Measuring the Interaction Force Between a Droplet and a Super-hydrophobic Substrate by the Optical Lever Method

This method can help answer key questions in the field of bionics and fluid mechanics, such as superhydrophobic structures of lotus leaves and the droplet coalescence processes. The main advantage of this technique is that a force on the scale of sub micro-newtons can be measured with a resolution of scale of nanonewtons. This method could provide insight into the contact process of droplet and superhydrophobic structures.

And it can also be applied to other fields of micro force measurement. Individuals who have never performed this technique will struggle because the distance between droplets and the superhydrophobic substrates is difficult to control accurately. Prepare the measurement system on an appropriate lab bench.

At the center of the system is the silicon cantilever, which is suspended over a nanopositioning Z stage. The five millimeter long cantilever is mounted at the end of the holder. A high speed camera is in place with the line of sight perpendicular to the cantilever.

The nanopositioning Z stage has a support that holds an electrode that at this point. Finally, a laser and a position sensitive detector are arranged to measure the cantilever deflection. After measuring the capacitance gradient, use a computer controlled DC power supply to calibrate the optical lever.

This schematic provides andoverview of the setup for calibration. A plate electrode is on the nanopositioning Z stage. It is 100 micrometers below the cantilever with an overlap length of one half millimeter.

The cantilever and the electrode form a capacitor, with the positive pull of the DC power supply connected with the cantilever and the negative pull with the plate electrode. Next, adjust the relative positions of the laser, the position sensitive detector, and the cantilever. Arrange them so the laser beam is reflected by the cantilever to the detector.

At the computer, set the data acquisition rate of the detector output voltage to one kilohertz. In the DC power supply control software, set the start voltage to zero volts. Set the end voltage to 125 volts.

Have the voltage increase in 25 volt increments. Hold each voltage value for five seconds. As the voltage is varied, monitor the voltage output of the position sensitive detector.

After this sequence of measurements, perform an analogous sequence starting with the voltage at 125 volts and decreasing to zero volts in increments of 25 volts. Use the data from five complete measurements to create a plot in which the slope is the constant of proportionality between the interaction force and the position sensitive detector output voltage. To prepare for the measurements, disconnect the DC power supply from the plate and cantilever.

Next, work with the nanopositioning Z stage. Identify the electrode on its plate support and remove both by unscrewing the support from the stage. In their place, screw a new plate support to the z stage before continuing.

Ensure the high speed camera's line of sight is perpendicular to the cantilever. Next, get a superhydrophobic structure for the plate support. Use the structure with a contact angle of almost 180 degrees to suspend the water droplet from the cantilever.

For the experiment, affix this structure to the plate support on the Z stage. With a micro pipette, place a two microliter droplet of water on the superhydrophobic structure. Now begin working with the nanopositioning stage software.

In this software, go to the speed dialogue box and set the speed to 10 micrometers per second. Click the forward button to start the droplet moving upward. Click stop when the droplet contacts the free end of the cantilever.

After one or two seconds, manually move the Z stage away from the cantilever. A hemispherical droplet of water should remain suspended from the lower surface of the cantilever. To continue, remove the superhydrophobic structure from the plate support and get a superhydrophobic substrate to replace it.

This substrate consists of a copper grit with sprayed on nanoparticles. The substrate is glued onto a cylinder. This substrate has a grid fraction of 46.18%Place the substrate on the plate support.

Adjust the position of the superhydrophobic substrate so it is 100 micrometers from the hemispherical droplet on the cantilever. With the detector, laser, and camera on, return to work with the nanopositioning control software. In the speed dialogue box, set the speed to 10 micrometers per second.

Click the forward button to start the substrate moving upward. Click stop when the substrate and droplet make contact. Click the back button to move the superhydrophobic substrate downward.

Click the stop button when the substrate and droplet are separated. There are different scenarios represented in these plots of Interaction Force verses Time. First focus on the data in the black curve which is for the substrate with grid fraction 46.18%Initially, the substrate and the droplet are far from contact.

At this point, the force is zero. As the distance between the substrate and the droplet decreases, a repulsive force arises. This is reflected by the increase in force.

Once the substrate and droplet come into contact, the force between the two becomes attractive, leading to a decrease in the curve as the droplet gradually wets the substrate through capillary action. Eventually, the cantilever oscillates around an equilibrium position. When other higher grid fractions are used, the magnitude of the force between the droplet and the substrate decreases.

While attempting this procedure, it is important to remember that the relevant position between laser, PSD, and the cantilever can't be changed. Which can ensure the accuracy of measurement results. After it's development, this technique paves a way for researches in the field of tension-force measurement to explore the force during droplets in contact with substrate in air.