Contact Electrification of Individual Dielectric Microparticles Measured

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Contact electrification of individual dielectric microparticles measured by optical tweezers in air Haesung Park, and Thomas W. LeBrun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12603 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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ACS Applied Materials & Interfaces

Contact Electrification of Individual Dielectric Microparticles Measured by Optical Tweezers in Air Haesung Park and Thomas W. LeBrun* Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States

KEYWORDS: Optical trapping, Optical levitation, charge measurement, surface study, contact electrification, Microparticles, Charging profile of individual microparticle

ABSTRACT

We measure charging of single dielectric microparticles after interaction with a glass substrate using optical tweezers to control the particle, measure its charge with a sensitivity of a few electrons and precisely contact the particle with the substrate. Polystyrene (PS) microparticles 15adhered to the substrate can be selected based on size, shape, or optical properties and repeatedly loaded into the optical trap using a piezoelectric (PZT) transducer. Separation from the substrate leads to charge transfer through contact electrification. The charge on the trapped microparticles is measured from the response of the particle motion to a step excitation of a

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uniform electric field. The particle is then placed onto a target location of the substrate in a controlled manner. Thus, the triboelectric charging profile of the selected PS microparticle can be measured and controlled through repeated cycles of trap loading followed by charge measurement. Reversible optical trap loading and manipulation of the selected particle leads to new capabilities to study and control successive and small changes in surface interactions.

INTRODUCTION

Charging of small particles is ubiquitous and arises in fields ranging from chemistry,1 to pharmaceutical drug delivery,2 and pneumatic transport technology.3 Measurements of charging can contribute to safety hazard control in transporting flammable agents and in successful drug delivery.2 For example, the surface charge on aerosol powders helps regulate the surface interaction of powders with internal organs through electrostatic interactions that dictate where and how particles deposit in the lungs and the respiratory tract.4 Also, maintenance of charges below a threshold of discharge prevents explosions and failures of electronic devices. Charges have been estimated with statistical or ensemble approaches for multiple particles using video measurement of trajectories under electrostatic fields5 or Faraday cup charge measurement6, but not on individual, pre-selected microparticles, nor the charge evolution over time. Unfortunately, measurement of charges on individual microparticles is not trivial due to many practical issues leading to inconsistency of the measurements. One of these issues is the difficulty inherent in manipulating and handling individual microparticles. Another fundamental difficulty is the triboelectric charging of microparticles, also known as contact electrification. This is the transfer of charge between two surfaces as they are contacted and then separated.7 Contact electrification (CE) is frequently underestimated and surprisingly poorly understood.


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Thus, any mechanical contacts during sample preparation and charge measurement changes the charge itself. For example, as we transfer powder particles from a container to another or as the moving particles touch a surface,7,8 these actions will lead to contact electrification of the particles with the substrate or with neighboring particles. Moreover, this triboelectric charging is neither predictable nor reversible since the charge transfer is affected by such environmental variables as temperature and moisture, and by the nature of the nano/microscale contact.9 In previous publications, Millikan oil drop techniques,10 magnetic levitometers,11 and optical trapping12 have shown that the levitation is an excellent way to measure the charge on an object down to the single electron level in a non-invasive manner. Because optical levitation combines femto-Newton

per

nanometer

stiffness

with

positional

detection

of

sub-nanometer

displacements,13–15 it can yield sensitivity of a single elementary charge under an E-field of 10 V/mm.12 Among the various levitation techniques, optical trapping is also one of the most convenient methods to levitate a small object.16–20 The technique allows manipulation and measurement of particle displacement with high spatial and temporal accuracy, and thus provides accuracy in the measurement of both the magnitude and the polarity of charge. Moreover, the reversible trap demonstrated here makes optical trapping an ideal candidate to measure contact electrification of an individual microparticle in air with a conventional optical microscope. In this paper, we will measure contact electrification of individual dielectric microparticles interacting with a substrate by using optical tweezers in air. While the trapping force holds the selected polystyrene microparticle in the radial direction, a piezoelectric transducer frees the particle from a glass substrate, and radiation pressure lifts the particle up into the trapping volume. While the particle is trapped, the charges upon it are measured from its transient


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response to a step excitation of electrostatic force generated between two parallel conducting plates. After the charge measurement, the trapped particle is replaced at the same location on the substrate. Thus, the target particle can be tracked and re-loaded. This process is repeated in a reversible and controlled manner which prevent unintentional particle to particle interactions allowing only more well-known particle and substrate interaction. As the particle develops charges through these repeated episodes of contact electrification, accumulations of charge are measured and recorded by optical tweezers, calibrated in near-real time. Moreover, our method can enforce contact electrification of the particle within a spatially confined area that is roughly the size of the contact area between the particle and substrate. This method, therefore, enables the study of micro/nanoscale contact electrification to understand the basic charging process under controlled conditions, which can have application in a wide range of areas from medical technology to more efficient triboelectric nanogenerators.

RESULTS AND DISCUSSION

Our technique is illustrated in figure 1. The measurement consists of repeated cycles of loading a particle into the trap, measuring its charge, and replacing the particle on the surface. Solid, dry particles are dispersed on a glass substrate (coverslip), which is translated to bring a selected particle into the focus of the trapping beam. A piezoelectric transducer (PZT) beneath the substrate is resonantly pulsed to vibrate the particle free from surface adhesion, and the particle is lifted to the trapping position by radiation pressure. Under the conditions here, particles freed from the surface are successfully trapped more than 95 % of the time. The trap is also used to move target particles from the area of initial deposition to a clear area of the substrate to avoid collisions with other particles.


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Reversible optical trap loading enables charging of the particle to be tracked as the particle is repeatedly trapped and placed (un-trapped under control) onto a glass substrate. This leads to charge transfer between the particle and the substrate through contact electrification. The charge on the particle is measured from the response (particle displacement) to an electric field while the particle is trapped. The response after removing an electric field corresponds to the homogenous solution of the governing equation of motion that yields the natural frequency and damping of the optical trap.5 This natural frequency and the calculated mass are used to calibrate trap stiffness for a quantitative measurement of the electrostatic force imposed on the charged particle in a uniform electric field (E-field). Knowing the field allows direct calculation of the charge on the microparticle. This measurement can be repeated hundreds of times for the same microparticle, once after each optical trap loading. Reversible optical trap loading enables us to study the entire contact electrification history of an individual 20 µ m diameter PS particle (DC-20, Thermo Scientific) interacting with a glass substrate in air. More information on this process is presented in the experimental detail section. To measure charge, an electrostatic field is applied to move the particle a few hundred nanometers away from trap center. The field is then blanked and the trajectory of the particle as it executes damped oscillations about the trap center is measured using a quadrant cell photodetector (QPD). For more efficient measurement, a square wave is used to drive the field and measure many cycles that may be averaged. Figure 2 (b) shows the measured trajectory of an 18.7 µ m diameter PS particle within an electric field driven by a 10 Hz square wave with duty cycle of 50 % and modulation depth of 100 % as shown in Figure 2 (a). The modulation frequency has been chosen to allow particle motion to completely damp during each 50 ms duty


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cycle (with a settling time of approximately 15 ms). The applied electric field (Eapp = Vapp./d) is monitored through the applied voltage signal (Vapp.) held by a high voltage amplifier between the two parallel conducting plates separated by d = 15 mm. As can be seen in the figure, the applied voltage signal is synchronously recorded with the particle trajectory captured by a QPD through a data acquisition (DAQ) system. The particle trajectory is recorded and averaged for 200 cycles, as shown by the blue circles in figure 2(c). It is then fitted to an ideal step response (red line, as given in equation S6 in the supporting materials),21 using a nonlinear least-square method, to yield the resonant frequency ( ωo ), damping ( β ), and induced steady-state displacement (XDC) as summarized in table 1. As can be seen from the data, the trapped particle shows nearly ideal behavior as a classic damped oscillator where the large ballistic motion (574 nm) dominates the random Brownian motion (~ 24 nm). (The target particle is first loaded about 100 µ m above the surface and then translated vertically about 10 mm above the substrate for the charge measurement to avoid any hydrodynamic or electrostatic interactions from the substrate.) To calibrate displacement and force, a video microscope is employed to calibrate the QPD and measure the particle diameter.22 With the mass m calculated from the diameter and material 2 density, the optical force (Fopt = -kx) is measured by the trap stiffness k (= ωo m ) multiplied by

the induced displacement (XDC), which then yields the charge q (= Fopt /Eapp) from the balance with the electrostatic force, as tabulated in table 1. With this simple technique optical tweezers can easily measure charges far below the minimum charge of 60,000 electrons measureable with a conventional digital electrometer.7 In the following, we perform the charge measurements on the microparticle as described above unless otherwise stated. The study of contact electrification begins with measuring initial charges on individual PS microparticles as above. During sample preparation the particles are charged by contact


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electrification between particles, from particle to container, from particle to glass capillary tube used to pick up the sample, and from particle to substrate. For each particle, the charge is measured four times with different strengths of electric fields and averaged. Once charge measurements on a particle are finished, it is placed to the side of the glass substrate to prevent possible interactions with other particles. Interestingly, we find that for particles prepared identically (i.e. from the same sample), not only can the initial charge vary by an order of magnitude, but both positive and negative polarities are observed. Figure 3 compares two PS microparticles found to be significantly different in both polarity and magnitude of charge. The trajectories captured by CCD camera and QPD are shown with the the applied voltage signals included for reference. The two particles exhibit anti-phase and in-phase responses to the applied E-field. Under the E-field modulation, a CCD camera captures the motion at 120 Hz. The trajectories are extracted by image analysis software (ImageJ) and confirm the steady-state QPD results. As clearly demonstrated in both the CCD-captured trajectory and the QPD signals, the two particles have opposite charges, positive in the first case (a to d) and negative in the second (a` to d`). Moreover, the total charge differs roughly by an order of magnitude. Figure 3 (d) and (d’) show steady-state displacements of 300 nm induced by E-fields of + 8 kV/m and + 1 kV/m, respectively. From the analysis of the transient response, figure 3 (d) and (d`), the particles have + 3520 ± 40 e and - 24900 ± 1560 e charges, respectively (see table 2). However in all cases, interaction with the glass substrate is observed to induce negative charging of the PS particle, up to statistical variations. This shows that ensemble charge measurements may not properly represent surface charges of microparticles, and that this method can provide valuable information obscured by ensemble measurement techniques. Although bi-polarity in particle


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charging has been previously observed between particles of very different sizes (polydisperse powder),23 we find bi-polarity among highly monodisperse particles. (All charges here are reported in units of the elementary charge e, which is positive. The results are reported with purely statistical standard deviations from the four repeated measurements.) Once the charge has been measured, the particle is returned to the surface by a motorized focusing block and placed at the same location so that the particle can be re-trapped. We repeatedly measured the charge over numerous trap loading cycles to study the contact electrification of the selected PS particle. The number of times the particle actually contacts the surface for each loading is not measured here because the substrate vibrates at least a few tens of cycles during a loading operation and multiple contacts are possible before the particle lifts into the trap. Though not the focus here, particle motion during substrate vibration has been measured by other groups and provides valuable insight into the particle dynamics on the surface.24 We assume the average number of surface interactions per detachment to be constant since we use the same PZT pulsing conditions throughout the experiment. We therefore report the average number of transferred charges per detachment (one loading cycle), defined as the difference in charge between subsequent charge measurements divided by the total number of particle detachments from the substrate since the previous measurement. For efficiency, about ten detachment and landing cycles are performed between charge measurements. The charging profiles in figure 4 show the change in charge on a PS particle due to contact electrification during repeated detachments from the substrate. The initial charge in figure 4 (a) is found to be + 4100 e, and it acquires enough negative charge from the glass substrate during 140 cycles of detachment to decrease the total charge by 30 % (+ 2840 e). The acquisition of negative charge from glass to PS material has been also observed on the macro scale.25


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The charge transfer per iteration decreases as the particle is repeatedly detached and placed on a fixed location of the substrate (without moving the xy translational stage). This part of the 0.43

charging profile (up to 90 detachments) can be fit to a power law ( y = −97.9 x

+ 4185 ), as

indicated by the gray line in figure 4 (a). However, the negative charge transfer jumps by more than an order of magnitude at around 130 detachments. This is representative of the behavior we typically observed: decrease of charge transfer, as though the charge available for transfer is exhausted at the donor site (or saturated at the acceptor site), punctuated by large jumps frequently correlated with contacting a new point on the substrate. In this case the jump coincided with escape of the particle to a new point on the substrate (change of contact point), from where it was subsequently trapped. But this was not always the case. Some observed jumps were not accompanied by interaction with new substrate locations, but it should be borne in mind that the rotation of the particle is not measured here. This saturation behavior recalls long-term charging found in measurement of rolling spheres on a surface.26 A gradual decrease in charge transfer rate has been reported in the kinetics of contact electrification between a dielectric surface and a metal sphere rolling in a circular path27,28 and also between a polymeric surface and polymeric cylinder-head touching each other discretely29. We find that a power law adequately describes the reduction in charge transfer per detachment, but don’t purport to show that it uniquely represents the underlying physics. Relatively few studies allow the progressive measurement of charge accumulation due to limited sensitivity or destructive charge measurement. Multiple models have been used to describe the change in charge transfer, but the reproducibility of measurements does not yet support critical comparison of models.


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The observed charge transfer to the PS particle remains persistently negative, independent of the polarity of the charge on the microsphere as shown in figure 4 (b), where the particle polarity changes from positive to negative after approximately 40 detachments. In fact, the charge transfer remains roughly constant in this case and a straight line would fit the data, highlighting the difference in overall form of this charging profile with respect to the previous case. While jumps are observed here, the saturation behavior is not. This is probably due to the “unconstrained” nature of the detachment process used for the data in figure 4 (b) as described below. Figure 4 (a) used a constrained detachment. To test the effect of varying the point on the substrate that a particle contacts, the charging profile can be measured using one of two processes. In the “constrained” process the particle is trapped and replaced onto the same nominal position on the substrate after detachment. The trap beam remains on while pulsing the PZT so that the particle only interacts with one position on the substrate as it is detached by the PZT. To test contact interaction with a larger area, we also used an “unconstrained” process in which the trap beam is turned off after the particle is replaced on the substrate. When the PZT launcher is pulsed multiple times, the particle is free to move about on the substrate. To re-measure the particle, the substrate is re-centered after trapping. The localization of the particle on the surface by the trapping beam during piezo actuation is illustrated in figure 5, which compares the locations where the particle stops during constrained and unconstrained detachment. In figure 5 (a), the large ring represents the particle diameter while the small circles give the locations of the particle after each cycle of launching and landing. The average standard deviation of the change in position is about 380 nm. Without the trapping beam, the particle interacts with a much larger region of the substrate. Pulsing the PZT without the trapping beam causes the particle to execute a random walk on the surface. The average


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distance per detachment is 1.98 µ m – roughly five times larger than the diameter of the area explored under constrained detachment. To determine whether the constrained particle is interacting with the same area of the substrate, we estimated the contact area by measuring the pull-off force of a 5 µ m diameter PS sphere on glass using an atomic force microscope (AFM) as shown in supporting figure S1. The measurements were only able to set a lower bound on the contact force of 1.2 µ N which gives a lower bound on the contact radius of 220 nm for the 20 µ m particle. Calculating the contact radius using the JKR (after John, Kendall, and Roberts)30 model gives a value of 276 nm. Together, these values indicate that under dry conditions the region of the substrate contacted is somewhat wider than the contact area, and that the particle would interact with largely the same area of the substrate during constrained detachment. While the contact mechanics may include local plastic deformation that increases the contact area,31,32 the JKR calculation gives a conservative value for the comparison here. In addition, to the extent that humidity influences the surface interaction, the associated meniscus would also increase the contact area,33,34 making the variation in particle position during constrained detachment smaller compared to the contact area. Figure 6 compares charge transfer for constrained and unconstrained detachment. Charging for unconstrained detachment (without the trapping laser) proceeds at a nearly constant rate of 26 electrons per detachment without significant deviation. The average constrained charging rate is lower, about 16 electrons per detachment. Interestingly, the observed rate of charge transfer is largely independent of the total charge on the particle. Measurements on otherwise identical but much more highly charged particles during constrained detachment (charge roughly 20,000) find a nearly identical charging rate of 15 electrons per detachment (data not shown).


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Constrained interaction also shows qualitatively different behavior, similar to that observed in figure 4 (a). The total charge appears to plateau or saturate where the charge transfer drops to about 4 electrons per detachment, while discrete jumps in charge punctuate the process, and an average charge transfer per detachment of 16 electrons is observed over 100 cycles. Jumps in charge transfer have been observed with multiple particles under varying conditions, but only during constrained detachment. While the saturation could be explained by charge localization that increases the energy required to transfer an electron, what causes the particle to move out of the the trap or rotate remains a question. And the jumps do not occur randomly. Indeed, for the constrained cases shown here, all three show jumps after approximately 100-120 detachments. A possible candidate to explain the change in particle position or rotation is the Coulomb force between localized areas of transferred charge which would be on the order of the optical force. Optical forces in our instrument range from about 10 pN at the trap position to 0.15 pN at the levitation position for the same optical power. Moreover, optical forces decrease as the particle descends for landing because the beam power is reduced. Assuming the transferred charge is localized6 and the particle moves normal to the surface, 1,000 electrons generate a force of hundreds of pN or greater at distances less than 1 µ m . As the initial charge on the particle can be positive or negative and vary by an order of magnitude, the phenomenology could also be quite varied. The resulting interaction will be attractive at short distances when dominated by transferred charge. But it may be attractive or repulsive at long distances depending on total charge, which can greatly complicate the dynamics of particle motion during launching and destabilize trap loading. Experimentally, we observe that as the charge transfer grows the particle either becomes too difficult to trap, or binds to the surface so strongly that that PZT vibration no longer dislodges it.


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Charge transfer could also be affected by local charge states of the glass substrate. Recent measurement using Kelvin force microscopy (KFM) reveals inhomogeneity (mosaics) of surface charge. According to the report by Baytekin et al., the local electric charge varied significantly both in magnitude and polarity over a several hundred nanometer scale.35 This variation could affect charge transfer so that sudden changes in the charging profile are not always predictable with spatial variation in every incident. However, the overall charging profiles, with and without the spatial confinement, show that the contact electrification can be efficiently suppressed by spatial confinement. Ultimately, this highlights the potential advantages and disadvantages of using trapping to measure contact electrification. Charge transfer can be measured with single electron resolution,12 but the low confinement forces may limit localization and hence the ability to study transfer between specific sites on a particle or surface without improved confinement. Given the low transfer rates of a few electrons per contact observed, single electron resolution is highly desirable. Although it is hard to directly compare individual results, because charge transfer is strongly affected by environmental conditions and by the nature of contact (e.g. pull-off vs rolling vs sliding), our observed charge transfer rate of 4 to 26 electrons comports surprisingly well with the previously reported value of 2 to 20 e per detachment of a 10 µm PS particle from a graphite surface using AFM.36 Measuring the induced displacement ( X o ) under a known E-field ( Eapp ) enables us to estimate the charge on the trapped particle once the trap stiffness (k) has been measured. The applied Coulomb force and thus the net charge ( Neff ) are then readily estimated from the equation of force balance, N eff = − kX o / eE app where e represents elementary charge. However, Brownian


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motion causes the induced displacement to fluctuate, limiting our displacement resolution and therefore charge resolution. To estimate the sensitivity of our charge measurement we first consider upper and lower bounds to the expected sensitivity, and then the calculated sensitivity based on our measurements under different assumptions. We define our charge sensitivity as the minimum charge that generates a displacement large enough to be distinguished from Brownian motion under our measurement conditions. An overly conservative estimate can be determined using the standard deviation of the position fluctuations from Brownian motion given by equipartition as

k BT / k

. This would give a position fluctuation of 23.6 nm for our stiffness, while the observed standard deviation is 24.0 nm, which corresponds to 154 electrons for a particle with a charge of 3.7 k electrons. This effectively represents a conservative estimate for sensitivity before data averaging. Improved estimates can be obtained by accounting for bandwidth limiting using a frequency domain analysis. We use the displacement fluctuations calculated from the theoretical form of the noise response for a harmonic oscillator in the frequency domain37 as the basis for comparing our measured Brownian response. (The noise response is considered in the accelerometer limit

f

f o , where fo is the resonant frequency). To estimate a theoretical lower bound for the sensitivity we consider the displacement noise

for a thermally-limited harmonic oscillator. The theoretical noise response for a harmonic oscillator in the accelerometer limit37 is given by

4k BT γ∆f / k , where γ , T , kB , ∆f , and k

are the coefficient of friction, temperature of the surrounding medium, Boltzmann constant, the measurement bandwidth, and trap stiffness, respectively. For our case (described in table 1) this yields a displacement noise of 0.958 nm in a 1 Hz bandwidth, which gives an estimated charge


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sensitivity of 6.1 electrons for an 18.7 µ m diameter sphere in air at 52 °C . Because the laser can heat the trapped particle, the temperature is calculated based on the temperature dependence of air damping retrieved from the transient response presented in figure 2 and table 1. Specifically, we estimate the air temperature to be uniform to within a few percent in the region the particle explores during displacement (assuming uniform heating) and that equilibration between the particle surface and surrounding air is fast compared to our measurement timescale. The temperature is then calculated from the damping based on the temperature dependence of air viscosity using Stokes law and Sutherland’s formula38 including a slip correction factor39 (Details of the temperature estimate is given in the supporting materials.) To estimate our measurement sensitivity we use the power spectal density of the recorded position fluctuations. The displacement due to Brownian motion of the trapped particle is measured using the quadrant-cell photodetector (QPD) over 4 s (concatenated from the 200 cycles of 20 ms) at the resting position at the center of trapping (a settling time to within 2 % variation is approximately 8.1 ms). This position fluctuation is then Fourier transformed to calculate the one-sided power spectral density (PSD) of the displacement normalized per root Hz, a displacement spectral density (DSD). As shown in figure S2, the low-frequency noise response (

f o ) of the trapped particle was retrieved by fitting the DSD with the full Langevin

equation (given in equation S5 in the supporting materials)40,41 over the frequency range from 10 Hz to 800 Hz. The trapped particle shows a roughly constant noise response of about 7.54 ( ± 2.15) ×10−10 m / Hz from a few Hz to 100 Hz (well below it’s typical resonance frequency of 250 Hz). Using this value to estimate the charge sensitivity we would expect our experimental value to be 4.8 ( ± 1.4) electrons with a measurement bandwidth of 1 Hz. However, this ignores ever-present low frequency variations such as drift. Integrating the PSD over a measurement


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bandwidth of 0.305 Hz to 10 Hz gives a more representative estimate of 1.81×10−9 m / Hz corresponding to a charge sensitivity of 11.5 electrons with a measurement bandwidth of 1 Hz. To improve sensitivity to the single electron level, pressure may be reduced, the electrostatic field increased, or a control system may be used.12,42 Reversible optical trap loading also should be extendable to smaller particles with improvements in the PZT driving circuit and the design of PZT launcher to create more ultrasonic power.43 However the transient response method is only applicable to underdamped systems which may require more highly focused beams or stronger optical power to yield an underdamped response of the trapped particles. We occasionally observed an interesting phenomenon that some charged particles form a crystalline array floating above the substrate under continuous PZT vibration, as shown in figure S3. This floating array is composed of hundreds of well-separated particles, arranged in a quasihexagonal pattern independently of whether the trapping beam is present. In fact, is was not possible to pull a particle from the array with a trapping beam or to move particles easily within the array. At most the trapping beam was able to induce exchange between adjacent particles with little or no rearrangement of the surrounding particles. The array is evidently a highly charged 2D colloidal crystal as the inter-particle spacing is quite large, consistent, and particleparticle contact is not observed. The forces that levitate the array and hold it in position remain to be studied, but the force of acoustic radiation pressure should be greater than the particle mass in this experiment, and therefore sufficient for levitation. Although acoustic levitation experiments typically hold particles about half a wavelength of sound (2.6 mm here) above a transducer,44 this is not consistent with our observation of levitation at less than 100 µ m above the surface. However, significant forces are also present at much shorter distances (e.g. less that λ /20) which is consistent with our observations.45 In addition, the force required to pull the particle


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close to the surface against the acoustic force is likely to be the same force that restricts the particles to one area of the coverslip – coulomb attraction to the positively charged area of the surface generated by contact electrification. Previous observations of highly ordered self-aligned charged particle arrays due to contact electrification have been reported.27,46 These levitated 2D colloidal crystal arrays may be useful in studying colloidal dynamics free of solvent-mediated hydrodynamic or surface effects that can limit measurements47, or for studying 2D arrays dominated by electrostatic interactions without the complications that may occur at interfaces48.

CONCLUSION

We have experimentally demonstrated reversible optical trap loading of a selected polystyrene (PS) particle in air over several hundreds of cycles with 96.5 % trapping efficiency. This reliable optical trap loading technique extends the utility of an optical trap to measure sample dependent properties and changes occurring beyond a single trapping time. As an example, using transient response, charges of the trapped PS particles were measured throughout numerous trap loadings. From these experiments, we measured a charging profile (a history of contact electrification) of individual PS microparticles interacting with a glass substrate, and found a significant variation in both polarity and amount of charge in the particles. Thus, our method can replace conventional ensemble charge measurement with accurate charge measurement on individual particles with traceability. Moreover, we were able to suppress charge transfer (i.e., contact electrification) onto the selected PS particle around 40 % of freely interacted particles by enforcing it to interact with an intended local area of a glass substrate using the trapping force. Our technique enables optical tweezers to be used to study samples with a limited quantity and to study particle and surface interactions made over confined areas.


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FIGURES

Figure 1. Schematics of the particle loading sequence (detachment, trapping, and landing) under illumination of focused IR light which holds the particle in the radial and axial directions so that the particle can be selectively loaded (and placed) in repeatable manner. Applying an external electric field, selective optical trap loading enables us to measure the charge accumulation developed by a single particle during contact electrification over repeated detachment events.


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Figure 2. Measurement of charges on the trapped particle: (a) a uniform electric field (E-field) applied along the x-axis is synchronously recorded with (b) the particle trajectory through the data acquisition (DAQ) system. While the particle returns from the induced displacement (from 50 to 100 ms), the transient (50 to 65 ms, within 2 % deviation from a steady-state) and static responses are analyzed to retrieve the resonant frequency, damping, and induced displacement (Xo). (c) Because the range of ballistic motion (> 600 nm) is much larger than the Brownian motion, the particle trajectory nearly fitted to an ideal transient response of a one dimensional oscillator with (d) small residuals (< ± 10 nm).


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Figure 3. Individual particles from the same sample show substantial variation in both polarity and net charge (one order of magnitude difference). For the highly charged particle, the polarity was easily verified with the particle trajectory ((a) and (a`)) captured by CCD camera under electrostatic fields. The direction of QPD displacement signals ((b) and (b`)) indicate the polarity of individual particles while the direction of the applied electric field (voltage divided by distance) is synchronously measured through the data acquisition system ((c) and (c`)). A high resolution transient response measurement of particle trajectory enables direct determination of the polarity and charge on the individual microparticles (positive-(d) and negative-(d`)). Both trajectories are very close to ideal transient response of an underdamped system with small residuals defined as difference between fitted ideal behavior and experimental trajectory ((e) and (e`)).


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Figure 4 A charge accumulations on the positively charged polystyrene (PS) particle over repeated contact electrification (detachment) with glass substrate. (a) The PS particle acquires negatively charged elements from the glass substrate. (b)The successive charge measurements on the PS microparticle indicate that the PS particle eventually becomes negatively charged regardless of their initial polarity.


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Figure 5. The trajectories of particle center are plotted as the particle repeats detachments using piezoelectric launcher excited by 10 pulses of square wave (600 V) (a) with (140 mW) and (b) without trapping beam (0 mW). The particle has been levitated and then actively landed by reducing the incident power. The particle has been landed over limited area of (a) σ x = 0.41

µ m and σ y = 0.36 µ m , whereas freely interacting particle randomly walk around over area of


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(b) σ x = 5.25 µ m , σ y = 5.21 µ m . The black arrow indicates time sequence of the landing trajectories.

Figure 6. Repeated interactions over same area hinder the transfer of charged element about 40 % (about 16 e/ detachment) compare to the unrestricted interaction of PS particle with glass substrate (about 26 e/ detachment).


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Figure 7. Schematics of experimental setup used in this study are comprised of three groups: optical illumination (HAL100-halogen illuminator, Nd:YVO4 trapping laser with EOM-electro optic modulator, and NIR-LWD-near infrared corrected long working distance objective lens), mechanical alignment group (TS-translational stage, MFS-motorized focusing stage), and positional detection group (QPD-quadrant cell photodetector, ND-neutral density filter). Over all setup including piezoelectric (PZT) launcher and sample enclosure covered with one pair of indium tin oxide (ITO) coated coverslips are installed on conventional inverted microscope (Eclipse TE2000).


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EXPERIMENTAL DETAILS

Sample preparation For the contact electrification experiment, dry polystyrene particles were used due to better sphericity and smaller size variation (DC-20, diameter = 20.0 µm ± 0.9 µm, Thermo Fisher Scientific Inc.). The PS microparticles were desiccated in a vacuum for few days before the experiment to remove residual moisture from the particle surface while the glass substrate was heated at 150 oC in a mini furnace. Once microparticles are dispersed on the substrate, the sample enclosure is secured with silicone rubber to seal the gap from the external flow of air. The temperature of our laboratory was kept at 23 oC with a relative humidity around 30 %.

Optical tweezer As shown in figure 7, the optical tweezer setup was comprised of three major functional groups to enable selective loading of a target particle in a reversible manner and to provide charge measurement. These groups are the trapping laser, the mechanical alignment, and the positional detection groups. The trapping laser group is composed of an infrared (IR) laser and electro-optic modulator (EOM). The fiber collimated 1064nm diode-pumped Nd:YVO4 laser (J20I-8S-12K/BL-106C, Spectra Physics) was delivered to the near-infrared (NIR) corrected long-working-distance (LWD) objective lens (20X, NA = 0.40, WD = 24.5 mm) with slightly under-filled output beam diameter of 8.0 mm (filling ratio = 0.8).49 The EOM (350-80LA/Driver 302RM, Con-Optics) regulates the optical power, demanded during the four different phases of particle/substrate interactions including resting, loading, trapping, and (re-)landing, as depicted in figure 1. The mechanical alignment group is designed to bring a selected particle relative to the fixed focus of the trapping beam so that the momentarily released particle can be trapped.


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Thus, careful alignment of the sample to the fixed trapping laser enables a selective loading while a motorized (vertical) translational stage (MFS) deliver the trapped particle to the measurement position, 10 mm above the substrate where the hydrodynamic and electrostatic couplings can be ignored. A quadrant cell photodetector (QPD, 2031 Newport) and CCD camera (Flea 3, Point Grey) were complementarily used to detect particle motion. A QPD was employed as a primary positional detector while a video microscope provided independent particle trajectory using video images captured by a Rochi-ruled CCD camera. The particle trajectories measured by QPD were mapped with the CCD recorded trajectories extracted by an image analysis (imageJ)50 and then converted into a displacement by multiplying the detection sensitivity α

as

the

ratio

of

particle

displacement

to

QPD

measured

voltage,

α = xlength (t ) / VQPD (t ) . These complimentary detectors allow us to measure the trapping force and electrostatic force through a simultaneous calibration of the QPD and trap stiffness and thus determine the charge on the particle.

Piezoelectric launcher and sample enclosure A ring-type PZT transducer (inner diameter = 13 mm, outer diameter = 38 mm, and thickness = 6.35 mm, purchased from APC International-USA) is installed on an XYZ linear translational stage (562-XYZ Newport). A copper ring (inner diameter = 12 mm, outer diameter = 15 mm, and thickness = 1 mm) is inserted between the PZT transducer and glass substrate. All driving signals are produced from a waveform generator (HP33250A) and amplified by a high voltage amplifier (PZD700A M/S, TREK Inc.) which provides an electrical gain of 300 V/V with maximum current ± 200 mA over a signal bandwidth of 150 kHz. The resonant frequency of the PZT launcher was found to be around 64 kHz by scanning the driving signal over a broad range


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of frequency. The sample enclosure (width × height × separation: 15 mm × 10 mm × 15 mm) was fabricated using a 3D printer. The two parallel faces of the sample enclosure are covered by ITO coated cover glass (0.7 mm thick, 10 mm ×15 mm, resistivity = 50 Ω) to produce a maximum electric field of 47 kV/m with estimated fringe fields less than 1% in magnitude. We have calculated the electric fields using a 3D electrostatic MoM (method of moments) solution distributed by NEVA Electromagnetics (module name E21).51 This module computes the capacitance of parallel plates, charge distribution on the plates, and the electric field at a point of interest. The electric field is found to change by less than 1% with 1mm of the cell center in any direction. The enclosure is attached to the top of the PZT holder. Figure S4 illustrates the overall construction of the PZT launcher.

Driving piezoelectric transducer To detach microparticles, the piezoelectric transducer can be driven in two different modes of operation: pulsed run and continuous run. In the pulsed run, the PZT transducer is driven by a gated driving voltage signal, composed of several cycles of a square wave. In the continuous run, it is driven by the continuous driving signal (square waves). The continuous mode is efficient in breaking up agglomerates of particles into smaller groups by transferring the excessive momentum of detached particles onto others. These multiple collisions of the continuous run cause particles to become highly charged which promotes their separation and isolation. Once a particle is isolated, the pulsed mode is used to detach it from the substrate and keep it with in the field of view. In the pulsed mode, the efficiency of detachment (or flying distance of freed particles) can be easily enhanced (controlled) by varying the total number of square waves or increasing their amplitude. About 1000 cycles of square wave are enough to take full


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advantage of resonant amplification of PZT vibration at its resonance frequency. Thus, the PZT launcher allows us to load the desired microparticle in a reversible and selective manner.

Radiometric force We haven’t estimated the photophoretic (aka radiometric) force here because it acts mainly perpendicular to the electrostatic field used for charge measurement. A shift of the particle away from the optical axis will cause the hot spot on the particle to shift as well and potentially give rise to a transverse force, but we expect it to be small. The particle moves at most by 500 nm and the hot spot by less than 140 nm, which would couple about 1% of the radiometric force transversely. But if so, the transverse trapping force experienced by the particle would still be linear for small displacements, just slightly modified (most likely reduced). That linear variation is just the stiffness measured during calibration, so the calibration already includes any linear effects due to radiometric forces.


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TABLES. Table 1. Transient response analysis and charge measurement

Transient response analysis Fitting Coefficient Value (95 % CB)

XDC [µm]

࣓o/2࣊ [1/s]

-0.5743 241.7 (± 0.0014) (± 0.5)

β

E(=V/d) [kV/m]

k [µN/m]

Fopt. [pN]

q(e) [electrons]

8.086

8.29

4.76

- 3680

[1/s] 967.5 (± 4.1)

Table 2. Comparison of oppositely charged particle Particle 1

Particle 2

Diameter (µm)

21.46

19.88

Polarity

Positive

Negative

Total charges (e)

3520 ± 40

- 24930 ± 1560

ASSOCIATED CONTENT Supporting Information. Figures showing (S1) the measurement of pull-off force to estimate the work of adhesion between the polystyrene and the silicon dioxide substrate using a 5 µ m PS particle attached to the apex of AFM cantilever (inset); (S2) the displacement spectral density (DSD) plot for the


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trapped microparticle ; (S3) formation of a floating crystal array over the substrate under continuous run of the PZT transducer; (S4) schematics of the PZT launcher: assembly of a ringtype PZT transducer, 3D printed holder for the PZT transducer with an interstitial copper ring, and an aluminum base plate. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources All work performed under the support of the National Institute of Standards and Technology. Notes The authors declare no competing financial interest. Copyright protection is not available for works of the U.S. Government, therefore NIST cannot license copyrights to these works. ACKNOWLEDGMENT Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.


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Charges on a polystyrene microparticle are measured over repeated interactions with a glass substrate using an optically trapped particle in air.


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Charges on a polystyrene microparticle are measured over repeated interactions with a glass substrate using an optically trapped particle in air. TOC 620x377mm (96 x 96 DPI)


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Schematics of the particle loading sequence (detachment, trapping, and landing) under illumination of focused IR light which holds the particle in the radial and axial directions so that the particle can be selectively loaded (and placed) in repeatable manner. Applying an external electric field, selective optical trap loading enables us to measure the charge accumulation developed by a single particle during contact electrification over repeated detachment events. Figure 1 486x205mm (96 x 96 DPI)



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Measurement of charges on the trapped particle: (a) a uniform electric field (E-field) applied along the xaxis is synchronously recorded with (b) the particle trajectory through the data acquisition (DAQ) system. While the particle returns from the induced displacement (from 50 to 100 ms), the transient (50 to 65 ms, within 2 % deviation from a steady-state) and static responses are analyzed to retrieve the resonant frequency, damping, and induced displacement (Xo). (c) Because the range of ballistic motion (> 600 nm) is much larger than the Brownian motion, the particle trajectory nearly fitted to an ideal transient response of a one dimensional oscillator with (d) small residuals (< ± 10 nm). Figure 2 511x577mm (96 x 96 DPI)


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Individual particles from the same sample show substantial variation in both polarity and net charge (one order of magnitude difference). For the highly charged particle, the polarity was easily verified with the particle trajectory ((a) and (a`)) captured by CCD camera under electrostatic fields. The direction of QPD displacement signals ((b) and (b`)) indicate the polarity of individual particles while the direction of the applied electric field (voltage divided by distance) is synchronously measured through the data acquisition system ((c) and (c`)). A high resolution transient response measurement of particle trajectory enables direct determination of the polarity and charge on the individual microparticles (positive-(d) and negative(d`)). Both trajectories are very close to ideal transient response of an underdamped system with small residuals defined as difference between fitted ideal behavior and experimental trajectory ((e) and (e`)). Figure 3 672x420mm (96 x 96 DPI)



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A charge accumulations on the positively charged polystyrene (PS) particle over repeated contact electrification (detachment) with glass substrate. (a) The PS particle acquires negatively charged elements from the glass substrate. (b)The successive charge measurements on the PS microparticle indicate that the PS particle eventually becomes negatively charged regardless of their initial polarity. Figure 4 408x549mm (96 x 96 DPI)


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The trajectories of particle center are plotted as the particle repeats detachments using piezoelectric launcher excited by 10 pulses of square wave (600 V) (a) with (140 mW) and (b) without trapping beam (0 mW). The particle has been levitated and then actively landed by reducing the incident power. The particle has been landed over limited area of (a) σx = 0.41 µm and σy = 0.36 µm, whereas freely interacting particle randomly walk around over area of (b) σx = 5.25 µm, σy = 5.21 µm. The black arrow indicates time sequence of the landing trajectories. Figure 5 183x305mm (96 x 96 DPI)



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Repeated interactions over same area hinder the transfer of charged element about 40 % (about 16 e/ detachment) compare to the unrestricted interaction of PS particle with glass substrate (about 26 e/ detachment). Figure 6 476x334mm (96 x 96 DPI)


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Schematics of experimental setup used in this study are comprised of three groups: optical illumination (HAL100-halogen illuminator, Nd:YVO4 trapping laser with EOM-electro optic modulator, and NIR-LWD-near infrared corrected long working distance objective lens), mechanical alignment group (TS-translational stage, MFS-motorized focusing stage), and positional detection group (QPD-quadrant cell photodetector, ND-neutral density filter). Over all setup including piezoelectric (PZT) launcher and sample enclosure covered with one pair of indium tin oxide (ITO) coated coverslips are installed on conventional inverted microscope (Eclipse TE2000). Figure 7 273x210mm (96 x 96 DPI)


Contact Electrification of Individual Dielectric Microparticles Measured

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