a superhydrophobic aluminium mesh to observe and understand the behaviour of oil and water droplets, including the unique phenomenon of mixing these liquids on the coated surface.

Background

Roughness

Among the many ways to create micro-nanoscale surface roughness, photolithography and acid etching are particularly important. Additionally, laser ablation and some additive methods are becoming increasingly widespread.

Photolithography

“Litho” is a Greek word meaning stone, so lithography means writing on stone. Photolithography is a technique similar to engraving a specific pattern on a surface. In this method, a UV-sensitive material called photoresist is first applied to a substrate, usually a silicon wafer. UV light is then exposed to the sample through a mask that has the pattern printed on it. As a result, the areas of photoresist exposed to UV light are baked and easily removed, leaving the desired micro-pattern on the surface. Although this method is somewhat complex, it allows us to create an ordered and precise pattern of roughness, as shown in Fig

Acid Etch

This method is based on the fact that the surface of most steels is not homogeneous and contains many impurities. For example, an aluminum alloy surface might have portions of zinc, magnesium, and manganese. When you immerse the aluminum in HCl acid, the acid removes these impurities, creating a rough surface (Fig. 2). Although controlling the pattern of roughness is not feasible with this method, it remains popular due to its ease of use.

Coating

Spin coating and Chemical Vapor Deposition (CVD) are popular techniques for adding a very thin layer of coating to a surface.

Spin Coating

When a droplet is placed on a spinning surface, centrifugal force drives it radially outward (Fig. 3). This process results in a very thin layer of the droplet on the surface, with thickness adjustable based on viscosity and spin velocity. Spin coating, a widespread technique in fabricating integrated circuits, nano-channels, and optical mirrors, allows us to apply a micro-nanoscale layer of coating to a surface.

Chemical vapor deposition (CVD)

Chemical Vapor Deposition (CVD) deposits a layer of material from the vapor phase onto a substrate by decomposing chemicals. The biggest advantage of CVD, which has made it very popular, is its ability to coat almost any metallic or ceramic compound, including elements, metals, and their alloys. All these introduced methods require skilled operators and are time- and cost-intensive. In the following section, we will explore an interesting technique to fabricate a superhydrophobic surface that satisfies both requirements (roughening and coating) in one step.

Concept

To achieve a superhydrophobic surface in one step, we will use inherently hydrophobic nanoparticles. These tiny particles can attach and deposit hierarchically rough surfaces (Fig. 4). In this experiment, we will use hydrophobic carbon nanoparticles (CNP) for this purpose. These particles result from the incomplete combustion of a candle, so they can be abundantly found in candle soot. Additionally, their size is less than 2 μm, making them ideal for creating hierarchical micro-nano scale roughness.

However, we must find a way to attach them to the surface because the structure of CNPs is very fragile and can be easily removed by a tiny external force due to their weak physical adhesion. To address this problem, we will use paraffin wax as a glue to bond the soot to the surface securely.

Experiment

Materials and Facilities: Candle, glass slide, lighter, Contact Angle Goniometer

Step 1: Apply Paraffin Wax

Rub the candle on the glass surface for 2 minutes.

Purpose: Apply a layer of paraffin wax as a glue for attaching CNPs to the substrate.

Step 2: Ensure Uniform Wax Layer

Rub a sponge or kitchen paper on the surface smoothly for 2 minutes.

Purpose: Ensure the paraffin wax layer is thin and uniform.

Step 3: Deposit CNPs

Light the candle and position the paraffin wax-coated side of the glass approximately 1 cm from the candle's wick.

Purpose: Deposit CNPs onto the surface.

Step 4: Distribute Paraffin Wax

Move the glass back and forth horizontally for 1 minute.

Purpose: Evenly distribute the paraffin wax layer.

Step 5: Remove Unbound Particles

Immerse the glass in water and shake it gently.

Purpose: Remove unbound particles from the surface.

Step 6: Measure Contact Angle

Dispense a 7 μL water droplet on the sample and measure the static contact angle using the Droplet Lab instrument.

Purpose: Measure the effectiveness of the superhydrophobic surface.

Step 7: Repeat for Untreated Surface

Rotate the glass and repeat step 6 for the untreated surface.

Step 8: Compare Results

Compare the results of step 6 with step 7.

Discovery

Discovery 1

From our experiment, we discovered that the substrate does not significantly impact the process. Test whether the same method can be applied to other materials such as aluminum, wood, or paper. Caution: To prevent wood and paper from burning, moisten them before starting the process.

Discovery 2

Rub a kitchen paper on the superhydrophobic surface for 20 seconds and measure the static contact angle again. Observe any changes. Explore adjustments to steps 1 to 4 to enhance the coating's resistance against rubbing.

Discovery 3

Repeat the experiment to create a superhydrophobic aluminum mesh. Place oil and water droplets on the surface and observe their behavior. Try mixing a droplet of oil with a droplet of water on the mesh. Analyze the phenomena observed.

Discovery 4

Create a superhydrophobic shaving blade and attempt to cut water droplets. Determine the size of the smallest water droplet that can be cut. Share your findings with us.

Discover and share

Now that you can create various superhydrophobic materials, design and conduct interesting experiments. Explore the world of superhydrophobic materials and share your results with us. Remember that "Discovery consists not in seeking new lands but in seeing with eyes.