Motile Micropump Based on Synthetic Micromotors for Dynamic

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Surfaces, Interfaces, and Applications

Motile Micropump based on Synthetic Micromotor for Dynamic Micropatterning Xiaocong Chang, Chuanrui Chen, Jinxing Li, Xiaolong Lu, Yuyan Liang, Dekai Zhou, Haocheng Wang, Guangyu Zhang, Tianlong Li, Joseph Wang, and Longqiu Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08159 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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ACS Applied Materials & Interfaces

Motile Micropump based on Synthetic Micromotor for Dynamic Micropatterning Xiaocong Chang,1,2 Chuanrui Chen,2 Jinxing Li,2 Xiaolong Lu,2 Yuyan Liang,2 Dekai Zhou,1 Haocheng Wang,1 Guangyu Zhang,1 Tianlong Li,1,* Joseph Wang,2,* and Longqiu Li,1,* 1

State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China 2 Department of Nanoengineering, University of California, San Diego, La Jolla, California 92093, United States E-mail: [email protected]; [email protected]; [email protected] KEYWORDS: micromotor, water-driven, motile micropump, micropatterning, rewritable

ABSTRACT: Micropump systems show great potentials on micropatterning process as a result of remarkable performance and functionality. However, existing micropumps cannot be employed as direct writing tools to perform complex micropatterning process owing to their lacking motility and controllability. Here we propose a motile micropump system based on the combination of water-driven ZnO/Ni/PS Janus micromotor with traditional immobilized micropump. This novel motile micropump system can translate the trajectory of Janus micromotors into predefined micropatterns by pumping away passive silica particles around micromotor under the effect of diffusiophoresis. The resolution and efficiency of micropatterning process can be regulated by controlling the diameters of Janus micromotors. Diverse surface micropatterns can be fabricated though remote magnetic control of the motile micropump system. Such ability to transform the versatile motile micropump into predetermined surface micropatterns creates new opportunities for mask-free micropatterning.

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1. Introduction A variety of micropatterning processes have been developed for micropatterns and microstructures fabrication.1 However, the costly processes and sophisticated equipment for these traditional micropatterning technologies have prompted the evolution of novel and unconventional routes to fabricate micropatterns. Advanced alternate strategies offer great promise to improve versatility and lower costs.2-5 Kim et al. reported a new photo-induced method to fabricated a spatial and temporal microparticle patterning.4 Kummel et al. demonstrated the behavior of microswimmers in confined environments and fabricated crystalline regions of passive particles by using active motor.5 It shows how microswimmers behave in crowded environments and finally to fabricate crystalline regions of passive particles. Previous active particles-induced micropatterning process usually require specific substrate, strict solvent conditions and specific kinds of colloids. Therefore, a light-free, mask-free, fuel-free, on-demand and highly precise way is demanded for fabricating complex micropatterns. Although needing UV light and fuels, the micropump system is still a promising candidate for micropatterning.6 Micropump systems can respond to external stimuli (e.g. ultraviolet light7) and then transfer chemical energy (such as N2H4,8 I2,9 fluoride10 and glucose11) and enzyme (such as glucose oxidase12) into mechanical momentum, showing exclusion or schooling behavior for the tracer particles.13 Consequently, micropumps can offer considerable potentials in patterning structures.14-16 Nevertheless, immobilized materials, e.g. supramolecular molecular,7 polymer17 or other stationary materials, are widely used in almost all existing micropump systems, which will hinder the versatility of micropump systems.8, 15 Hence, it is difficult to use the traditional micropump to produce micropatterns as a result of lacking motility and controllability. Therefore, it is essential to develop a motile and steerable micropump system to serve as a novel method for dynamic and controllable micropatterning.


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Synthetic micromotor can propel itself by transforming magnetic,18-20 light,21-24 ultrasound25 or chemical energy26-30 into mechanical movement, which plays a key role in various potential applications, such as cargo delivery,31-34 drug delivery,35 crack repair,36 environment remediation,37-41 assembly,42-46 swarm,47 synthesis of nanoparticles,48 chemotactic behavior49-51 and interaction with other micromotors52-53 or living biosystems54. Besides their self-propulsion ability, synthetic micromotors possess the ability of autonomous navigation and speed regulation through tuning the external physical field.55-57 These advances include a kind of precise autonomous navigation for smart micromotors in complicated and dynamically changing environments through external magnetic field,58 a facile and precise directional control of micromotor under the navigation of light source59 and a precise and reversible control of the speed of bubble-propelled micromotor through external ultrasound fields60. Therefore, the remarkable motility and controllability of synthetic micromotors have paved the way to serve as a succedaneum of immobilized materials in existing micropump system. However, there is no relevant research reported yet for combing self-propelled micromotor and traditional micropump in the literatures. Here, we are trying to fill this gap by proposing a water-driven motile micropump, based on a ZnO/Ni/PS Janus micromotor for complex surface micropattern. The autonomously moving ZnO/Ni/PS Janus micromotor is driven by self-generated ion concentration gradient, while pumps away surround silica particles through diffusiophoresis. The motile micropump provides an attractive route toward rapid arbitrary 2D shape patterning along the locomotion trajectory of the Janus micromotor. To modulate both the resolution and efficiency of micropatterning process based on the motile micropump system, the corresponding pump width and micropatterning area in unit time are regulated and tailored by tuning the size of ZnO/PS Janus microspheres. Complex predefined surface



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micropatterns in microscale areas can be fabricated by combining the motile micropump system with external magnetic guidance. Particularly, after showing messages micropatterns can disappear naturally, followed by rewriting different messages by fabricating another micropatterns on the same area. Such versatile motile micropump system offers simple, cost-effective micropatterning process, paving the way for new opportunities for novel functional surface science and potential applications, such as producing secret messages, dynamically changeable microchannels, adaptive optics and microfluidics. 2. Results and Discussion 2.1. Mechanism of motile micropump. As shown in Figure 1a, the self-propelled Janus micromotor is fabricated by coating polystyrene (PS) microspheres with a zinc oxide (ZnO) layer. The ZnO side of the Janus micromotor reacts with water spontaneously at room temperature to release zinc ions and hydroxide ions. Consequently, a high local ion concentration of Zn2+ and OH- is formed around the ZnO surface, leading to an ion gradient across the ZnO/PS Janus micromotor. Unlike the concentration gradient of molecules, the electrolyte gradient causes a chemophoretic flow with the direction opposite to the osmophoresis.50-51, 59, 61-63 The induced chemophoretic flow propels the micromotor toward the ZnO side. The schematic diagram in Figure 1b illustrates the pumping mechanism of ZnO/PS Janus micromotor in water mixed with passive silica particles. A diffusion-induced electric field, resulting from the large difference in diffusion coefficients between the cation ‘Zn2+’ (D (Zn2+) = 0.703 × 10-5 cm2/s64) and the anion ‘OH-’ (D (OH-) = 5.273 × 10-5 cm2/s64), is established pointing outward the ZnO/PS Janus micromotor. The existence of electric double layer on the negatively charged sodium borosilicate glass slide sets up an electroosmotic flow around the ZnO/PS micromotor. The direction of electroosmotic flow is the same as that of the electric field.17 Consequently, the negatively charged


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silica particles are pumped away from the ZnO/PS micromotor as a result of the competition of electric force and electroosmotic flow force, explained by the following equation13:

c D   D  kBT  (p   w ) c 2 kB2T 2 U [(  )( ) ] [( ){ln(1   w2 )  ln(1   p2 )}]  2 c0 D  D e  c0 e Electrophoretic term

(1)

Chemophoretic term

where▽c is the concentration gradient; c0 is the bulk concentration of ions; D+ and D- are the diffusion coefficients of the cation and anion respectively; kB is Boltzmann constant; T denotes the absolute temperature; ε is the dielectric permittivity of the solution; η is the viscosity; ζp and ζw are the zeta potentials of the particle and wall, respectively; γw = tanh(eζw/4kBT) and γp = tanh(eζp/4kBT).13 As reported by Wang, the chemophoretic term can be neglected.13 Therefore, the motion direction of passive silica particles is governed by the electrophoresis term in Eq.(1).17 The zeta potential of passive silica particles is around -15mv to -40mv, which is lower than that of glass (ζglass = -62.2 mv). It means that the electroosmotic force overbears the electric field force based on the above equation.13, 17, 65 Consequently, the surround negatively charged silica particles can be pumped away from the ZnO/PS Janus micromotor with the direction of electroosmotic flow force. Meanwhile the distribution of silica particles pumped by ZnO/PS Janus micromotor depends on the ion concentration distribution.13 To further understand the distribution of passive silica particles in motile micropump system, we performed the simulation of ion concentration distribution using COMSOL Multiphysics, a commercially available finite element software. Figure 1c displays the simulation results of ion concentration distribution changing with time around the ZnO/PS Janus micromotor. A higher ion concentration distribution appears along with the track line of ZnO/PS Janus micromotor at 15 s, while ions just distribute around the micromotor initially. Note that the passive silica particles among the higher ion concentration distribution area can be pumped away from ZnO/PS



Janus micromotor resulting in a clear exclusion region along with the movement of ZnO/PS Janus micromotor, while the silica particles in other area where ion concentration is very low cannot move and just stay where they are. The optical microscope image of Figure 1d (captured from Video S1) shows the experimental result of motile micropump. The micropattern fabricated by static micromotor is similar to the pattern created by the traditional micropump, as indicated in Figure S2a. Based on the cooperation between the property of traditional micropump and the self-powered ZnO/PS Janus micromotor in water, the micromotor (the larger black particle in Figure 1d) can travel through the negatively-charged silica particles (the dense smaller particles in Figure 1d) and produce a micropattern in a silica particle suspension.

a

PS/Ni/ZnO

Zn2+

b

OH-

SiO2

Electric force (E)

Electroosmotic force (EOF) Electric field

Move direction

E

Electroosmotic flow

EOF ZnO + H2O →

Zn2+ +

Ion Gradient

2OH-

c

t=3s

t = 15 s

nM 22.0

Ion Concentration

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d

20.0

Move direction

18.0

16.0

14.0

PS/Ni/ZnO

ZnO + H2O → Zn2+ + 2OH-

12.0 10.0

Figure 1. Mechanism of motile micropump based on ZnO/PS Janus micromotors. (a) Schematic of motion mechanism: ZnO/PS Janus micromotor is powered by ion gradient generated through the redox reaction at the ZnO surface. As a result, the Janus motor moves toward the ZnO side. (b) Schematic of pumping mechanism: An outward electric field (the red arrows) generated in response to the different diffusion speed of Zn2+ and OH- released from the reaction between ZnO/PS Janus micromotor and water. An electroosmotic flow (the blue arrows) is created at the same direction with


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the electric field owing to the double electric layer of the glass slide. The negatively charged silica particles (the green microspheres) are pumped away from the ZnO/PS micromotor, since the electroosmotic force overbears the electric field force. (c) Theoretical simulation of ion concentration distribution released by ZnO/PS Janus micromotor at 3 s and 15 s. (d) Optical microscope images of the motile micropump based on water-driven ZnO/PS Janus micromotor in suspension of 1.21 μm silica particles. Scale bar, 25 μm. 2.2. Micropatterning resolution of motile micropump. Similar to the traditional micropatterning method, resolution is an important factor of micropatterning process for motile micropump system. Therefore, the influence of the diameters of Janus micromotor on the resolution of micropatterning process has been investigated experimentally and theoretically. Here the ZnO/PS Janus micromotors with diameters of 2, 5 and 10 μm are used for micropatterning, as displayed in Figures 2a-c, respectively. The optical microscope images in Figures 2d-f show the surface micropatterns with different widths along the track line of ZnO/PS Janus micromotors. As expected, ZnO/PS Janus micromotor with smaller diameters result in higher-resolution micropatterns. The widths of surface micropatterns increase from 24.38 to 38.87 and 153.57 μm as the diameters of ZnO/PS Janus micromotor increase from 2 to 5 and 10 μm, suggesting that the resolution of surface micropatterns can be regulated and controlled by adjusting the diameters of ZnO/PS Janus micromotors. In order to further investigate the mechanism of variable resolution capability of motile micropump system, simulated ion concentration distribution around ZnO/PS Janus micromotors with different diameters are provided. Figures 2g-i show the distribution of the ion concentration released from water-driven micromotors with different diameters (2, 5 and 10 μm) at 25 s, respectively. Note that the variable widths of ion concentration distribution are function of the diameters of ZnO/PS Janus micromotors. As expected, the ion distribution becomes wider with an increase of the diameter of ZnO/PS Janus micromotors. The distribution of passive silica particles around the ZnO/PS Janus micromotors with



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different diameters at 25 s are illustrated in Figures 2j-l (captured from Video S2), respectively. It shows that the widths of surface micropatterns vary with the widths of the ion distribution dominated by the diameter of ZnO/PS Janus micromotor. The simulation results are consistent with the experimental results displayed in Figures 2d-f. In addition, the density of the passive silica particles in the substrate is crucial to the quality and resolution of the resulting micropatterns. As illustrated in Figure S3, the shape of resulted micropatterns becomes more and more clear upon increasing the density of silica particles from 0 to 34×106 particles/cm2. As a matter of fact, a minimum density of 1-2×106 particles/cm2 particles is required to generate a clear micropattern and the density of 30-34 ×106 particles/cm2 that we are using in our experiment is already reaching the theoretical maximum density as the substrate has been packed with passive SiO2 particles, shown in Figures 2d-f and Figure S3f. To study the relationship between resolution and particle density, various densities of silica particles are employed to evaluate the resolution of micropatterns fabricated by the ZnO/PS Janus micromotors with diameter of 2 μm. The width of micropatterns can be tuned to change from 24.38 ± 2.18 μm to 38.87 ± 3.98 μm and 153.57 ± 12.23 μm upon increasing the diameter from 2 μm to 5 μm and 10 μm, respectively, as demonstrated in Figure S4b. However, the width of the micropatterns changes slightly along with changing the particle density for the micromotor with the same size, as shown in Figure S4a.


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a

nM 15

g

d

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25 μm

25 μm

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20 μm

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e

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t=25s

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9.5 80

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l

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20 μm 40

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Figure 2. Micropatterning resolution of motile micropump based on water-driven ZnO/PS Janus micromotor. (a-c) Scanning electron microscope (SEM) images of ZnO/PS Janus micromotor with diameters of 2 μm (a), 5 μm (b) and 10 μm (c), respectively. (d-f) Optical microscope images of surface micropatterns fabricated by motile micropump system based on ZnO/PS Janus micromotor in 1.21 μm silica particles suspension. (g-i) Simulation of ion concentration distribution released from ZnO/PS Janus micromotor with different diameters of 2 μm (g), 5 μm (h) and 10 μm (i), at 25s. (j-l) Simulation of the distribution of 1.21 μm passive silica particles in dynamic motile micropump system at 25 s in response to the ion concentration distribution of ZnO/PS micromotor with different diameters. 2.3. Micropatterning efficiency of motile micropump. In addition to resolution, the micropatterning efficiency of the motile micropump system should also be considered. The micropatterning efficiency is defined as the working area per unit time, which is mainly attributed to the velocity and pump width of motile micropump system. Figures 3a-c illustrate the random trajectories of ZnO/PS Janus micromotor with diameters of 2, 5 and 10 μm in suspension of 1.21 μm passive silica particles over a 15 s period, respectively. Although all ZnO/PS Janus micromotors with different diameters move



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toward ZnO side resulting from chemophoretic flow, the track lines of Janus motor are fluctuant caused by Brownian movement and decrease with the increase of the diameter of ZnO/PS Janus micromotors in same period. Note that the track lines of ZnO/PS Janus micromotors in motile micropump system are 72, 50.4 and 27 μm respectively, as the diameters of ZnO/PS Janus micromotors increasing from 2, 5 to 10 μm. Corresponding velocities of motile micropump systems are quantified by studying the trajectories of the ZnO/PS Janus micromotor with different diameters. It should be noted from the blue bars shown in Figure 3d that the velocities of ZnO/PS Janus micromotors show a downward trend as the diameters increases from 2 to 5 and 10 μm, showing by that the velocity of the ZnO/PS Janus micromotor decreases from 5.17 to 3.71 and 2.01 μm/s, respectively. To tune the speed of ZnO/Ni/PS Janus micromotors with the same diameters, hydrogen peroxide with various concentrations are added into silica particle suspension. As indicated in Figure S5, the speed of ZnO/Ni/PS Janus micromotors with different diameters increase dramatically with the concentration of hydrogen peroxide increasing from 0% to 0.5%. The speed of 2 μm diameter micromotor increases gradually while the speeds of micromotors with diameters of 5 μm and 10 μm remain stable. In addition, the speed of micromotors has an effect with the passive silica particles. As indicated in Figure S6d, the speeds of ZnO/Ni/PS Janus micromotors in water without silica particle decrease from 4.25 μm/s to 3.18 μm/s and 1.88 μm/s along with the diameter of micromotors increasing from 2 μm to 5 μm and 10 μm, respectively. However, all the speed of Janus micromotors with different diameters in silica particle suspension are higher than that in water without silica particles. The accelerated motion is attributed to the threedirection restrictions of the grooves micropattern made from the passive silica particles. To further investigate the micropatterning efficiency of motile micropump system in same condition, working area in unit time is quantified in terms of the velocity and the pump width of micropump,


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demonstrated by the red bars in Figure 3d. Note that the micropatterning area of the motile micropump system increases from 115.79 to 144.32 and 315.59 μm2/s along with the increase of the diameter from 2 to 5 and 10 μm, respectively. Accordingly, diverse diameters of ZnO/PS Janus micromotor give freedom for varying the velocity and pump width of the motile micropump system and hence regulating the efficiency of micropatterning process.

a

b

c

d

Figure 3. Micropatterning efficiency of motile micropump system. (a-c) Tracking lines of ZnO/PS Janus micromotor with diameters of 2 μm (a), 5 μm (b) and 10 μm (c), respectively over a 15 s interval in the silica particles suspension. Scale bar, 20 μm. (d) The velocity and micropatterning area of motile micropump system with different diameters, both measured in Milli-Q water mixed with 1.21 μm silica particles. They show different trends along with the increasing diameter of the ZnO/PS Janus micromotor. 2.4. Remote magnetically guided motile micropump. Versatile micropatterns and sophisticated geometry call for direction control for micropatterning process. However, the common Janus micromotor based on motile micropump move randomly in water. In order to create complex micropatterns, it is essential to develop the method to control motile micropump moving in a



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designated direction. Oriented motile micropump can be realized by incorporating a ferromagnetic nicker layer into Janus micromotor, e.g., to form a trilayer ZnO/Ni/PS Janus micromotor. Figure 4a illustrates the schematic depiction of directional movement of ZnO/Ni/PS Janus micromotor in an obtuse angle through changing the orientation of external magnet. The optical microscope image in Figure 4b displays a surface micropattern in a shape of an obtuse angle fabricated through the remote magnetically-guided motion of motile micropump in 1.21 silica particles suspension. A sharp turn illustrated in Figures 4c-d is produced in the same method without slowing down in an acute corner. The arbitrary orientation of this micropatterning process illustrates the potential of using motile micropump system to fabricate various micropatterns through continuously controlling the direction of Janus micromotor. A predefined ‘square-wave’ micropattern is generated through turning the external magnet successively in a 90-degree angle for three times, as illustrated in Figures 4e-f. Therefore, the motile micropump can perform obtuse-angled turn, acute-angled turn and successive right-angled turns through turning the external magnet at same obtuse angle, acute angle and successive right angle at the corner of the micropatterns shown in Figure 4. Such ability of executing precise and successive direction control of motile micropump through turning the direction or angles of external magnet, makes motile micropump system possible to generate more complex and predefined surface micropatterns.


b

c

θ2

θ2

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Obtuse-angle turn

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N

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f

90°

‘Square-curve’

b

θ1

d

θ2

f

First turn

Second turn

90° 90° Obtuse-angle turn

90°

Acute-angle turn Third turn

Figure 4. Remote magnetically guided motile micropump. (a, b) Schematic depiction and optical microscope image of direction guidance of ZnO/Ni/PS Janus micromotor motion by turn external magnetic field in an obtuse angle in silica particles suspension. (c, d) An oriented acute turn of motile micropump. (e) Specific right angle trajectory (‘square-wave’) of ZnO/Ni/PS Janus micromotor guided by successively rotating magnetic at 90° three times. (f) Optical microscope images of the process to produce ‘square-wave’ micropattern through remote direction control of motile micropump. Scale bar, 50 μm. The diameter of ZnO/Ni/PS Janus micromotor and silica particles is about 5 and 1.21 μm, respectively. 2.5. Complex micropatterns produced by motile micropump. Based on continuous directional movement of micropatterning process, complex two-dimensional surface micropatterns with sharp vertices and smooth connection can be fabricated to show messages. The diagrams shown in Figures 5a-d illustrate the predefined track lines of surface micropatterns in a shape of ‘NANO’ letters through directional control of Janus micromotor. The final surface micropatterns of ‘NANO’ letters are manufactured by a 5-μm-diameter ZnO/Ni/PS micromotor along the above predefined track lines in 1.21-μm-diameter silica particles suspension captured from Video S3, shown in in Figures 5e-h. The ZnO/Ni/PS Janus micromotors follow the predefined track lines (N, A, N, O) at a constant velocity under the control of external magnetic field. And the micropatterning cycle of each letter (N, A, N, O) is approximately 40.6, 51.8, 60.2 and 63 s, respectively. The widths of those letters are around 35 μm,



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which is approximately 7 times wider than the size of ZnO/Ni/PS Janus micromotor, resulting from the widths of ion distribution of motile micropump system. Based on the above continuously directional magnetic orientation of motile micropump, Figures 5i-l illustrate the predefined track lines of a complex star-shape surface micropattern. The ZnO/Ni/PS micromotor departs from point 1, passes by the points 2-4 and finally reaches point 5. It just need to turn an obtuse angle of the external magnetic field in each corner of the ‘star’ pattern. The optical microscope images shown in Figures 5m-p demonstrate the time-lapse micropatterning process of the star-shape surface micropattern through the remote magnetically guided ZnO/Ni/PS Janus micromotor pumping away the passive silica particles, captured from Video S4. Note that the micropatterning process of motile micropump system demonstrates stable fabricating speed and shows no obvious speed loss when the Janus micromotor moves through every sharp vertex.


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a

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Figure 5. Letters micropatterns and complex geometry micropatterns produced by direction controlled motile micropump. (a-d) Predefined ‘NANO’ track lines of motile micropump. (e-h) Optical microscope images of “NANO” letters micropattern fabricated by motile micropump based on direction controlled ZnO/Ni/PS Janus micromotor in silica particles suspension. (i-l) Predefined ‘star’ trajectory of ZnO/ Ni/PS Janus micromotor. (m-p) Optical microscope images of the micropatterning process of ‘star’ micropattern through motile micropump guided by external magnetic field. The diameter of ZnO/Ni/PS micromotor and silica particles is about 5 and 1.21 μm, respectively.

2.6. Rewritable property of motile micropump. Similar to secret code or verification code, message need to disappear or to change to another message after a period of time. The micropatterns produced



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by the motile micropump system can used for showing secret messages (such as letters or figures). After acquiring the messages, the micropattern can disappears gradually. And same or different message can be rewritten in the same region. As demonstrated in SI Video 5, a ‘circular’ micropattern is fabricated in the silica suspension incipiently, and then disappear gradually as result of the Brownian motion. Later, another ‘circular’ micropattern can be rewritten by the same Janus micromotor in the same region. To better understand the disappearing process of micropattern, a straight-line micropattern is fabricated by motile micropump in the silica particles suspension. The width of the straight-line micropattern decreases gradually over time and the micropattern will disappear eventually after about 260 s, as shown in Figures 6a. In order to quantitatively analyze the relationship between micropattern width and time, the widths of the straight-line micropattern are measured at different moments. Figure 6b illustrates the width of micropattern in the shape of a straight line as a function of time. Note that the micropattern width of dynamic motile micropump decreases from 39.26 to 1.9 μm upon increasing the time from 0 s to 260 s. After that, the glass substrate where micropatterns have disappeared is covered with silica particles again. We can rewrite any other messages on the same area.

a

Initial State

0s

65 s

195 s

260 s

b

Move Direction 130 s

15 μm

Figure 6. Disappearing process of micropattern. (a) Time-lapse microscope images of erasure process of micropattern in the shape of a straight line in the silica particles suspension. (b) The width of micropattern in the shape of a straight line fabricated by motile micropump system based on ZnO/Ni/PS Janus micromotor changes over time.


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3. Conclusions In summary, we have demonstrated a novel and versatile motile micropump system for micropatterning through integrating water-driven ZnO/Ni/PS Janus micromotor and catalytic micropump system. The autonomously moving ZnO/Ni/PS Janus micromotor is driven by selfgenerated ion concentration gradient, while pumps away nearby silica particles through diffusiophoresis. Consequently, the dynamic motile micropump can fabricate complex surface micropatterns in a particle suspension through remote magnetic control. Adjustable resolution and micropatterning efficiency are realized through tuning diameters of the Janus micromotor. In future, higher micropatterning efficiency can be realized through controlling multiple micromotors in the same time. Predetermined surface micropatterns with different shapes and sizes, corresponding to the predefined track lines and diameters of ZnO/Ni/PS Janus micromotors, are also generated to show various messages. The new capability of generating complex geometric micropatterns in silica particles suspension through dynamic motile micropump opens up new opportunities for new functional surface science and potential applications, such as producing secret messages, dynamically changeable microchannels, dynamically variable masks for lithography, reconfigurable circuit elements, adaptive optics and microfluidics. 4. Experimental Section Materials: Commercially polystyrene microparticles with a diameter of 5 μm were purchased from Polysciences, Inc. And commercially silica microparticles with a diameter of 1.21 μm were purchased from Bangs Laboratories, Inc. The water used in all experiments was prepared in a Milli-Q purification system with the resistivity higher than 18.2 MΩ cm-1. Preparation of ZnO/Ni/PS Janus micromotors: Polystyrene microbeads were dispensed on a glass slide, then coated with 30 nm Ni layer by electron beam evaporation on a Temescal BJD 1800 system



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with deposition rate of 0.5 Å/s. ZnO layer was sputtered with a Denton Discovery 18 system at room temperature with an output power of 200 W for 10 min. The process of preparation is show in the Figure S1a-d. The ZnO/Ni/PS motors were separated from the glass slide and dispensed in ultrapure water before use. Characterization: Scanning electron microscopy (SEM) images and EDX analyses were obtained with a Phillips XL30 ESEM instrument, using an acceleration potential of 3 kV. Videos were captured by an inverted optical microscope (Nikon Instrument Inc. Ti-S/L100), coupled with 10× and 20× objectives, a Hamamatsu digital camera C11440 using the NIS-Elements AR 3.2 software. Supporting Information Supporting Information is available from the ACS Publications website or from the author. The Supporting Information includes the description of supporting videos, supporting figures and simulations. ACKNOWLEDGMENT X.C., C.C. and J. L. contributed equally to this work. This work was supported by the National Natural Science Foundation of China (Grant Nos. 51705108, 51822503, 51875141, 51505222 and 51522003), Program of Introducing Talents of Discipline to Universities (No. B07018), Self-Planned Task of State Key Laboratory of Robotics and System (HIT) (No. SKLRS201706A), and National Science and Technology Major Project (No. 2016ZX0510-006). Dr. T.L would like to thank the China Postdoctoral Science Foundation Grant (No. 2017M621257), General Financial Grant from the Heilongjiang Postdoctoral Science Foundation (No. LBH-Z17055), and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) (No. 2019-KF4). REFERENCES [1] [2] [3] [4] [5]

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Motile Micropump Based on Synthetic Micromotors for Dynamic

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