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: &amp;plusmn;5 &amp;mu;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 &amp;micro;m to 250 &amp;micro;m (350 &amp;micro;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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6.6K Views.  Tianjin University. 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.

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

The Visual Colorimetric Detection of Multi-nucleotide Polymorphisms on a Pneumatic Droplet Manipulation Platform

A simple and visual method to detect multi-nucleotide polymorphism (MNP) was performed on a pneumatic droplet manipulation platform on an open surface. This approach to colorimetric DNA detection was based on the hybridization-mediated growth of gold nanoparticle probes (AuNP probes). The growth size and configuration of the AuNP are dominated by the number of DNA samples hybridized with the probes. Based on the specific size- and shape-dependent optical properties of the nanoparticles, the number of mismatches in a sample DNA fragment to the probes is able to be discriminated. The tests were conducted via droplets containing reagents and DNA samples respectively, and were transported and mixed on the pneumatic platform with the controlled pneumatic suction of the flexible PDMS-based superhydrophobic membrane. Droplets can be delivered simultaneously and precisely on an open-surface on the proposed pneumatic platform that is highly biocompatible with no side effect of DNA samples inside the droplets. Combining the two proposed methods, the multi-nucleotide polymorphism can be detected at sight on the pneumatic droplet manipulation platform; no additional instrument is required. The procedure from installing the droplets on the platform to the final result takes less than 5 min, much less than with existing methods. Moreover, this combined MNP detection approach requires a sample volume of only 10 &#181;l in each operation, which is remarkably less than that of a macro system.

This work presents a simple and visual method to detect multi-nucleotide polymorphisms on a pneumatic droplet manipulation platform. With the proposed method, the entire experiment, including droplet manipulation and detection of multi-nucleotide polymorphisms, can be performed near 23 &#176;C without the aid of advanced instruments.

A simple and visual method to detect multi-nucleotide polymorphism (MNP) was performed on a pneumatic droplet manipulation platform on an open surface. ...

Genetics

Quantitative and Qualitative Examination of Particle-particle Interactions Using Colloidal Probe Nanoscopy

Colloidal Probe Nanoscopy (CPN), the study of the nano-scale interactive forces between a specifically prepared colloidal probe and any chosen substrate using the Atomic Force Microscope (AFM), can provide key insights into physical interactions present within colloidal systems. Colloidal systems are widely existent in several applications including, pharmaceuticals, foods, paints, paper, soil and minerals, detergents, printing and much more.1-3 Furthermore, colloids can exist in many states such as emulsions, foams and suspensions. Using colloidal probe nanoscopy one can obtain key information on the adhesive properties, binding energies and even gain insight into the physical stability and coagulation kinetics of the colloids present within. Additionally, colloidal probe nanoscopy can be used with biological cells to aid in drug discovery and formulation development. In this paper we describe a method for conducting colloidal probe nanoscopy, discuss key factors that are important to consider during the measurement, and show that both quantitative and qualitative data that can be obtained from such measurements.

Colloidal probe nanoscopy can be used within a variety of fields to gain insight into the physical stability and coagulation kinetics of colloidal systems and aid in drug discovery and formulation sciences using biological systems. The method described within provides a quantitative and qualitative means to study such systems.

Colloidal Probe Nanoscopy (CPN), the study of the nano-scale interactive forces between a specifically prepared colloidal probe and any chosen substrate ...

Chemistry

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; ...

Investigating Single Molecule Adhesion by Atomic Force Spectroscopy

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. At the same time the AFM tip is passivated to prevent unspecific interactions between the tip and the substrate, which is a prerequisite to study single molecules attached to the AFM tip. Analyses to determine the adhesion force, the adhesion length, and the free energy of these molecules on solid surfaces and bio-interfaces are shortly presented and external references for further reading are provided. Example molecules are the poly(amino acid) polytyrosine, the graft polymer PI-g-PS and the phospholipid POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine). These molecules are desorbed from different surfaces like CH3-SAMs, hydrogen terminated diamond and supported lipid bilayers under various solvent conditions. Finally, the advantages of force spectroscopic single molecule experiments are discussed including means to decide if truly a single molecule has been studied in the experiment.

A protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. Procedures and examples to determine the adhesion force and free energy of these molecules on solid supports and bio-interfaces are provided.

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single ...

Fabrication of Superhydrophobic Metal Surfaces for Anti-Icing Applications

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in industry. The wettability of the produced surface was analyzed by bouncing drop experiments and the topography was analyzed by confocal microscopy. In addition, we show various methodologies to measure its durability and anti-icing properties. Superhydrophobic surfaces hold a special texture that must be preserved to keep their water-repellency. To fabricate durable surfaces, we followed two strategies to incorporate a resistant texture. The first strategy is a direct incorporation of roughness to the metal substrate by acid etching. After this surface texturization, the surface energy was decreased by silanization or fluoropolymer deposition. The second strategy is the growth of a ceria layer (after surface texturization) that should enhance the surface hardness and corrosion resistance. The surface energy was decreased with a stearic acid film.
The durability of the superhydrophobic surfaces was examined by a particle impact test, mechanical wear by lateral abrasion, and UV-ozone resistance. The anti-icing properties were explored by studying the ability to repeal subcooled water, freezing delay, and ice adhesion.

We illustrate several methodologies to produce superhydrophobic metal surfaces and to explore their durability and anti-icing properties.

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in ...

Measurement of Dynamic Force Acted on Water Strider Leg Jumping Upward by the PVDF Film Sensor

This study aimed to make an explanation for the phenomenon in nature that water strider usually jumps or glides on the water surface easily but quickly, with its peak locomotion speed reaching 150 cm/s. First of all, we observed the microstructure and hierarchy of water strider legs using the scanning electron microscope. On the basis of the observed morphology of the legs, a theoretical model of the detachment from water surface was established, which explained water striders' capability to slide on water surface effortlessly in terms of energy reduction. Secondly, a dynamic force measurement system was devised using the PVDF film sensor with excellent sensitivity, which could detect the whole interaction process. Subsequently, a single leg in contact with water was pulled upward at different speeds, and the adhesion force was measured at the same time. The results of the departing experiment suggested a deep understanding of the fast jumping of water striders.

The protocol here is dedicated to investigating the free and quick maneuvering of water strider on water surface. The protocol includes observing the microstructure of legs and measuring the adhesion force when departing from water surface at different speeds.

This study aimed to make an explanation for the phenomenon in nature that water strider usually jumps or glides on the water surface easily but quickly, ...

Safe Experimentation in Optical Levitation of Charged Droplets Using Remote Labs

The work presents an experiment that allows the study of many fundamental physical processes, such as photon pressure, diffraction of light or the motion of charged particles in electrical fields. In this experiment, a focused laser beam pointing upwards levitate liquid droplets. The droplets are levitated by the photon pressure of the focused laser beam which balances the gravitational force. The diffraction pattern created when illuminated with laser light can help measure the size of a trapped droplet. The charge of the trapped droplet can be determined by studying its motion when a vertically directed electrical field is applied. There are several reasons motivating this experiment to be remotely controlled. The investments required for the setup exceeds the amount normally available in undergraduate teaching laboratories. The experiment requires a laser of Class 4, which is harmful to both skin and eyes and the experiment uses voltages that are harmful.

Optical levitation is a method for levitating micrometer-sized dielectric objects using laser light. Utilizing computers and automation systems, an experiment on optical levitation can be controlled remotely. Here, we present a remotely controlled optical levitation system that is used both for educational and research purposes.

The work presents an experiment that allows the study of many fundamental physical processes, such as photon pressure, diffraction of light or the motion ...

Fluorescence Recovery after Merging a Droplet to Measure the Two-dimensional Diffusion of a Phospholipid Monolayer

We introduce a new method to measure the lateral diffusivity of a surfactant monolayer at the fluid-fluid interface, called fluorescence recovery after merging (FRAM). FRAM adopts the same principles as the fluorescence recovery after photobleaching (FRAP) technique, especially for measuring fluorescence recovery after bleaching a specific area, but FRAM uses a drop coalescence instead of photobleaching dye molecules to induce a chemical potential gradient of dye molecules. Our technique has several advantages over FRAP: it only requires a fluorescence microscope rather than a confocal microscope equipped with high power lasers; it is essentially free from the selection of fluorescence dyes; and it has far more freedom to define the measured diffusion area. Furthermore, FRAM potentially provides a route for studying the mixing or inter-diffusion of two different surfactants, when the monolayers at a surface of droplet and at a flat air/water interface are prepared with different species, independently.

We present a new technique to measure the lateral diffusion of a surface active species at the fluid-fluid interface by merging a droplet monolayer onto a flat monolayer.

We introduce a new method to measure the lateral diffusivity of a surfactant monolayer at the fluid-fluid interface, called fluorescence recovery after ...

Bioengineering

Fabrication and Characterization of Thickness Mode Piezoelectric Devices for Atomization and Acoustofluidics

We present a technique to fabricate simple thickness mode piezoelectric devices using lithium niobate (LN). Such devices have been shown to atomize liquid more efficiently, in terms of &#64258;ow rate per power input, than those that rely on Rayleigh waves and other modes of vibration in LN or lead zirconate titanate (PZT). The complete device is composed of a transducer, a transducer holder, and a &#64258;uid supply system. The fundamentals of acoustic liquid atomization are not well known, so techniques to characterize the devices and to study the phenomena are also described. Laser Doppler vibrometry (LDV) provides vibration information essential in comparing acoustic transducers and, in this case, indicates whether a device will perform well in thickness vibration. It can also be used to &#64257;nd the resonance frequency of the device, though this information is obtained more quickly via impedance analysis. Continuous &#64258;uid atomization, as an example application, requires careful &#64258;uid flow control, and we present such a method with high-speed imaging and droplet size distribution measurements via laser scattering.

Fabrication of piezoelectric thickness mode transducers via direct current sputtering of plate electrodes on lithium niobate is described. Additionally, reliable operation is achieved with a transducer holder and &#64258;uid supply system and characterization is demonstrated via impedance analysis, laser doppler vibrometry, high-speed imaging, and droplet size distribution using laser scattering.

We present a technique to fabricate simple thickness mode piezoelectric devices using lithium niobate (LN). Such devices have been shown to atomize liquid ...

Direct Force Measurements of Subcellular Mechanics in Confinement using Optical Tweezers

During the development of a multicellular organism, a single fertilized cell divides and gives rise to multiple tissues with diverse functions. Tissue morphogenesis goes in hand with molecular and structural changes at the single cell level that result in variations of subcellular mechanical properties. As a consequence, even within the same cell, different organelles and compartments resist differently to mechanical stresses; and mechanotransduction pathways can actively regulate their mechanical properties. The ability of a cell to adapt to the microenvironment of the tissue niche thus is in part due to the ability to sense and respond to mechanical stresses. We recently proposed a new mechanosensation paradigm in which nuclear deformation and positioning enables a cell to gauge the physical 3D environment and endows the cell with a sense of proprioception to decode changes in cell shape. In this article, we describe a new method to measure the forces and material properties that shape the cell nucleus inside living cells, exemplified on adherent cells and mechanically confined cells. The measurements can be performed non-invasively with optical traps inside cells, and the forces are directly accessible through calibration-free detection of light momentum. This allows measuring the mechanics of the nucleus independently from cell surface deformations and allowing dissection of exteroceptive and interoceptive mechanotransduction pathways. Importantly, the trapping experiment can be combined with optical microscopy to investigate the cellular response and subcellular dynamics using fluorescence imaging of the cytoskeleton, calcium ions, or nuclear morphology. The presented method is straightforward to apply, compatible with commercial solutions for force measurements, and can easily be extended to investigate the mechanics of other subcellular compartments, e.g., mitochondria, stress-fibers, and endosomes.

Here, we present a protocol to investigate the intracellular mechanical properties of isolated embryonic zebrafish cells in three-dimensional confinement with direct force measurement by an optical trap.

During the development of a multicellular organism, a single fertilized cell divides and gives rise to multiple tissues with diverse functions. Tissue ...