Loss-Free Photo-Manipulation of Droplets by Pyroelectro-Trapping on

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Loss-Free Photo-Manipulation of Droplets by Pyroelectro-Trapping on Superhydrophobic Surfaces Xin Tang, and Liqiu Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02470 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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Loss-Free Photo-Manipulation of Droplets by Pyroelectro-Trapping on Superhydrophobic Surfaces Xin Tang1,2 and Liqiu Wang1,2* 1

2

Department of Mechanical Engineering, the University of Hong Kong, Hong Kong

HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), Hangzhou, Zhejiang 311300, China *Correspondence and requests for materials should be addressed to L.Q.W. ([email protected])

Keywords: droplets, loss-free manipulation, photo-control manoeuvre, superhydrophobic surfaces, pyroelectricity; Abstract Manipulation of tiny amounts of liquid is a fundamental technique for miniaturized diagnostic, analysing and synthetic processes. Among diverse manoeuvre methods, lightcontrolled liquid manipulation outstands because of its ready controllability, high spatial precision, and noncontact feature of light. As light controls the motion of liquid droplets, substantial liquid loss frequently accompanies, leading to reduced sample volume, contaminated devices, and erroneous results. Here, we report a light-controlled droplet-manoeuvre method based on pyroelectro-trapping on superhydrophobic surfaces. On such platform, a light source traps and guides the droplet on a non-wetting surface remotely, offering a precise and loss-free droplet transport that eliminates inter-sample cross-contaminations. Our approach provides a simplified, facile and compact platform which is suitable for repeated and multi-step usages. Droplet-based micro-reactions and enhanced mixing inside tiny droplets are effectively

1 ACS Paragon Plus Environment

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demonstrated using the platform. The photo-controlled loss-free droplet manoeuvre is very promising for applications like chemical/bio-assays, microfluidics, and liquid transfer.

Tiny liquid droplets or flows appear widely in various processes, such as high-throughput chemical reactions, point-of-care diagnostics, and accurate biological analyses, providing numerous benefits, including well-controlled reaction condition, fast heat transfer, and reduced reagent consumption.1-3 The manoeuvre towards tiny amounts of liquid determines the analysing accuracy as well as the synthesizing efficiency notably, and gives rise to the demand for high manipulating precision and flexibility.4-7 On planar surfaces, the motion of droplets can be controlled using diverse developed approaches

such

as

magnetic-controlled

surfaces,8

thermocapillary

surfaces,9,10

and

electrowetting-on-dielectric (EWOD) chips.11,12 Photo-controlled liquid motion attracts intensive attention because of light-offered remarkable advantages such as contactless interaction, high spatio-temporal precision, and ready controllability. A number of devices and chemicals are developed to convert light irradiation into liquid motion by exploiting various photo-related energy conversions which include photo-electric (optoelectrowetting platform),13,14 photochemical

(light-responsive

surfaces

and

surfactants),15-17

and

photo-mechanical

(photodeformation of liquid crystal polymers) types.18 Such conversions create imbalanced capillary forces across liquid droplets or slugs by inducing surface energy gradient,13,15,16 variation of interfacial tension,17 and asymmetric channel deformation,18 respectively. As a result, using the light-controlled methods, liquid droplets/slugs spatially follow the asymmetricallyilluminated guiding light sources at a relatively low speed, typically 0.03-0.5 mm s-1.


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As light actuates liquid transports, liquid droplets or slugs deposit substantial residues on the partial-wetting surfaces or sidewalls, resulting in sample cross-contaminations and limited device lifetime.18-20 Even when droplets are manoeuvred in immiscible carrier fluid, the analytes in droplet vehicles are prone to diffuse into surrounding environment.21 Apart from the limited sample accuracy during transports, the existing photo-controlled methods often involve stringent stimuli such as high voltage,13,14 UV light,15 and direct droplet heating,22 making them incompatible with biological applications. Moreover, the fabrication processes of available methods are expensive owing to the time-consuming micro/nano fabrications and the complex organic chemical synthesis. Consequently, the high device cost as well as the limited repeated usage restrict their accessibility and scalability. To dexterously manipulate tiny liquid droplets in a photo-controlled and loss-free manner, we utilize a superhydrophobic surface as a pedestal. On the non-wetting surface, spherical aqueous droplets roll frictionlessly with minimized substrate resistance, leaving no visible trace of liquid residues.23 To precisely manoeuvre the mobile droplets, we place an electrodeless control panel above the superhydrophobic platform. The control panel consists of two contacting transparent layers which are a glass wafer coated with Prussian blue (PB) nanocubes and a lithium niobate (LN) wafer. As near infrared (NIR) light irradiates the control panel, PB nanocubes trigger thermogenesis photothermally. In response to the temperature rise, LN wafer pyroelectrically generates non-uniform electric field which drives surrounding droplets towards the irradiated region, thereby, realizing precise spatial control and loss-free transportation of liquid droplets. Droplets motion on the pyroelectro-trapping on superhydrophobic surface (PETOS) platform can be continuously guided through NIR light at a speed as high as 1 mm s-1. The PETOS circumvents electrodes/voltage circuits, external pumps/valves, and addition of


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photo-responsive chemicals for droplets actuation. The method provides a miniaturized, compact, and reusable platform for remote and lossless photo-control of tiny liquid droplets.

Results The configuration of the PETOS. The PETOS platform consists of three components, a superhydrophobic substrate, a pyroelectric crystal wafer, and a layer of photothermal material (Fig. 1a). To fabricate a strongly water-repellent pedestal, we electrodeless-galvanically deposit silver nanocrystals onto a copper substrate, and then functionalize the silver nanostructures with a self-assembly monolayer of fluorinated thiol (Fig. 1b).24 On the superhydrophobic surface, in addition to a high contact angle of 157°, water droplets exhibit high mobility as a 10-µl water droplet instantaneously rolls off at a sliding angle as small as 0.5° (Fig. 1d). Therefore, on the as-fabricated superhydrophobic pedestal, water droplets experience negligible contact-angle-hysteresis (CAH) forces Fγ = (l / 2)γ (cos θ r − cos θ a ) , where l, γ, θ r , θ a denote length of contact line, surface tension, receding and advancing contact angles, respectively.25 To actively control the droplets mobility, we then install a planar panel termed “pyroelectro-trapping (PET) panel” above the superhydrophobic pedestal and separate the panel and pedestal using spacers of thickness L. The transparent PET panel is readily fabricated by placing a layer of photothermal material on top of a pyroelectric LN crystal wafer. Layer-bylayer (LbL) assembly of PB nanocubes onto a glass wafer produces a photothermal material layer with both high optical transparency and superior thermogenesis capacity (Fig. 1c). At equilibrium, external surface charge compensates the spontaneous polarization Ps of the LN.26-28 Upon irradiation of a 785-nm laser, because of charge transfer transition between Fe(Ⅱ) and


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Fe(Ⅲ) in PB, the PB nanocubes effectively absorb the optical energy and generate heat,29,30 leading to a local temperature rise in LN. Such temperature increase ∆T induces a polarization variation ∆Ps, causing a net surface charge density σ=Pc∆T on the irradiated region, where Pc is the pyroelectric coefficient (for LN at 25 °C, Pc = −8.3 × 10 −5 C m-2 °C-1). Such pyroelectric effect has been utilized for reversible liquid crystal fragmentation,31 microlenses fabrication,32 and material dispensing.26 The non-uniform electric field associated with the net surface charges exerts dielectrophoretic (DEP) attraction FE on surrounding liquid droplets.33 When the light-triggered attractive force applies, the sliding angle of a 10-µl water droplet increases to be higher than 2° (Fig. 1d). On a level PETOS platform, once the DEP force FE overcomes the contact-linepinning force Fγ , the liquid droplet is driven towards and further immobilized under the irradiation. As the droplet moves on a superhydrophobic surface, the micro-capillary bridges situated at the receding contact line either recede or pinch off, leaving minute amounts of liquid on top of nanoscale asperities. Such minute amount of residue is negligible compared with macroscopic one on partial-wetting surfaces.34-37 In this way, we lossfreely transport and precisely control droplets by readily shifting the illumination on the PETOS (Fig. 1e).

Fabrication of the PET panel. The biocompatible PB nanocubes feature strong thermal stability in addition to high light-to-heat conversion efficiency, thereby, serving as photothermal agent of the PET panel. We synthesize the PB nanocubes by mixing aqueous solution of FeCl3 and K4[Fe(CN)6] with citric acid as the surface capping agent.29 The X-ray diffraction pattern of the as-prepared particles shows the diffraction peaks of PB crystals (Fig. 2a). As shown in the UV-Vis spectrum of the PB


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aqueous dispersion, the strong light absorption of PB nanocubes ranges from wavelengths of 650 nm to 800 nm and tops at 710 nm, making the 785-nm light a suitable stimulus to excite the thermogenesis (Fig. 2b). Analysing from transmission electron microscopy images, the synthesized PB nanocubes have an average side length of roughly 34.5 nm with a standard deviation of 4.5 nm, indicating a relatively high uniformity (Fig. 2c). Then we coat the PB nanocubes onto glass through the layer-by-layer assembly deposition.38 Negative charges are firstly created by treating the glass wafer with oxygen plasma. The glass wafer is then immersed into an aqueous solution of positively charged polyelectrolyte (Polyethylenimine, PEI), rinsed and subsequently immersed into negatively charged PB aqueous dispersion. Fig. 2d shows the scanning electron microscopy images of PB-coated glass after different repeated deposition cycles. The PB nanocubes pack more closely as the alternate cycles increase, producing gradually enhancing photothermal performance. However, PB-deposition layers compromise the transmittance of the glass wafer, resulting in a trade-off between the photothermal performance and transparency. The optical properties of the PET panel modules are examined through UV-Vis-NIR transmittance measurement (Fig. 2e). The LN crystal shows roughly 70% transmittance over the visible spectrum. Although deposited PB nanocubes colour the glass wafer blue, the PB-coated glasses still maintain high transparency over visible spectrum (above 70% for the glass coated with 3-layer PB). The high transmittance of both LN and the PB-coated glass contributes to high optical transparency of PET panel. Such superior optical property of the control panel is crucial for real-time observation of droplets motion and in-situ detection of chemical reactions inside droplets.


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Photothermal performance of LbL-assembled PB nanocubes. As a 785-nm laser irradiates on the PB-coated glass wafer, local heating rapidly occurs. Using infrared thermal imaging, the light-responsive heating is measured. The laser spot has a diameter of roughly 1.9 mm. As shown in Fig. 3a, when being exposed to a laser with a power density P of 12 W cm-2, LN wafer shows no thermal response and the temperature of a bare glass wafer only slightly increases by 5 °C. Unlike such weak photothermal responses, 3-layer, 5-layer, and 7-layer PB-coated glass rapidly gain temperatures as high as 40.7, 47.9, and 49.7 °C, respectively, in 20 s. The increasing PB layers on glass wafers result in stronger thermogenesis responses. Fig. 3b shows the equilibrium temperature distributions for different investigated wafers under the laser illumination. The photo-responsive thermogenesis linearly depends on the laser power density (Fig. 3c). When the 14-W cm-2 laser applies, the temperature of the 3-layer PB-coated glass wafer reaches a maximum value of 49 °C which is a sufficient level to drive droplets motion. Therefore, we use the 3-layer PB-coated glass wafer to construct the PET panel of high transparency. The strong heat generation are limited within the PET panel, causing no obvious heating on manipulated droplets (Figure S1 and Supporting Note S1). As shown in Fig. 3d, to investigate the thermal stability of the PB coating, we repeatedly switch the laser on/off for 10 cycles. The maximum temperature of the PB coating remains the same without any performance decay. Thereby, the fabricated PET panel is free from any thermal degradation when it is repeatedly used.

Damping oscillation mode. Installing the PET panel above the superhydrophobic surface completes the fabrication of the PETOS platform. Approaching the irradiated spot, the electric field strength E monotonically


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strengthens in space. Because the water dielectric constant is higher than that of air, the DEP force drives polarized water droplets in a direction of stronger field strength. Once the surrounding droplet is actuated towards the illumination, the DEP force resembles the Hookean restoring force which brings the droplet into a decaying oscillation with kinetic energy damped through viscous force and CAH force (Fig. 4a and Movie S1).39 Such electric potential well attracts the nearby liquid droplet and finally immobilizes it below the laser spot. Tiny water droplets of varied volumes (4-µl, 10-µl, and 20-µl) are used to probe the manoeuvrability of the system. As shown in Fig. 4b, when the spacing L is below 3.5 mm, strong electric field readily lifts droplets up, resulting sticking droplets on the above LN wafer. Contrarily, weak electric field cannot overcome the pinning force on droplets. The applicable parameters are identified and shaded in orange in the phase diagram. The light spot and manoeuvre domain define the centre and radius of the potential well, respectively. Droplets inside the manoeuvring range can be driven towards the laser. As shown in Fig. 4c, droplets of larger volume appear to have a wider manoeuvre domain. The manoeuvre domains are similar for 3.5-mm and 4-mm separated systems and decrease when the spacing is 4.5 mm. Therefore, we fix the spacing at 4 mm for subsequent measurements. Stronger laser power density brings about wider domains (Fig. 4d). Using a 14-W cm-2 laser, the domain reaches 9.2 mm for a 20-µl droplet. However, after the laser is switched on, a temporal delay exists for a droplet to start its motion. The response time is measured as a function of initial distances between droplets and the laser spot. Droplets of different volumes have very similar response time (Fig. 4e). However, droplets placed at a larger distance take longer time to initiate their motion. Strong laser power density reduces such temporal delay, thereby facilitating rapid responses (Fig. 4f). Using a 14-W cm-2 laser, the response time of a 20-µl water droplet at 8-mm


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position is 8 s and the response time decreases to be 1 s when the droplet is placed at 2-mm position. Unlike most photo-control methods which require precisely asymmetric illumination on one side of tiny droplets, our manoeuvre method has a finite manoeuvring range, making it more convenient and pragmatic to control the droplet motion.

Droplet motion dynamics. Although we focus on manipulating droplets with a radius r smaller than the capillary length ( lc = γ ρ g ),40 the PETOS platform remains effective for even liquid puddles (Movie S2). Fig. 5a shows the spacing for controllable droplets of volume ranging from 4 to 500 µl using a 14-W cm-2 laser. When Lmin < L < Lmax , the droplets can be manoeuvred without upward attraction (Figure S2). As NIR light irradiates, the triggered electric field exerts a DEP force on drops. If the induced electric dipole is small compared with the non-uniformities of the field, the DEP force can be expressed as FE = 2π r 3ε 0 K∇E 2 , where K is the Clausius-Mossotti factor ( K = (ε1 − ε 0 ) (ε1 + 2ε 0 ) ), ε0 and ε1 are the permittivity of continuous phase and droplets, respectively. The laser power, spacing, and droplet position determine E whose magnitude is estimated through the droplet deformation (Figure S2 and Supporting Note 2). However, the aforementioned DEP force equation is not strictly valid for our system as the droplet size and device geometry (spacing) are on the same scale. By analysing the droplet force balance on a tilted PETOS, we can estimate the maximum DEP force in x direction FEx (Fig. 5b). When the PETOS is free from illumination, the sliding angle α0 relates CAH force Fγ = (4π 3)r 3 ρ g sin α 0 .


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With NIR light, the sliding angle increases to be α1, giving FEx = (4π 3) r 3 ρ g (sin α1 − sin α 0 ) . As shown in Fig. 5c, the CAH force increases with the increasing droplet volume, owing to a longer receding contact line length. The DEP force shows a similar increasing trend which is in consistent with the DEP force equation. Once the droplet is in oscillatory motion, apart from the pinning CAH force at the receding contact line, the viscous dissipation within the contact region contributes to the moving resistance (Fig. 5d). To examine the impact of viscosity on droplet motion, the droplet viscosities are tuned from 1 to 1150 mPa s by mixing water with glycerol. Unlike viscosities, the surface tension of water-glycerol mixture approximately remains constant. As shown in Fig. 5e, drop of higher viscosity has less oscillation cycles. When viscosity increases to be 1150 mPa s, no oscillatory movement can be observed. On the basis of droplets’ moving trajectories, we analyse the energy dissipation ratio between the viscous and CAH one. Because the Clausius-Mossotti factor for water (0.963) and glycerol (0.939) are roughly the same, thus we assume the DEP force and electric potential U are independent of glycerol concentration. For pure water droplets, the average velocity of the oscillatory motion is 7.53 mm s-1, giving a capillary number

Ca = µV γ 95%), tetraethyl orthosilicate (Sigma-Aldrich, ≥99%), ammonium hydroxide (Acros, 28-30%), toluene (SigmaAldrich, 99.8%), octadecyltrichlorosilane (Acros, 95%), silicone oil (Aladdin, 10 mPa·s). To fabricate the silver-nanocrystal-based superhydrophobic surfaces, we cleaned the polished copper sheet through successive ultrasonic rinse in ethanol, acetone, and isopropanol. Silver nanostructures were deposited onto the copper by immersing the copper sheet into aqueous silver solution (0.01 M) for 45 s. Then, the sheet was washed with deionized water and dried under nitrogen flow. Then, fluorinated self-assembly monolayer was deposited onto the surface by immersing the sheet into a 1H, 1H, 2H, 2H-perfluorodecanethiol-in-dichloromethane solution (0.001 M) for 15 min. The fabrication was completed by washing the surface with fresh dichloromethane and dried it in ambient condition for 5 min.


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To synthesize the Prussian blue nanocubes, 0.49 g citric acid was firstly added to 100 ml 1.0 mM FeCl3 aqueous solution under 500-rpm mechanical stirring at 60 °C. Then, 100 ml 1.0 mM potassium ferrocyanide solution containing the same amount of citric acid was added into the FeCl3 solution. After stirring for 1 min, the solution was allowed to cool to room temperature. The Prussian blue nanocubes were subsequently washed for 3 times through repeated centrifugation (at 10000 rpm for 60 min) and ultrasonic dispersion (for 15 min) in fresh deionized water. To coat PB nanocubes onto glass wafer, the glass wafer was firstly treated with oxygen plasma (Potentlube, PT-5S) for 5 min to render it negatively charged. Then the negatively charged glass wafer was immersed into the positively charged 0.5 wt% PEI aqueous solution for 2 min. The glass wafer was subsequently washed through water rinse and immersed into the negatively charged 0.05 wt% PB aqueous dispersion. Such immersion cycles were repeated for several times to complete the coating process. The PB coated glass wafer was then placed onto the LN wafer with the coated side contacting the LN crystal. Then the fabricated control panel was placed above and separated from the superhydrophobic surface using two spacers of thickness L. To fabricate the TiO2-nanoparticle-based superhydrophobic surface for wafer-scale PETOS, we coated silicon wafers with a superhydrophobic paint-like solution. To prepare the superhydrophobic solution, 100 µl triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane was firstly added into 12 ml ethanol and stirred for 2 h. Then, 0.6 g ~100 nm in size TiO2 nanoparticles and 0.6 g of Degussa P25 TiO2 nanoparticles were placed into the solution and stirred at 700 rpm for 1 h. Then, roughly 1-ml prepared superhydrophobic solution was evenly


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distributed onto the silicon wafer through a sharp blade and the coating was dried in ambient condition for 20 min. To fabricate the SLIPS, we prepared a substrate of sintered silica nanoparticle porous networks. To fabricate the 120 nm in size silica nanoparticles, 50 ml tetraethyl orthosilicate-inethanol solution (0.44 M) was firstly prepared; then another solution containing 2.76 ml ammonium hydroxide, 28.44 ml water, and 18.8 ml ethanol was added into the tetraethyl orthosilicate ethanol solution and mechanically stirred at 300 rpm for 6 hr at room temperature. Solvent of 50-ml reacted silica nanoparticle colloid was allowed to evaporate to reduce the volume to be 15 ml. Then 7.5 ml glycerol was then added and mixed with the 15 ml concentrated silica colloid to increase the solution viscosity. The mixed solution was spin coated onto oxygenplasma-treated silicon wafers at 800 rpm for 10s, then the coated wafers were baked on a hot plate at 150 °C for 1 min. The coating and baking processes were repeated for 5 times and the coated substrates were subsequently sintered at 600 °C for 1 h. The sintered porous substrates were immersed in an octadecyltrichlorosilane-in-toluene solution (10 mM) for 15 min. Treated substrates were subsequently flushed with toluene and baked in a furnace at 100 °C for 30 min to make them hydrophobic. 10 mPa s silicone oil was then spin coated onto the functionalized porous substrate at 1000 rpm for 15 s to complete the fabrication of SLIPS.

Instrument and characterization. A 785-nm near infrared laser (SLOC Lasers, ADR-1805) was used to manipulate the droplet on the PETOS platform. The synthesized PB particles were identified through X-ray diffraction (Rigaku, SmartLab). The strong absorbance of the PB nanocubes towards NIR light was identified by measuring the UV-Vis absorbance (Thermo Scientific, Nanodrop 2000c) of PB


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aqueous dispersion. We observed the synthesized PB nanocubes using a transmission electron microscope (Philips, CM100). The PB coated glass wafers were observed using a scanning electron microscope (Hitachi, S4800). The transparency of the PB coated glass wafers were measured using a spectrophotometer (PerkinElmer, Lambda 35). The light-triggered thermogenesis of the PB coated glass wafer was determined using an infrared thermal camera (Fluke, Ti40). We used a high-speed camera (Phantom, Miro 110) coupled with a camera lens (Sigma, 30 mm/F1.4/DC/HSM) to record the movement of manipulated droplets. To continuously guide the droplet motion, we fixed the laser on a precise motion control platform (Aerotech, Planar DL) to control the laser moving velocities.

Data availability. The authors declare that the data supporting the findings of this study are provided in the article and its supplementary information.


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Acknowledgements The authors wish to thank Prof. Yuguo Li, Dr. Shien-Ping Feng, Dr. Wen-Di Li for equipment support. The financial support from the Research Grants Council of Hong Kong (GRF 17237316, 17211115, 17207914 and 717613E), the University of Hong Kong (URC 201511159108, 201411159074 and 201311159187) is gratefully acknowledged. This work was also supported in part by the Zhejiang Provincial, Hangzhou Municipal and Lin’an County Governments.

Author Contributions X.T. and L.W. conceived the project. X.T. and L.W. designed the project. X.T. performed the experiments. X.T. and L. W. analyzed the data. X.T. and L.W. wrote the manuscript. L.W. supervised the study.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Temperature of superhydrophobic substrate with NIR light irradiation, infrared thermal photographs of irradiated PETOS platform, images of an evaporating droplet with NIR light irradiation, images of deformed water droplets in electric field, droplets’ deformations and electric field strengths varies with the spacings, images of upward droplets attraction.

Competing financial interests: The authors declare no competing financial interests.


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Figures and captions

Figure 1. The design of the pyroelectro-trapping on superhydrophobic surfaces platform. (a) Schematics showing the configuration of the PETOS platform; (b) Scanning electron microscopy images showing the assembled photothermal PB nanocubes (top) and deposited water-repellent silver nanocrystals (down); (c) Schematics of the mechanism for photocontrol droplets manipulation; (d) Distinct droplets sliding angles with/without pyroelectro-trapping. Dashed rectangles denote the outof-focus control panels; (e) Sequential images showing a consecutive droplet manoeuvre using a 785-nm laser.


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Figure 2. Fabrication of the PET control panel. X-ray diffraction (a) and UV-Vis (b) spectra of the as-prepared PB nanocubes. Inset in (b) shows the aqueous colloid of PB nanocubes; (c) Size distribution of as-prepared PB nanocubes based on transmission electron microscopy images. The orange line is the Gaussian fitting line. Inset in (c) shows a transmission electron microscopy image of the PB nanocubes; (d) Scanning electron microscopy images showing the deposited PB nanocubes for different assembly cycles; (e) UV-Vis-NIR transmittance spectra of LN and PB-nanocubes-coated glass wafers. Insets in (e) show images of LN and PB-nanocubescoated glass wafers.


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Figure 3. Photothermal performance of the control panel. (a) Temporal maximum temperature profiles of the laser-irradiated regions on PB-coated wafers and the LN crystal wafer. Blue and red shaded regions denote off and on states, respectively, of the 785-nm laser; (b) Spatial temperature distributions of laser-irradiated wafers. Insets are corresponding infrared thermal photographs of the illuminated regions. Dashed lines in insets show the measuring lines for temperature distributions; (c) Temporal maximum temperature profiles of the 3-layer-PB glass wafer irradiated with different laser power densities. The shaded colour code is the same as in (a); (d) Temperature profile of the 3-layer-PB glass wafer for 10-cycle switching of a 12-W cm-2 laser. The shaded colour code is the same as in (a).


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Figure 4. Damping oscillation mode of droplet manoeuvre. (a) A typical decaying oscillation trajectory of a 20-µl water droplet using a 14-W cm-2 NIR laser. After 5 oscillations, the droplet is precisely immobilized below the laser whose position is denoted by the orange dashed line; (b) Dependence of manoeuvrability on the laser power density and the panel-to-substrate spacing. Cross, diagonal cross, and circle denote 4-µl, 10-µl, and 20-µl water droplets, respectively. Green, red, and blue colour code of the symbol denote upward attracted, uncontrollable, and manoeuvrable droplets, respectively; (c) Manoeuvre domains increase with the increasing droplet volume for PETOS of different panel-to-substrate spacings. The 12-W cm-2 laser is utilized; (d) Dependence of manoeuvre domains on the droplet volume and laser power densities. The panel-to-substrate spacing is 4 mm; (e) Response time appears to be independent of droplet volume. The 12-W cm-2 laser is utilized; (f) Response time decreases with the increasing laser power densities (droplets placed out of manoeuvre domain cannot response to the NIR light).


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Figure 5. Droplet motion on the PETOS. (a) The allowable panel-to-substrate spacing for liquid droplets and puddles using a 14-W cm-2 NIR laser. Green, red, and blue region colour denote upward attracted, uncontrollable, and manoeuvrable regions, respectively; (b) Schematic showing the force balance of a droplet on a tilted PETOS; (c) The CAH forces and maximum DEP forces in x direction derived from the droplets’ sliding angles; (d) Schematic showing the force balance of a droplet actuated on the PETOS; (e) Typical decaying oscillation trajectories for droplets of various viscosities. Orange dashed line denote laser spot position; (f) The average oscillating velocities and ratios of viscous to CAH energy dissipation for droplets of various viscosities.


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Figure 6. Continuously guiding mode of droplet manoeuvre. (a) Temporal positions of 4-µl droplets guided by a 6-W cm-2 laser moving at different velocities; (b) The maximum laser velocity increases with the increasing laser power density and increasing droplet volume; (c) The maximum

velocity

decreases

with

the

increasing

droplet

viscosity;

(d)

coloured

chronophotograph shows a 4-µl droplet continuously follows a irradiating laser moving at a speed of 0.15 mm s-1. Dashed rectangles denote the out-of-focus control panel.


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Figure 7. Droplet-based micro-reaction on the PETOS platform. (a) Sequential images showing the loss-free manipulation of FeCl3 and KSCN aqueous droplets and triggered chemical reaction upon their coalescence; (b) Enhanced mixing inside a 10-µl water droplet by shaking the PETOS system.


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Figure 8. Wafer-scale droplet manipulation. (a) Image of a 4-inch PETOS platform; (b) Lateral-view image of the 4-inch PETOS platform; (c) Enlarged region in (b) showing the components of the 4-inch PETOS; (d) Scanning electron microscopy image showing the nanostructures of TiO2-nanoparticles-based superhydrophobic coating; (e) Chronophotographs showing the wide-range droplet motion manipulated through a laser on the PETOS. The droplet is dyed red for visualization. Magenta arrows denote the moving trajectories of laser.


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Figure 9. Pyroelectro-trapping on SLIPS. (a) Schematic showing the configuration of the pyroelectro-trapping on SLIPS system; (b) Scanning electron microscopy image showing the cross section of a porous substrate; (c) Distinct droplets behaviour on a SLIPS with/without pyroelectro-trapping; (d) Sequential images showing droplet manoeuvre on a SLIPS using the PETOS strategy. Purple dashed line denotes the initial droplet position.


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Loss-Free Photo-Manipulation of Droplets by Pyroelectro-Trapping on

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