The authors thank the Tianjin Natural Science Foundation (No. 18JCQNJC04800), Tribology&#160;Science&#160;Fund&#160;of&#160;State&#160;Key&#160;Laboratory&#160;of&#160;Tribology (No. SKLTKF17B18) and National Natural Science Foundation of China (Grant No. 51805367) for their support.

1. Assembly of the SOL calibration system
<ol>
	<li>Assemble the SOL calibration system according to the schematic diagram shown in Figure 2.</li>
	<li>Fix the laser to a support, making the angle between the laser and the horizontal direction be 45&#176;.</li>
	<li>Fix the PSD to another support, making the PSD perpendicular to the laser. Connect the PSD to the data acquisition device and the data acquisition device to the computer. 
	NOTE: These angles are determined by the experimenter&#39;s visual measurement and need not be exactly 45&#176; or 90&#176;.</li>
	<li>Fix the wider end of the cantilever to a holding device while the other end is suspended. Fix the holding device to a two-dimensional high-precision displacement stage. 
	NOTE: The dimensions of the cantilever are shown in Figure 3.</li>
	<li>Fix the plate electrode to the nanopositioning z-stage by a clamping device. 
	NOTE: The nanopositioning z-stage can bring the electrode to move along the z-axis with a displacement resolution of 1 nm.</li>
	<li>Connect the positive pole of the capacitive bridge with the cantilever and the negative pole with the plate electrode.</li>
	<li>Install a high-speed camera, whose line of sight is perpendicular to the cantilever.</li>
	<li>Adjust the position of the plate electrode, making the vertical distance between the plate electrode and the cantilever be about 100 &#956;m, and the overlap length about 0.5 mm. 
	NOTE: These distances are checked by image processing.</li>
</ol>
2. Measurement of the capacitance gradient
<ol>
	<li>Use the computer to control the capacitance bridge to collect the capacitance changes between the plate electrode and the cantilever in real time. Set the sampling rate to 0.5 Hz.</li>
	<li>Control the nanopositioning z-stage by computer to drive the plate electrode to step upward with a step of 10 &#956;m and a step number of 6 and stay for 10 s after each movement.</li>
	<li>Change the movement direction of the plate electrode to downward, and repeat step 2.2.</li>
	<li>Determine the relationship between the capacitance and displacement of the plate electrode in the measurement result, and obtain the value of P according to the Eq. (3).</li>
	<li>Repeat steps 2.1&#8211;2.4 5x and calculate the average value of P.</li>
</ol>
3. Calibration of the optical lever
<ol>
	<li>Disconnect the connection between the capacitive bridge and the cantilever/plate electrode.</li>
	<li>Connect the positive pole of the DC power supply with the cantilever and the negative pole with the plate electrode.</li>
	<li>Adjust the relative position between the laser, PSD and cantilever to make the laser reflected on PSD by cantilever. 
	NOTE: The laser spot is a circle about 2 mm in diameter.</li>
	<li>Control the DC power supply by computer to apply voltage varying with time on the parallel plate capacitor. At the same time, collect the output voltage of PSD in real time by the data acquisition device.
	<ol>
		<li>Set the sampling rate of the data acquisition device to 1,000 Hz.</li>
		<li>Set the initial voltage of the DC power supply to 0 V and hold for 5 s.</li>
		<li>Increase the voltage by 25 V and hold for 5 s.</li>
		<li>Repeat step 3.4.3 4x until the voltage increases to 125 V.</li>
		<li>Decrease the voltage by 25 V and hold for 5 s.</li>
		<li>Repeat step 3.4.5 4x until the voltage decreases to 0 V.</li>
	</ol>
	</li>
	<li>Determine the relationship about the output voltage of PSD and the DC supply voltage in the measurement result, and obtain the value of SOL according to Eq. (8).</li>
	<li>Repeat steps 3.4&#8211;3.5 5x and calculate the average value of SOL.</li>
</ol>
4. Preparation of super-hydrophobic substrates
<ol>
	<li>Prepare three circular copper grids with the same diameter of 3 mm and different grid fractions. Their grid fractions are 46.18%, 51.39% and 58.79% respectively. 
	NOTE: These copper grids are commercial products that were purchased.</li>
	<li>Spray the nanoparticles onto three copper grids to obtain super-hydrophobic substrates with micro structure and hydrophobicity.
	<ol>
		<li>Spray the base coat onto the copper grid.</li>
		<li>Spray the top coat onto the copper grid when the first coat is dry. 
		NOTE: The nanoparticles are packed in a can with a spray head. The nanoparticles will be sprayed by pressing the spray head when used.</li>
	</ol>
	</li>
	<li>Glue the copper grids on the side of cylinders with a diameter of 3 mm to obtain a surface super-hydrophobic structure with a curvature of 1/3 mm-1.</li>
</ol>
5. Measurement of the interaction force between droplets and super-hydrophobic substrates
<ol>
	<li>Disconnect the connection between the DC power supply and the cantilever/plate electrode. Remove the plate electrode from the nanopositioning z-stage.</li>
	<li>Fix a plate support to the nanopositioning z-stage.</li>
	<li>Install a high-speed camera, whose line of sight is perpendicular to the cantilever.</li>
	<li>Suspend a droplet on the lower surface of the free end of the cantilever.
	<ol>
		<li>Place a super-hydrophobic structure with a contact angle of almost 180&#176; on the plate support.</li>
		<li>Place a 2 &#956;L droplet on the super-hydrophobic structure using a micropipettor.</li>
		<li>Control the nanopositioning z-stage using software (e.g., PIMikroMove) to drive the droplet to move upward.
		<ol>
			<li>In the dialog box, set the velocity to 10 &#956;m/s.</li>
			<li>Click the Forward button and the droplet begins to move upwards.</li>
			<li>Click the Stop button when the droplet contacts with the free end of the cantilever.</li>
		</ol>
		</li>
		<li>Stay for 1 or 2 s, and then control the nanopositioning z-stage to drive the super-hydrophobic structure away from the cantilever. 
		NOTE: Since the silicon cantilever is hydrophilic, the droplet is suspended on the lower surface of the free end of the cantilever, forming a hemispherical droplet with a diameter of about 0.5 mm.</li>
		<li>Remove the super-hydrophobic structure with a contact angle of almost 180&#176; from the plate support.</li>
	</ol>
	</li>
	<li>Place the super-hydrophobic substrate with a grid fraction of 46.18% on the plate support.</li>
	<li>Adjust the position of the plate support, making the vertical distance between the super-hydrophobic substrate and the hemispherical droplet be about 100 &#956;m. 
	NOTE: The distance is checked by image processing.</li>
	<li>Turn on the PSD, laser, and high-speed camera.</li>
	<li>Control the data acquisition device by computer to collect the output voltage of PSD in real time. Set the sampling rate to 100 kHz.</li>
	<li>Set the velocity to 10 &#956;m/s in the software, and then click the Forward button, so that the super-hydrophobic substrate moves gradually closer to the droplet.</li>
	<li>Click the Stop button when the super-hydrophobic substrate and the droplet contact.</li>
	<li>Set the velocity to 10 &#956;m/s in the software, and then click the Back button to drive the super-hydrophobic substrate to move downward.</li>
	<li>Click the Stop button when the super-hydrophobic substrate is separated from the droplet.</li>
	<li>Draw the curve of the output voltage of PSD varying with time.</li>
	<li>Repeat steps 5.4&#8211;5.13 using super-hydrophobic substrates with grid fractions of 51.39% and 58.79%.</li>
	<li>Analyze the relationship between the interaction force and the grid fraction of super-hydrophobic substrate.</li>
</ol>

<ol>
	<li>Guo, Z., Liu, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biomimic+from+the+superhydrophobic+plant+leaves+in+nature:+Binary+structure+and+unitary+structure.">Biomimic from the superhydrophobic plant leaves in nature: Binary structure and unitary structure.</a> Plant Science. 172 (6), 1103-1112 (2007).</li><li>Koch, K., Bhushan, B., Barthlott, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diversity+of+structure,+morphology+and+wetting+of+plant+surfaces.">Diversity of structure, morphology and wetting of plant surfaces.</a> Soft Matter. 4 (10), 1943-1963 (2008).</li><li>Gao, X., Jiang, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biophysics:+Water-repellent+legs+of+water+striders.">Biophysics: Water-repellent legs of water striders.</a> Nature. 432 (7013), 36 (2004).</li><li>Yin, W., Zheng, Y. L., Lu, H. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-dimensional+topographies+of+water+surface+dimples+formed+by+superhydrophobic+water+strider+legs.">Three-dimensional topographies of water surface dimples formed by superhydrophobic water strider legs.</a> Applied Physics Letters. 109 (16), 163701 (2016).</li><li>Zheng, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Elegant+Shadow+Making+Tiny+Force+Visible+for+Water-Walking+Arthropods+and+Updated+Archimedes&#8217;+Principle.">Elegant Shadow Making Tiny Force Visible for Water-Walking Arthropods and Updated Archimedes&#8217; Principle.</a> Langmuir. 32 (41), 10522-10528 (2016).</li><li>Hu, D. L., Chan, B., Bush, J. W. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+hydrodynamics+of+water+strider+locomotion.">The hydrodynamics of water strider locomotion.</a> Nature. 424 (6949), 663-666 (2003).</li><li>Jiang, C. G., Xin, S. C., Wu, C. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Drag+reduction+of+a+miniature+boat+with+superhydrophobic+grille+bottom.">Drag reduction of a miniature boat with superhydrophobic grille bottom.</a> AIP Advances. 1 (3), 032148 (2011).</li><li>Guo, Z., Liang, J., Fang, J., Guo, B., Liu, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Novel+Approach+to+the+Robust+Ti6Al4V-Based+Superhydrophobic+Surface+with+Crater-like+Structure.">A Novel Approach to the Robust Ti6Al4V-Based Superhydrophobic Surface with Crater-like Structure.</a> Advanced Engineering Materials. 9 (4), 316-321 (2007).</li><li>Guo, Z., Liu, W., Su, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+stable+lotus-leaf-like+water-repellent+copper.">A stable lotus-leaf-like water-repellent copper.</a> Applied Physics Letters. 92 (6), 063104 (2008).</li><li>Liu, K., Zhang, M., Zhai, J., Wang, J., Jiang, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bioinspired+construction+of+Mg-Li+alloys+surfaces+with+stable+superhydrophobicity+and+improved+corrosion+resistance.">Bioinspired construction of Mg-Li alloys surfaces with stable superhydrophobicity and improved corrosion resistance.</a> Applied Physics Letters. 92 (18), 61 (2008).</li><li>Biance, A. L., Clanet, C., Qu&#233;r&#233;, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=First+steps+in+the+spreading+of+a+liquid+droplet.">First steps in the spreading of a liquid droplet.</a> Physical Review E Statistical Nonlinear &amp; Soft Matter Physics. 69 (1), 016301 (2004).</li><li>Eddi, A., Winkels, K. G., Snoeijer, J. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Short+time+dynamics+of+viscous+drop+spreading.">Short time dynamics of viscous drop spreading.</a> Physics of Fluids. 25 (1), 77-177 (2017).</li><li>Paulsen, J. D., Burton, J. C., Nagel, S. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Viscous+to+Inertial+Crossover+in+Liquid+Drop+Coalescence.">Viscous to Inertial Crossover in Liquid Drop Coalescence.</a> Physical Review Letters. 106 (11), 114501 (2011).</li><li>Paulsen, J. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Approach+and+coalescence+of+liquid+drops+in+air.">Approach and coalescence of liquid drops in air.</a> Physical Review E. 88 (6), 063010 (2013).</li><li>Vakarelski, I. U., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bubble+colloidal+AFM+probes+formed+from+ultrasonically+generated+bubbles.">Bubble colloidal AFM probes formed from ultrasonically generated bubbles.</a> Langmuir. 24 (3), 603-605 (2008).</li><li>Shi, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+forces+and+spatiotemporal+evolution+of+thin+water+films+between+an+air+bubble+and+solid+surfaces+of+different+hydrophobicity.">Measuring forces and spatiotemporal evolution of thin water films between an air bubble and solid surfaces of different hydrophobicity.</a> ACS Nano. 9 (1), 95-104 (2015).</li><li>Shi, C., Chan, D. Y. C., Liu, Q., Zeng, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probing+the+Hydrophobic+Interaction+between+Air+Bubbles+and+Partially+Hydrophobic+Surfaces+Using+Atomic+Force+Microscopy.">Probing the Hydrophobic Interaction between Air Bubbles and Partially Hydrophobic Surfaces Using Atomic Force Microscopy.</a> The Journal of Physical Chemistry C. 118 (43), 25000-25008 (2014).</li><li>Chung, K., Shaw, G. A., Pratt, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Accurate+noncontact+calibration+of+colloidal+probe+sensitivities+in+atomic+force+microscopy.">Accurate noncontact calibration of colloidal probe sensitivities in atomic force microscopy.</a> Review of Scientific Instruments. 80 (6), 930 (2009).</li><li>Zheng, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improving+environmental+noise+suppression+for+micronewton+force+sensing+based+on+electrostatic+by+injecting+air+damping.">Improving environmental noise suppression for micronewton force sensing based on electrostatic by injecting air damping.</a> Review of Scientific Instruments. 85 (5), 055002 (2014).</li><li>Song, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Highly+sensitive,+precise,+and+traceable+measurement+of+force.">Highly sensitive, precise, and traceable measurement of force.</a> Instrumentation Science &amp; Technology. 44 (4), 15 (2016).</li><li>Zheng, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+multi-position+calibration+of+the+stiffness+for+atomic-force+microscope+cantilevers+based+on+vibration.">The multi-position calibration of the stiffness for atomic-force microscope cantilevers based on vibration.</a> Measurement Science &amp; Technology. 26 (5), 055001 (2012).</li></ol>

The author has nothing to disclose.

In this protocol, a measurement system based on optical lever method is assembled and calibrated, which is designed for measuring the interaction force between the droplets and super-hydrophobic substrates. Among the all steps, it is critical to calibrate SOL using electrostatic force. The results of the calibration experiment verify Eq. (8): P(1/Vp1-1/Vp2) is proportional to 2(1/Vs12-1/Vs22) and make it possi...

The contact between a droplet and a super-hydrophobic surface is very common in daily life and industrial production: water droplets sliding from the surface of lotus leaf1,2, and a water strider traveling rapidly over the water3,4,5,6. A super-hydrophobic coating on the exterior surface of a ship can help reduce the corrosion degree of the ship and reduce the resistance of the navigation7,8,9,10. There is great value for industrial production and bionics research in studying the contact process between a droplet and a super-hydrophobic surface.
To observe the spreading process of droplets on a solid surface, Biance used a high-speed camera to photograph the contact process and found that the duration of the inertial regime is mainly fixed by the drop size11. Eddi photographed the contact process between the droplet and the transparent plate from the bottom and side using a high-speed camera, which comprehensively revealed the variation of the contact radius of the viscous droplet with time12. Paulsen combined an electrical method with high-speed camera observation, thus reducing the response time to 10 ns13,14.
Atomic force microscopy (AFM) has also been used to measure the interaction force between the droplet/bubble and solid surfaces. Vakarelski used an AFM cantilever to measure the interaction forces between two small bubbles (approximately 80-140 &#956;m) in aqueous solution during controlled collisions on the scale of micrometers to nanometers15. Shi used a combination of AFM and reflection interference contrast microscopy (RICM) to simultaneously measure the interaction force and the spatiotemporal evolution of the thin water film between an air bubble and mica surfaces of different hydrophobicity16,17.
However, since commercial cantilevers used in AFM are too small, the laser spot irradiated on the cantilever would be submerged by droplets or bubbles. The AFM has difficulties in measuring the interaction force between droplets and droplets/substrates in the air.
In this paper, a measurement system based on an optical lever method is designed to measure the interaction force between droplets and super-hydrophobic substrates. The force sensitivity of the optical lever (SOL) is calibrated by electrostatic force18, and then the interaction forces between droplets and different super-hydrophobic substrates are measured by the measurement system.
The schematic diagram of the measurement system is shown in Figure 1. The laser and position sensitive detector (PSD) constitute the optical lever system. A millimetric silicon cantilever is used as a sensitive component in the system. The substrate is fixed on the nanopositioning z-stage, which can move in vertical direction. When the substrate approaches the droplet, the interaction force causes the cantilever to bend. Thus, the position of the laser spot on PSD will change, and the output voltage of PSD will change. The output voltage of PSD Vp is proportional to the interaction force Fi, as shown in Eq. (1).
<img alt="Equation 1" src="/files/ftp_upload/59539/59539eq1.jpg" /> (1)
In order to acquire the interaction force, SOL must be calibrated first. The electrostatic force is used as the standard force in the calibration of SOL. As shown in Figure 2, the cantilever and the electrode make up a parallel plate capacitor, which could generate electrostatic force in a vertical direction. The electrostatic force Fes is determined by the voltage of the DC power supply Vs, as shown in Eq. (2)19,20,21.
<img alt="Equation 2" src="/files/ftp_upload/59539/59539eq2.jpg" /> (2)
where C is the capacitance of the parallel plate capacitor, z is the displacement of the cantilever free end, and dC/dz is called capacitance gradient. The capacitance could be measured by the capacitance bridge. The mathematical relationship between C and z can be fitted by a quadratic polynomial, as shown in Eq. (3).
<img alt="Equation 3" src="/files/ftp_upload/59539/59539eq3.jpg" /> (3)
where Q, P and CT are the coefficients of the quadratic term, the primary term and the constant term respectively. Therefore, the electrostatic force Fes can be expressed as Eq. (4).
<img alt="Equation 4" src="/files/ftp_upload/59539/59539eq4.jpg" /> (4)
Since the overlap area of two plates of the capacitor is very small, the elastic force acted on the cantilever can be expressed as Eq. (5), according to Hooke&#39;s law:
<img alt="Equation 5" src="/files/ftp_upload/59539/59539eq5.jpg" /> (5)
where k is the stiffness of the cantilever.
When the elastic force and electrostatic force applied on the cantilever are equal (i.e., Fi = Fes), the cantilever is in equilibrium. Eq. (6) can be derived from Eqs. (1), (2) and (5):
<img alt="Equation 6" src="/files/ftp_upload/59539/59539eq6.jpg" /> (6)
In order to reduce the uncertainty of calibration results, a difference method is used to calculate SOL. The results of two experiments are taken as Vs1, Vp1 and Vs2, Vp2, and are substituted into Eq. (6):
<img alt="Equation 7" src="/files/ftp_upload/59539/59539eq7.jpg" /> (7)
Transforming the equations and subtracting the lower equation from the upper equation in Eq. (7), the parameters Q and k are eliminated. Then the calibration formula of SOL is obtained, as shown in Eq. (8):
<img alt="Equation 8" src="/files/ftp_upload/59539/59539eq8.jpg" /> (8)
Performing a series of experiments, the curve is drawn with P(1/Vp1-1/Vp2) as the ordinate and 2(1/Vs12-1/Vs22) as the abscissa. The slope of the curve is SOL.
After obtaining SOL, the electrode will be replaced by different super-hydrophobic substrates. The interaction forces between droplets and super-hydrophobic substrates will be measured by the system shown in Figure 1.

<table>
	<tbody>
		<tr>
			<td>Camera</td>
			<td>Shenzhen Andonstar Tech Co., Ltd</td>
			<td>digital microscope A1</td>
			<td>Frame rate: 30 frames/sec; Focal distance: 5 mm - 30 mm</td>
		</tr>
		<tr>
			<td>Capacitive bridge</td>
			<td>Andeen-Hagerling</td>
			<td>AH2550A</td>
			<td>The capacitive bridge is used to measure the capacitance between the cantilever and the plate electrode.</td>
		</tr>
		<tr>
			<td>Data acquisition device</td>
			<td>National Instruments</td>
			<td>USB-4431</td>
			<td>The data acquisition device is used to read the output voltage data.</td>
		</tr>
		<tr>
			<td>DC power supply</td>
			<td>Keithley</td>
			<td>2410</td>
			<td>Voltage range: &#177;5 &#956;V; Accuracy: 0.012%</td>
		</tr>
		<tr>
			<td>Grid</td>
			<td>Electron Microscopy China</td>
			<td>AGH100, AGH150, AGH300</td>
			<td>The grid fractions of AGH100, AGH150 and AGH300 are 46.18%, 51.39% and 58.79% respectively</td>
		</tr>
		<tr>
			<td>Laser</td>
			<td>Shenzhen Infrared Laser Technology Co., Ltd.</td>
			<td>HW650AD100-10BD</td>
			<td>Laser wavelength: 650 nm</td>
		</tr>
		<tr>
			<td>Nanoparticle</td>
			<td>Rust-Oleum</td>
			<td>274232</td>
			<td>NeverWet Multi-Surface Liquid Repelling Treatment is a revolutionary super hydrophobic coating.</td>
		</tr>
		<tr>
			<td>Nanopositioning z-stage</td>
			<td>Physik Instrumente</td>
			<td>P622.ZCD</td>
			<td>Travel ranges 50 &#181;m to 250 &#181;m (350 &#181;m open loop); Resolution to 0.1 nm; Linearity error only 0.02%</td>
		</tr>
		<tr>
			<td>Position sensitive detector</td>
			<td>Hamamatsu Photonics K.K.</td>
			<td>S1880</td>
			<td>The two-dimensional PSD is used to translate optical signals into electrical signals.</td>
		</tr>
	</tbody>
</table>

measuring the interaction force between a droplet and a super-hydrophobic substrate by the optical lever method

The goal of this paper is to investigate the interaction force between droplets and super-hydrophobic substrates in the air. A measurement system based on an optical lever method is designed. A millimetric cantilever is used as a force sensitive component in the measurement system. Firstly, the force sensitivity of the optical lever is calibrated using electrostatic force, which is the critical step in measuring interaction force. Secondly, three super-hydrophobic substrates with different grid fractions are prepared with nanoparticles and copper grids. Finally, the interaction forces between droplets and super-hydrophobic substrates with different grid fractions are measured by the system. This method can be used to measure the force on the scale of sub-micronewton with a resolution on the scale of nanonewton. The in-depth study of the contact process of droplets and super-hydrophobic structures can help to improve the production efficiency in coating, film and printing. The force measurement system designed in this paper can also be used in other fields of microforce measurement.

The displacement of the plate electrode and the corresponding capacitance between the cantilever and the electrode measured in one experiment are shown in Table 1. The relationship between capacitance C and displacement z is fitted by quadratic polynomial using the polyfit function in MATLAB, as shown in Figure 4. The first order coefficient P can be obtained by the fitting function. The final value of P is 0.2799 pF/mm, which is the average calculated from...

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

The protocol aims to investigate the interaction between droplets and super-hydrophobic substrates in the air. This includes calibrating the measurement system and measuring the interaction force at super-hydrophobic substrates with different grid fractions.

The goal of this paper is to investigate the interaction force between droplets and super-hydrophobic substrates in the air. A measurement system based on ...

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Kinetics of a Particle: Force and Acceleration

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6.6K Görüntüleme.  Tianjin University. Bu yöntem, lotus yapraklarının süperhidrofobik yapıları ve damlacık birleştirme süreçleri gibi biyonik ve akışkanlar mekaniği alanındaki anahtar soruların yanıtlanmasına yardımcı olabilir. Bu tekniğin en büyük avantajı, alt mikro-newton ölçeğindeki bir kuvvetin nanonewton ölçeği ile ölçülebilen bir kuvvet olmasıdır. Bu yöntem damlacık ve süperhidrofobik yapıların temas süreci hakkında bilgi sağlayabilir.Ve aynı zamanda mikro kuvvet ölçümü ...

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

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves 2. This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks 3, negative refractive index materials 4, and lenses that resolve objects below the diffraction limit 5. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size 6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response 8. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber 9 with the Taylor-wire process 10, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 &deg;C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 &mu;m diameter indium wires separated by ~100 &mu;m, which exhibit a terahertz plasmonic response.

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

Taking Advantage of Reduced Droplet-surface Interaction to Optimize Transport of Bioanalytes in Digital Microfluidics

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of &#8220;lab-on-a-chip&#8221; platforms. Often, droplet motion relies on the wetting of a surface, directly associated with the application of an electric field; surface interactions, however, make motion dependent on droplet contents, limiting the breadth of applications of the technique.
Some alternatives have been presented to minimize this dependence. However, they rely on the addition of extra chemical species to the droplet or its surroundings, which could potentially interact with droplet moieties. Addressing this challenge, our group recently developed Field-DW devices to allow the transport of cells and proteins in DMF, without extra additives.
Here, the protocol for device fabrication and operation is provided, including the electronic interface for motion control. We also continue the studies with the devices, showing that multicellular, relatively large, model organisms can also be transported, arguably unaffected by the electric fields required for device operation.

The protocol for fabrication and operation of field dewetting devices (Field-DW) is described, as well as the preliminary studies of the effects of electric fields on droplet contents.

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of &#8220;lab-on-a-chip&#8221; ...

Patterning via Optical Saturable Transitions - Fabrication and Characterization

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.4 Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization5,6, beam pen lithography (BPL)7, scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities1-3, opening the door to massive parallelization.

We report that the diffraction limit of conventional optical lithography can be overcome by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.1-3 This paper outlines our fabrication technique and two locking mechanisms, namely: dissolution of one photoisomer and electrochemical oxidation.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

Experiments on Ultrasonic Lubrication Using a Piezoelectrically-assisted Tribometer and Optical Profilometer

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at a frequency above the acoustic range (20 kHz). As a solid-state technology, ultrasonic lubrication can be used where conventional lubricants are unfeasible or undesirable. Further, ultrasonic lubrication allows for electrical modulation of the effective friction coefficient between two sliding surfaces. This property enables adaptive systems that modify their frictional state and associated dynamic response as the operating conditions change. Surface wear can also be reduced through ultrasonic lubrication. We developed a protocol to investigate the dependence of friction force reduction and wear reduction on the linear sliding velocity between ultrasonically lubricated surfaces. A pin-on-disc tribometer was built which differs from commercial units in that a piezoelectric stack is used to vibrate the pin at 22 kHz normal to the rotating disc surface. Friction and wear metrics including effective friction force, volume loss, and surface roughness are measured without and with ultrasonic vibrations at a constant pressure of 1 to 4 MPa and three different sliding velocities: 20.3, 40.6, and 87 mm/sec. An optical profilometer is utilized to characterize the wear surfaces. The effective friction force is reduced by 62% at 20.3 mm/sec. Consistently with existing theories for ultrasonic lubrication, the percent reduction in friction force diminishes with increasing speed, down to 29% friction force reduction at 87 mm/sec. Wear reduction remains essentially constant (49%) at the three speeds considered.

We present a protocol for using a piezoelectrically-assisted tribometer and optical profilometer to investigate the dependence of ultrasonic wear and friction reduction on linear velocity, contact pressure, and surface properties.

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at ...

Methods for Measuring the Orientation and Rotation Rate of 3D-printed Particles in Turbulence

Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing technology is used to fabricate particles with slender arms connected at a common center. Shapes explored are crosses (two perpendicular rods), jacks (three perpendicular rods), triads (three rods in triangular planar symmetry), and tetrads (four arms in tetrahedral symmetry). Methods for producing on the order of 10,000 fluorescently dyed particles are described. Time-resolved measurements of their orientation and solid-body rotation rate are obtained from four synchronized videos of their motion in a turbulent flow between oscillating grids with R&#955; = 91. In this relatively low-Reynolds number flow, the advected particles are small enough that they approximate ellipsoidal tracer particles. We present results of time-resolved 3D trajectories of position and orientation of the particles as well as measurements of their rotation rates.

We use 3D printing to fabricate anisotropic particles in the shapes of jacks, crosses, tetrads, and triads, whose alignments and rotations in turbulent fluid flow can be measured from multiple simultaneous video images.

Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing ...

Standardized Method for Measuring Collection Efficiency from Wipe-sampling of Trace Explosives

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common procedure for collecting traces of explosives, and standardized measurements of collection efficiency are needed to evaluate and optimize sampling protocols. The approach described here is designed to provide this measurement infrastructure, and controls most of the factors known to be relevant to wipe-sampling. Three critical factors (the applied force, travel distance, and travel speed) are controlled using an automated device. Test surfaces are chosen based on similarity to the screening environment, and the wipes can be made from any material considered for use in wipe-sampling. Particle samples of the explosive 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) are applied in a fixed location on the surface using a dry-transfer technique. The particle samples, recently developed to simulate residues made after handling explosives, are produced by inkjet printing of RDX solutions onto polytetrafluoroethylene (PTFE) substrates. Collection efficiency is measured by extracting collected explosive from the wipe, and then related to critical sampling factors and the selection of wipe material and test surface. These measurements are meant to guide the development of sampling protocols at screening venues, where speed and throughput are primary considerations.

Optimized sampling protocols and the development of new wipe materials can be facilitated by standardized measurements of collection efficiency from wipe-sampling. Our approach for sampling trace explosives uses an automated device to control speed, force, and distance during wipe-sampling followed by extraction of collected explosives.

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common ...

Experimental Implementation of a New Composite Fabrication Method: Exposing Bare Fibers on the Composite Surface by the Soft Layer Method

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional component that should have high electrical conductivity, high mechanical properties, and high productivity.
In this regard, a carbon fiber/epoxy resin composite can be an ideal material to replace the conventional graphite bipolar plate, which often leads to the catastrophic failure of the entire system because of its inherent brittleness. Though the carbon/epoxy composite has high mechanical properties and is easy to manufacture, the electrical conductivity in the through-thickness direction is poor because of the resin-rich layer that forms on its surface. Therefore, an expanded graphite coating was adopted to solve the electrical conductivity issue. However, the expanded graphite coating not only increases the manufacturing costs but also has poor mechanical properties.
In this study, a method to expose fibers on the composite surface is demonstrated. There are currently many methods that can expose fibers by surface treatment after the fabrication of the composite. This new method, however, does not require surface treatment because the fibers are exposed during the manufacture of the composite. By exposing bare carbon fibers on the surface, the electrical conductivity and mechanical strength of the composite are increased drastically.

A protocol to expose bare fibers on the composite surface by eliminating resin rich area is presented. The fibers are exposed during fabrication of the composites, not by the post surface treatment. The exposed carbon composites exhibit high electrical conductivity in the through-thickness direction and high mechanical property.

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional ...

Microfluidic Imaging Flow Cytometry by Asymmetric-detection Time-stretch Optical Microscopy (ATOM)

Scaling the number of measurable parameters, which allows for multidimensional data analysis and thus higher-confidence statistical results, has been the main trend in the advanced development of flow cytometry. Notably, adding high-resolution imaging capabilities allows for the complex morphological analysis of cellular/sub-cellular structures. This is not possible with standard flow cytometers. However, it is valuable for advancing our knowledge of cellular functions and can benefit life science research, clinical diagnostics, and environmental monitoring. Incorporating imaging capabilities into flow cytometry compromises the assay throughput, primarily due to the limitations on speed and sensitivity in the camera technologies. To overcome this speed or throughput challenge facing imaging flow cytometry while preserving the image quality, asymmetric-detection time-stretch optical microscopy (ATOM) has been demonstrated to enable high-contrast, single-cell imaging with sub-cellular resolution, at an imaging throughput as high as 100,000 cells/s. Based on the imaging concept of conventional time-stretch imaging, which relies on all-optical image encoding and retrieval through the use of ultrafast broadband laser pulses, ATOM further advances imaging performance by enhancing the image contrast of unlabeled/unstained cells. This is achieved by accessing the phase-gradient information of the cells, which is spectrally encoded into single-shot broadband pulses. Hence, ATOM is particularly advantageous in high-throughput measurements of single-cell morphology and texture &#8211; information indicative of cell types, states, and even functions. Ultimately, this could become a powerful imaging flow cytometry platform for the biophysical phenotyping of cells, complementing the current state-of-the-art biochemical-marker-based cellular assay. This work describes a protocol to establish the key modules of an ATOM system (from optical frontend to data processing and visualization backend), as well as the workflow of imaging flow cytometry based on ATOM, using human cells and micro-algae as the examples.

Microfluidic Imaging Flow Cytometry by Asymmetric-detection Time-stretch Optical Microscopy ATOM

This protocol describes the implementation of an asymmetric-detection time-stretch optical microscopy system for single-cell imaging in ultrafast microfluidic flow and its applications in imaging flow cytometry.

Scaling the number of measurable parameters, which allows for multidimensional data analysis and thus higher-confidence statistical results, has been the ...

A Pipette-Tip Based Method for Seeding Cells to Droplet Microfluidic Platforms

Amongst various microfluidic platform designs frequently used for cellular analysis, droplet-microfluidics provides a robust tool for isolating and analyzing cells at the single-cell level by eliminating the influence of external factors on the cellular microenvironment. Encapsulation of cells in droplets is dictated by the Poisson distribution as a function of the number of cells present in each droplet and the average number of cells per volume of droplet. Primary cells, especially immune cells, or clinical specimens can be scarce and loss-less encapsulation of cells remains challenging. In this paper, we present a new methodology that uses pipette-tips to load cells to droplet-based microfluidic devices without the significant loss of cells. With various cell types , we demonstrate efficient cell encapsulation in droplets that closely corresponds to the encapsulation efficiency predicted by the Poisson distribution. Our method ensures loss-less loading of cells to microfluidic platforms and can be easily adapted for downstream single cell analysis, e.g., to decode cellular interactions between different cell types.

This article presents a protocol for seeding scarce population of cells using pipette-tips to droplet microfluidic devices in order to provide higher encapsulation efficiency of cells in droplets.

Amongst various microfluidic platform designs frequently used for cellular analysis, droplet-microfluidics provides a robust tool for isolating and ...

A Random-displacement Measurement by Combining a Magnetic Scale and Two Fiber Bragg Gratings

Long-distance displacement measurements using optical fibers have always been a challenge in both basic research and industrial production. We developed and characterized a temperature-independent fiber Bragg grating (FBG)-based random-displacement sensor that adopts a magnetic scale as a novel transferring mechanism. By detecting shifts of two FBG center wavelengths, a full-range measurement can be obtained with a magnetic scale. For identification of the clockwise and counterclockwise rotation direction of the motor (in fact, the direction of movement of the object to be tested), there is a sinusoidal relationship between the displacement and the center wavelength shift of the FBG; as the anticlockwise rotation alternates, the center wavelength shift of the second FBG detector shows a leading phase difference of around 90&#176; (+90&#176;). As the clockwise rotation alternates, the center wavelength shift of the second FBG displays a lagging phase difference of around 90&#176; (-90&#176;). At the same time, the two FBG-based sensors are temperature independent. If there is some need for a remote monitor without any electromagnetic interference, this striking approach makes them a useful tool for determining the random displacement. This methodology is appropriate for industrial production. As the structure of the whole system is relatively simple, this displacement sensor can be used in commercial production. In addition to it being a displacement sensor, it can be used to measure other parameters, such as velocity and acceleration.

A protocol to create a full-range linear displacement sensor, combining two packaged fiber Bragg grating detectors with a magnetic scale, is presented.

Long-distance displacement measurements using optical fibers have always been a challenge in both basic research and industrial production. We developed ...