Bioinspired Photodetachable Dry Self-Cleaning Surface - Langmuir

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Bio-inspired photo-detachable dry self-cleaning surface Pei Chen, Xudong Li, Junfei Ma, Rui Zhang, Fei Qin, Jiaojiao Wang, Travis Shihao Hu, Yilin Zhang, and Quan Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04310 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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Bio-inspired photo-detachable dry self-cleaning surface Pei Chen†, Xudong Li†, Junfei Ma‡, Rui Zhang‡, Fei Qin†, Jiaojiao Wang§, Travis Shihao Hu#, Yilin Zhang⊥ and Quan Xu*,‡ † Institute

of Electronic Packaging Technology & Reliability, College of Mechanical Engineering

& Applied Electronics Technology, Beijing University of Technology, Beijing 100124, China ‡

State Key Laboratory of Petroleum Resources and Prospecting, Harvard SEAS-CUPB Joint

Laboratory on Petroleum Science, Beijing Key Laboratory of Biogas Upgrading Utilization, China University of Petroleum, Beijing 102249, China §

School of Materials Science and Engineering, East China University of Science and

Technology, Shanghai 200237, China #

Department of Mechanical Engineering, California State University, Los Angeles, CA 90032,

USA ⊥C.

Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, WV

26506-6045, USA Author information Corresponding Author *E-mail: [email protected] (Q.X.). Abstract Geckos have adapted to the complicated natural environment with its excellent climbing ability. Current artificial Gecko-inspired Synthetic Adhesives (GSAs) mimic gecko’s attach-detach

ACS Paragon Plus Environment

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mechanism by creating anisotropic and hierarchical structures. Easy detachment and high selfcleaning capability are still the unsolved problems in GSAs. This study presents an unprecedented photodetachable mechanism of making bio-inspired smart surfaces utilizing carbon dots (CDs) doped PDMS composite. Under ultraviolet (UV) irradiation, it could be triggered up to 80.46% reduction of adhesion force between PDMS-CDs bio-inspired surface and contaminating particles. A load-drag-pull (i.e., LDP) test mimicking gekco’s locomotion was adopted to test the dry selfcleaning capabilities of these bio-inspired surfaces, where the falling rate of the model contaminates (PS micopellets; average size in diameter ~8 μm) can reach up to 54.83% after seven repeated steps under UV irradiation. The significantly improved dry self-cleaning capability is attributed to the photothermal effect of CDs inside the PDMS matrix. This work and proposed mechanism will find its applications in the realms of climbing robot, space adhesive devices and self-cleaning, advanced gripping technologies for pick and place or assembly. Keywords: Carbon Dots, Bio-Inspired Surfaces, Dry Self-Cleaning, Photodetachable Adhesion

Introduction Countless animals in nature have evolved smart and multifunctional surfaces/structures on the exterior of their bodies, adapting to a rather complex and dynamic environment of their local habitats. 1 Some of the amazing examples include, shark skin, 2 mussel threads, 3 and gecko toe pads. 4 Of particular interests are the excellent adhesive properties of geckos, namely, the strong adhesion, easy-to-detach ability (or reversibility), and self-cleaning.

5-8

Geckos can

control its adhesion and detachment with ease through a critical angle between the adhesive


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fibrils and the contact surfaces. 9-11 When the angle between the setal stalk and the contact surface is less than 30°, the gecko's pad will show strong adhesion to facilitate attachment. Otherwise, the adhesion of the gecko's pad will decrease rapidly,

12-13

leading to an easy

detachment from the contacting surface. Conventionally, polymeric materials and carbon nanotubes are two major candidates in making artificial Gecko-inspired Synthetic Adhesive (GSAs); each offers distinctive advantages and disadvantages. 14-15 Polymers were more widely used in fabricating GSAs at the early stage due to relatively easier geometrical control and a wide range of selectable modulus (e.g., polyimide [PI],16 polypropylene [PP],17 poly(urethane acrylate) [PUA], Polydimethylsiloxane [PDMS], 18 Poly(methyl methacrylate) [PMMA] and more). Carbon nanomaterials, on the other hand, have excellent, mechanical, thermal and electrical properties and could be synthesized as high performance and multi-functional adhesives.19 In the attempt of making high-performance GSAs, many researchers have focused on controlling the adhesion on and off effectively, and incorporating excellent dry contact selfcleaning capacities.

20-23

In general, a combination of structural anisotropy and different

external stimuli (i.e., mechanical, thermal, electrical, magnetic and chemical) are used to control the adhesion engagement/disengagement, 24-26 but many of them are either one-time control or needs stickily controlled environments, limiting their practical applications. A recent study shows that photo-detachable adhesion can be achieved by breaking the covalent polymer network of two hydrogels, however it suffers from low levels of reusability and lack


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of active self-cleaning. 27 In the present study, we propose a new bio-inspired photo-detachable surface from the PDMS-CDs composites. PDMS is easy to modify, thermally stable, and possesses excellent optical properties, which can transmit UV light with wavelengths above 300 nm. 28 It has unique advantages as the matrix to hold together the nanoparticles (i.e., CDs) and transfer loads, due to its relatively large molecular gaps. CDs known as the 0-dimension carbon materials with specific size under 10 nm, are a prominent filler material can provide strong adhesive force via Van der Waals interaction.,29 hydrogen bonds that are ubiquitous at the interface.30 In addition, CDs are well known for their photoluminescent and photothermal effects

31-32,

which may offer an

alternative method to control adhesion. Herein, we demonstrate that ,under 365 nm UV radiation, the adhesion force of the bio-inspired PDMS-CDs surface could be reduced substantially, facilitating the easy detachment and self-cleaning. A load-drag-pull test was adopted to test the dry self-cleaning capability, where UV irradiation had improved the PS microparticles (i.e., the model containments; ~8 μm) dislodging rate up to 54.83% after seven repeated steps, one of the highest self-cleaning efficiency recorded so far.33 This work provides a promising route of making switchable adhesives and/or manipulators. Results and Discussion Adhesion Force and Self-Cleaning Measurement of Bio-Inspired Surface


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The fabrication process is illustrated in Figure 1(a). The prepared PDMS-CDs solutions were poured onto the silicon template, and casted in the shape of micropillars. The bio-inspired surfaces, filled with arrayed micropillars, were obtained by carefully demolding the samples from the silicon template after curing. The samples with the mass fraction of 0%, 2%, 4% and 6% of CDs were fabricated and labeled as a, b, c and d, respectively. The SEM images, from Figure 1(b) to (d), show the geometrical features of the bio-inspired surface at different scales and perspectives. The surface consists of evenly distributed vertical micro-pillars with a diameter of about 15 μm and a height of about 17 μm. Energy Dispersive Xray spectroscopy (EDX) is adopted to map the distribution of Calcium (Ca) in PDMS-CDs samples as shown in the Figure 1(e). Initially, no Ca existed in the PDMS. When fabricating CDs, 0.22g of calcium chloride (CaCl2) is added to speed up the reaction. Therefore, the EDX results provide convincing evidence to reveal the successful doping of CDs, and furthermore, indirectly demonstrate the well distribution of the CDs. The distribution was relatively uniform as shown in Figure 1 (e). Figure S1-4 show the morphology and photoluminescence characterization of asprepared CDs.


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Figure 1 (a) Schematics of the preparation of bio-inspired adhesive samples; (b) Top view SEM image of the sample surface; (c) zoomed in image of (b); (d) SEM image of the sample surface from side view; (e) EDX mapping shows the elements of Si, O, Ca and C from left to right. Atomic Force Microscope (AFM) was adopted to measure the adhesion force of the bioinspired surface, as shown in Figure 2 (a). The AFM probe was modified by gluing a single SiO2 nanoparticle underneath the end of a tipless AFM cantilever. The AFM measurement (known as force curve) is illustrated in Figure 2(b). The adhesion force is determined by the difference of extended and retract force curves, and the elastic modulus can be obtained from the extending curve. The dry self-cleaning capability was measured by a load-drag-pull (LDP) procedure, as shown in Figure 2(c). The preload of the self-cleaning experiments was 2N, and the velocity of the self-cleaning experiments was 1cm/s. The self-cleaning experiment is conducted by simulating the self-cleaning path of gecko. Furthermore, anti-static electricity is considered in the design of the test device to prevent static electricity generated by the mechanical movement. More specifically, the counter surface was covered with a glass slide coated with conductive adhesive(FS05006) and


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placed on a metal frame. The dislodging rate of adhered polystyrene (PS) pellets after each step was recorded as a measure of the self-cleaning capability. A detailed description is shown in experimental section.

Figure 2 (a) Schematic of the principle of atomic force microscopy (AFM); (b) Typical AFM force curve in adhesion measurement (inset shows an AFM probe with the nanoparticle modified tip); (c) The schematic of the dry self-cleaning measurements.

UV triggered photothermal effect on adhesion force To characterize the effect of UV irradiation on adhesive performance of the bio-inspired surfaces, 365 nm UV irradiation was used on samples of different CDs concentration. AFM in-


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situ measurements of the adhesion force were conducted employing four different retraction velocities. For low concentration samples a to c, 0.1 μm/s, 1 μm/s, 10.9 μm/s and 119μm/s retraction velocities are chosen. However, in the case of sample d with 6% of CDs, no data was successfully obtained at the retraction speed of 119μm/s with UV irradiation. Therefore, four retraction velocities of 0.01 μm/s, 0.1 μm/s, 1 μm/s and 10.9 μm/s were chosen for the sample d. Figure 3(d) shows that UV irradiation significantly reduced the adhesion force of sample d for all retraction velocity, and the average adhesion force were reduced by 80.46% of its initial value. The inset shows the bio-inspired surface was excited to light blue color. The infrared thermal imager was used to record the real-time temperature change during UV irradiation. As shown in Figure 4 (a) - (d) and Video S1, the temperature increased by 10.84 ºC within 3 m for sample d. For low concentration samples a, b and c, as shown in Figure 3 (a), (b) and (c), no significant change of adhesion force with and without UV irradiation was observed, and the corresponding infrared thermal images were also shown in Figure S5 with no significant temperature increase. The results indicate that the amount of CDs doping is nontrivial to UV controllable adhesion.


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Figure 3 Effect of UV on the adhesion of samples (a), (b), (c) and (d) under different retraction velocities.

Figure 4 (a) Infrared thermal image of sample d under UV irradiation at 30 s, the maximum temperature reached 29.03 ºC; (b) the maximum temperature reached 32.22 ºC at 1 m; (c) the


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maximum temperature reached 36.67 ºC at 2 m; (d) the maximum temperature reached 39.87 ºC at 3 m; Although temperature increase was observed, whether or not the temperature will influence the adhesion force still needs to be determined experimentally. When the temperature increased from 30 ºC to 50 ºC, the adhesion force of samples with different CD concentrations decreased significantly, as shown in Figure 5 (a). The AFM adhesion forces on samples b, c and d decreased 82.0%, 40.34% and 20.74%, respectively, when temperature decreased from 50 ºC to 30 ºC. This observation shows that change of temperature has significant effect on the adhesion performances. FTIR spectrum was used to analyze the structural transformation of the bio-inspired surfaces at a range of temperatures (from -5 to 50 º C), as shown in Figure 5 (b). The peak at 3500 cm-1, corresponds to O-H bond, has shifted to higher wavenumbers, when temperature increases, and the other peaks stay the same. It can be seen that the shift of -OH group occurred without formation of new peaks, indicating no new bond formation. Furthermore, the decrease in the number of hydrogen bonds will increase the electron cloud density in the chemical bonds, which increases the chemical bond force constant and causes the peak attributed to the stretching vibration of -OH group shifted. Therefore, the decrease of -OH group is caused by the formation of hydrogen bonds, resulting in the increase of hydrogen bonds, which may cause a decrease of adhesion force.


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Figure 5 (a) Influence of temperature on adhesion force of PDMS-CDs surfaces; (b) FTIR of the sample d at temperatures of -5, 10, 40, and 50 ºC.

Dry self-cleaning ability of the bio-inspired surface Although mechanical shear effect exists in the self-cleaning process, the efficiency of the process is still greatly impacted by the material properties. The surface structure of our sample was inspired by gecko. Therefore, its self-cleaning performance was examined according to the trajectory of gecko in the actual movement process. The dry self-cleaning capability of the PDMSCDs bio-inspired surface was tested under three experimental conditions with the study of UV effect on the bio-inspired surfaces: i) without UV at 30 ºC, ii) with UV at 30 ºC, and iii) without UV at 50 ºC. The pellets dislodging ratio (i.e., after a LDP action, the ratio of the number of pellets transferred on the target/contacting surface to the initial number of pellets on the bio-mimicking surfaces) become stabilized after the seven steps, as shown in Figure 6 (a). Strongest self-cleaning capability was recorded as 54.83% of pellets dislodging ratio for the condition of 30 ºC with UV. In contrast, the dislodging ratio without UV under the same temperature is only 26.88%. On another hand, temperature increase can also improve the self-cleaning capability to 46.23% as shown in the condition of 50 º C without UV. Figure 6 (b) to (e) illustrate the self-cleaning capability under different conditions, and additional images were shown in Figure S6. It is shown that both UV irradiation and high temperature can improve the self-cleaning capability.


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Figure 6 (a) Dislodging rate vs. steps for three different conditions; Schematics of (b) the initial contaminated bio-inspired surface, (c) stabilized self-cleaning state at 30 ºC, (d) stabilized self-cleaning state at 50 ºC and (d) Stabilized self-cleaning state 365 nm UV at 30 º C.

Key factors influencing adhesion properties of the bio-inspired surfaces The dry adhesive test between the bio-inspired surface and nanosphere modified AFM tip were carried out under a variety of conditions. The contact time of AFM tip to the bio-inspired surface, the preloading, and the retraction velocity were precisely controlled in this study. From Figure 7 (a), (c) and (e), it can be seen that as the concentration of CDs increases under all experimental conditions, the AFM adhesion on bio-inspired surface increase substantially.


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Figure 7 Adhesion performance of bio-inspired surface under different factors (a) Effect of contact time on the adhesion of bio-inspired surface; (b) Maximum and minimum adhesion force of sample a, b, c, and d for contact time changes; (c) Effect of load force on the Adhesion of bio-inspired surface; (d) Maximum and minimum adhesion force of sample a, b, c, and d for load force changes; (e) Effect of retraction velocity change on the adhesion of bio-inspired surface; (f) Maximum and minimum adhesion force of sample a, b, c, and d for retraction velocity changes.


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The contact time versus adhesion force is shown in Figure 7 (a) and (b), where the contact time changed from 0.2 s to 2.4 s, while the retraction velocity was kept as 1 μm/s and the preloading force was kept 100 nN. The adhesion forces on samples a, b, c, and d changed by 6.62%, 7.18%, 2.80%, and 7.88%, compared with their initial forces, respectively. Similarly, Figure 7 (c) and (d) represents the relationship between preloading force and adhesion force, and the contact time and retraction velocity were fixed at 3 s and 1 μm/s. With the preloading force increased from 80 nN to 400 nN, the final adhesion forces of samples a, b, c, and d changed from their initial adhesion forces by -8.53%, -3.70%, -5.814%, and 6.448% respectively. Since the preloading force and contact time did not cause chemical changes inside or on the surface of the sample, the polar groups that play an important role in the surface adhesion force are not affected. Therefore, these two factors do not have a significant impact on the adhesion force of the bio-inspired surface. Four retracting velocities (0.1 μm/s, 1 μm/s, 10.9 μm/s and 119 μm/s) were applied to study the velocity effect on adhesive force, as shown in Figure S7 (e) and(f), when contact time and load force were kept at 3 s and 100 nN. The adhesion force of samples a, b, c, and d increased by 89.31%, 89.92%, 71.80%, and 60.20% compared with their initial adhesion forces, respectively. This phenomenon has been fully discussed in our previous established dynamic models. 34 Photo-Detachable Mechanism of Bio-Inspired Surface Figure 8 (a) shows the adhesion force tested by AFM before UV, under UV and repeated tested without UV. The adhesion force basically did not recover after UV irradiation, which means irreversible chemical bond contributed to the adhesion. In order to understand the effect of UV irradiation on the -OH group from the surface of samples, FTIR analysis was performed on all samples before and after UV irradiation. Peaks observed at 3500 cm-1 could be attributed to the stretching vibration of O-H. No significate change is found for samples a, b and c, as shown in


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Figure S7. PDMS-CDs doped with 6 wt% CDs (sample d) exhibits more intramolecular -OH leading to a decreased amount of free -OH on the surface of the sample, which is indicated by the peak shift as shown in the Figure 8 (b). Therefore, less hydrogen bonds should exist in these samples, which leads to the reduction of adhesive force.

Figure 8 (a) The surface adhesion force of 6% PDMS-CDs bio-inspired surface with UV, without UV and repeated test after UV; (b) The FTIR results of the 6% PDMS-CDs with UV and without UV The significant adhesion force reduction may be attributed to the distinctive photothermal effect of the doped CDs. The UV radiation energy could be absorbed and converted into the kinetic energy of the CDs’ lattice, resulting in a temperature rise. 35 Infrared images in Figure 4 (a) to (d) show only sample d with 6 wt% CDs possesses significant temperature increase. FTIR illustrated that the number of hydrogen bonds could decrease when the surface temperature is increased from -5 to 50 ºC. As can be seen from Figure 1(e), the main components of the bio-inspired surface are C, O, and Si, which can form hydrogen bonds such as C-O-H-O-C and C-O-H-O-C. When the temperature increases, the adhesion force may be affected by the decrease of hydrogen bonding content across the interfaces, as schematically shown in Figure 9.


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Figure 9 Schematics of (a) the amount of hydrogen bonding at low temperatures, and (b) the amount of hydrogen bonding at higher temperatures.

The temperature influence on the bio-inspired surface adhesion of PDMS-CDs indicated that all of the samples showed switching effect with the temperature increase. Although 6 wt% PDMSCDs samples exhibited less temperature effect, they all showed a clear switching at least. From temperature test, we can confirm temperature increase will decrease the interfacial adhesion, but we can’t confirm concentration will play a role in temperature effect. Different from the global heating during temperature test, UV irradiation test targets on the luminescent CDs which is the local heat source. Only 6 wt% samples can be effectively heated by CDs under UV irradiation, as illustrated by Figure 4, and others failed to show temperature change under UV irradiation. Therefore, concentration of CDs could affect the surface temperature under UV irradiation, but may not play a role in the temperature test. In addition, contact angle measurement was adopted to characterize the change of the surface free energy of the bio-inspired surface under different temperature. The contact angle of both water and glycol (i.e., two probing liquids using the sessile drop method) on the bio-inspired surfaces,


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as shown in the Figure 10 and the Table S1, S2, S3. The free surface energy can be obtained by the following Equations (1) and (2), 36 p

γl(1 + cos θ) = 2 γdsγdl +2 γpsγl 𝛾𝑠 = γsd +γsp

（1）

（2）

where γs is the surface free energy, γl is the liquid surface tension, γds is the dispersion part of the solid surface free energy, γps is the polar part of the solid surface free energy. γdl is the dispersion part of the liquid surface tension, γpl is the polar part of the liquid surface tension, θ is the contact angle of the corresponding probing liquid on the bio-inspired surface. The surface free energy also can be expressed as follows, 𝛾𝑠 = 𝑈𝑠 ― 𝑇𝑠𝑠

(3)

The relationship between surface free energy 𝛾𝑠, surface internal energy 𝑈𝑠, surface temperature 𝑇 and surface entropy 𝑠𝑠is shown in equation (3). With the increase of temperature, the surface entropy gradually increased, and the internal energy of PDMS-CDs samples also slightly increased. However, part of the internal energy was consumed to change the chemical bonds on the surface of the samples. Therefore, the increase of internal energy could not offset the increase of surface entropy. In this case, the surface free energy will decrease with the increase of temperature, which can be speculated from equation (3). The change in the contact angle experiment in the Figure 10 confirms this hypothesis.


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Figure 10 Contact angle test (a) bio-inspired adhesion surface to water at 30 ºC; (b) bio-inspired adhesion surface to ethylene glycol at 30 ºC; (c) bio-inspired adhesion surface to ethyl alcohol at 40 ºC; (d) bio-inspired adhesion surface to ethyl alcohol at 50 ºC.

With the increase of doping concentration of CDs, the surface free energy gradually increased, as illustrated in Table S1, S2 and S3, which is consisted with the doping effect of CDs reported in Figure 10. We found that the surface free energy of the bio-inspired surface decreased with the increase of temperature from 30 to 50 ºC, specifically from 6.755 to 5.961 mN/m by contact angle test. Similar decrease treads were found for all of the samples regardless of the concentration of CDs. Therefore, the reduction of surface free energy on the bio-inspired surface is another explanation for the adhesion force decrease. Moreover, another possible cause is, that the high energy of UV photons could directly cause the chemical bonds break, such as the breakage of C=C, O-H, and Si-O, which may affect the surface adhesion force. The energy of O-H bond is 465 kJ/mol, and the photon energy of 365 nm UV light is reported as 544.9 kJ/mol. 37 Therefore, the UV photon at 365 nm can possibly break the O-H bond at the interface, resulting in a decrease in the adhesion force measured on the bioinspired surface.


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Table 1 The elastic modulus of the bio-inspired surface with different CDs doping concentrations

Elastic modulus

Pure PDMS

2% PDMSCDs

4% PDMS -CDs

6% PDMS -CDs

2.73 MPa

2.77 MPa

2.79 MPa

2.82MPa

Additionally, the elastic modulus of the bio-inspired surface was studied through AFM and the results are shown in the Table 1. The elastic modulus of samples increased slightly from 2.73 MPa to 2.82 MPa with the increase of the CDs doping concentration. As the elastic modulus increase, the pull-off strength should increase. However, the increase of elastic modulus is not significant. Therefore, the increase of adhesion force detected by AFM should be partially attributed to the increase of elastic modulus. Conclusions We have demonstrated a novel bio-inspired photo-detachable adhesive with high selfcleaning capabilities using a PDMS-CDs combined materials system. Doping of CDs into the PDMS micro-pillars significantly improves the adhesive properties of the bio-inspired surface. Under 365 nm UV irradiation the adhesion force of the 6 wt% PDMS-CDs sample decreased by 80.46% on average compared with its initial adhesion force. The sample temperature increased with the prolonged exposure time, which is mainly attributed to the photothermal effect of CDs. This temperature increase has led to a decrease of the adhesion force. UV irradiation and temperature increase both contributed to improving the capabilities of self-cleaning, where UV irradiation performed better with a highest detachment efficiency of 54.83%. The principle of photo-detachment is attributed to direct breakage of the hydrogen bonding by UV irradiation and the decrease of associated free surface energy with increasing temperature. Overall, different from the existing adhesion switching methods, the presented PDMS-CDs bio-inspired surface has a non-


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contact adhesion force controllability based on UV irradiation, which can also ensure a high selfcleaning efficiency. Experimental section Sample preparation. CDs were prepared by using the following reagent, 0.5582 g of sodium citrate, 0.2402 g of urea, and 0.22 g of calcium chloride powder. Their mixture with 20 ml of toluene was sufficiently stirred using a pipette, and was placed in a 50 ml Teflon-lined stainless steel autoclave at 160 ºC for 36 hours. The final CDs solution was obtained after filtering the mixture by using a cylindrical filtration membrane filter (0.22 mm). PDMS was mixed with curing agent(Sylgard 184) at a ratio of 10:1. CDs solution (0%, 2%, 4%, and 6% weight percentage) was added into the PDMS mixture before curing. Then the mixture was put into a vacuum pump to degas. About 3 ml of trimethylsiloxane (TMCS) is dropped onto the silicon template for 30 min. After that, the mixed solution of PDMS and CDs (PDMS-CDs) was poured on the silicon template and then cured in the oven at a temperature of 60ºC for 9 h. Adhesion tests of bi-inspired surface. For the adhesion measurement of the bio-inspired surface, the test is performed by AFM (Dimension Icon AFM, Bruker Co., Inc.), and the detailed description of AFM testing method is mentioned in our previous study.29 Conventional AFM probe is extremely sharp, and easy to penetrate the bio-inspired surface. To ensure non-destructive contact between AFM tip and sample surface, a nanosphere is attached to a tipless AFM probe (Probe type: RTESPA-150, Bruker, Inc.). Nanospheres, made from silicon dioxide (SiO2), were deposited in a clean silicon wafer. The wafer was washed by DI water and kept in Argon (Ar) environment for 24h to eliminate the possible water bridging effect between the nanospheres and wafer. To fabricate the nanosphere modified probe, a nanosphere was picked by a nanomanipulator,


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and transferred to the right location of a tipless AFM probe under scanning electron microscope (SEM, FEI HeliosNanoLab, Cleveland, USA), followed by a permanent gluing process. Self-clearing capability test. The load-drag-pull process, that is similar to the gecko’s foot motions, is adopted in the evaluation of the self-cleaning capabilities of the bio-inspired surface, as shown in Figure2(c). The hydrophobic PS pellets have fully covered to the bio-inspired surface in the beginning. The preload of the self-cleaning experiments was 2N approximately, and the velocity of the self-cleaning experiments was 1cm/s. After each step of load-drag-pull motion, the shedding of pellets from the bio-inspired surface was observed and counted. The dislodging rate of the PS pellets is used to characterize the self-cleaning capability. In this study, the self-cleaning capability, at 30°C without UV light, 30°C with UV light, and 50°C without UV light, was tested. Characterizations The UV-vis absorption spectrum of the as-prepared CDs was obtained by a UV-Vis spectrometer (Jasco V-570) (Figure S1). The as-prepared CDs demonstrated an intense emission centered at 565 nm when excited at different wavelengths (400–500 nm) (Figure S2). The photoluminescence of the optimized CDs was investigated using a fluorescence spectrometer (Figure S3). The TEM image of CDs were showed in Figure S4. Scanning electron microscopy (SEM) was used to observe the surface morphology of the bioinspired surfaces (Figure 1(b), (c), (d)), and Energy dispersive X-ray spectroscopy (EDX) was also adopted to analyze the energy spectrum of the bio-inspired surface doped with different concentrations of CDs (Figure 1(e)). The morphology of the CDs were investigated using transmission electron microscopy (TEM) (Model JEM-2100) operated at 200 kV. Fourier Transform infrared spectroscopy (FTIR) is adopted to analyze the PDMS-CDs as well. It was performed on a Bruker Tensor 27 spectrophotometer. In addition, the surface temperature of the


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sample was recorded by an infrared thermal imager (FLIR SC7300M), the contact angle experiment (Solon Tech Co., Lid SL200B) was applied to investigate the surface free energy of the PDMS-CDs bio-inspired surface.

Supporting Information A description of various characterizations, experimental mechanisms and explanations of this material can be seen in Tables S1-3, Figures S1-6 and Video S1.

Acknowledgement We thank National Natural Science Foundation of China (No. 51875577 and 11502005), Beijing Nova Program (No. Z181100006218138) and Tribology Science Fund of State Key Laboratory of Tribology(No. SKLTKF16A06) for the financial support. References (1) Zhang, C.; McAdams, D. A.; Grunlan, J. C. Nano/MicroManufacturing of Bioinspired Materials: a Review of Methods to Mimic Natural Structures. Adv. Mater. 2016, 28, 6292−6321. (2) Kirschner, C. M.; Brennan, A. B. Bio-Inspired Antifouling Strategies. Annu. Rev. Mater. Res. 2012, 42, 211−229. (3) Lee, B. P.; Messersmith, P. B.; Israelachvili, J. N.; Waite, J. H. Mussel-Inspired Adhesives and Coatings. Annu. Rev. Mater. Res. 2011, 41, 99−132.


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(4) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Adhesive Force of a Single Gecko Foot-hair. Nature 2000, 405, 681−685. (5) Izadi, H.; Stewart, K. M.; Penlidis, A., Role of contact electrification and electrostatic interactions in gecko adhesion. J R Soc Interface 2014, 11 (98), 20140371. (6) Xu, Q.; Zhang, W.; Dong, C.; Sreeprasad, T. S.; Xia, Z., Biomimetic self-cleaning surfaces: synthesis, mechanism and applications. J R Soc Interface 2016, 13 (122), 20160300. (7) Gillies, A. G.; Puthoff, J.; Cohen, M. J.; Autumn, K.; Fearing, R. S., Dry self-cleaning properties of hard and soft fibrillar structures. ACS Appl Mater Interfaces 2013, 5 (13), 6081-6088. (8) Autumn, K.; Niewiarowski, P. H.; Puthoff, J. B., Gecko Adhesion as a Model System for Integrative Biology, Interdisciplinary Science, and Bioinspired Engineering. Annu Rev Ecol

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(12) Autumn, K.; Hsieh, S. T.; Dudek, D. M.; Chen, J.; Chitaphan, C.; Full, R. J., Dynamics of geckos running vertically. J Exp Biol 2006, 209 (Pt 2), 260-272. (13) Birn-Jeffery, A. V.; Higham, T. E., Geckos significantly alter foot orientation to facilitate adhesion during downhill locomotion. Biol Lett 2014, 10 (10), 20140456. (14) Geim, A. K.; Dubonos, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov, A. A.; Shapoval, S. Y., Microfabricated adhesive mimicking gecko foot-hair. Nat Mater 2003, 2 (7), 461-463. (15) Murphy, M. P.; Kim, S.; Sitti, M., Enhanced adhesion by gecko-inspired hierarchical fibrillar adhesives. ACS Appl Mater Interfaces 2009, 1 (4), 849-855. (16) Liu, K.; Du, J.; Wu, J.; Jiang, L., Superhydrophobic gecko feet with high adhesive forces towards water and their bio-inspired materials. Nanoscale 2012, 4 (3), 768-772. (17) Autumn, K.; Majidi, C.; Groff, R. E.; Dittmore, A.; Fearing, R., Effective elastic modulus of isolated gecko setal arrays. J Exp Biol 2006, 209 (18), 3558-3568. (18) Yu, J.; Chary, S.; Das, S.; Tamelier, J.; Turner, K. L.; Israelachvili, J. N., Friction and adhesion of gecko-inspired PDMS flaps on rough surfaces. Langmuir 2012, 28 (31), 1152711534. (19) Xu, M.; Du, F.; Ganguli, S.; Roy, A.; Dai, L,; Carbon nanotube dry adhesives with temperature-enhanced adhesion over a large temperature range. Nat. Commun., 2016, 7, 13450. (20) Stark, A. Y.;

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(21) Hawkes, E. W.; Eason, E. V.; Christensen, D. L.; Cutkosky, M. R., Human climbing with efficiently scaled gecko-inspired dry adhesives. J R Soc Interface 2015, 12 (102), 20140675. (22) Jin, K.; Tian, Y.; Erickson, J. S.; Puthoff, J.; Autumn, K.; Pesika, N. S., Design and fabrication of gecko-inspired adhesives. Langmuir 2012, 28 (13), 5737-5742. (23) Xu, Q.; Wan, Y.; Hu, T. S.; Liu, T. X.; Tao, D.; Niewiarowski, P. H.; Tian, Y.; Liu, Y.; Dai, L.; Yang, Y.; Xia, Z., Robust self-cleaning and micromanipulation capabilities of gecko spatulae and their bio-mimics. Nat Commun 2015, 6, 8949. (24) Huber, G.; Gorb, S. N.; Hosoda, N.; Spolenak, R.; Arzt, E., Influence of surface roughness on gecko adhesion. Acta Biomater 2007, 3 (4), 607-610. (25) Stark, A. Y.; Klittich, M. R.; Sitti, M.; Niewiarowski, P. H.; Dhinojwala, A., The effect of temperature and humidity on adhesion of a gecko-inspired adhesive: implications for the natural system. Sci Rep 2016, 6, 30936. (26) Wang, B. L.; Heng, L.; Jiang, L., Temperature-Responsive Anisotropic Slippery Surface for Smart Control of the Droplet Motion. ACS Appl Mater Interfaces 2018, 10 (8), 7442-7450. (27) Gao, Y.; Wu, K.; Suo, Z., Photodetachable Adhesion. Adv Mater 2018, 1806948. (28) Zhang, E.; Wang, Y.; Lv, T.; Li, L.; Cheng, Z.; Liu, Y., Bio-inspired design of hierarchical PDMS microstructures with tunable adhesive superhydrophobicity. Nanoscale 2015, 7 (14), 6151-6158. (29) Zhang, K.; Ji, B., Why Are Superhydrophobic Surfaces So Sticky? The Crucial Roles of van der Waals Force at Nanoscale in Wet Adhesion. Journal of Computational and Theoretical

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(30) Xu, Q.; Wu, X.; Wang, Z. H.; Hu, T. S.; Street, J.; Luo, Y.; Xia, Z. H., Temperatureinduced tunable adhesion of gecko setae/spatulae and their biomimics. Mater Today-Proc 2018, 5 (12), 25879-25893. (31) Ge, J.; Jia, Q.; Liu, W.; Guo, L.; Liu, Q.; Lan, M.; Zhang, H.; Meng, X.; Wang, P., RedEmissive Carbon Dots for Fluorescent, Photoacoustic, and Thermal Theranostics in Living Mice. Adv Mater 2015, 27 (28), 4169-4177. (32) Yuan, F.; Li, S.; Fan, Z.; Meng, X.; Fan, L.; Yang, S. Shining Carbon Dots: Synthesis and Biomedical and Optoelectronic Applications. Nano Today 2016, 11, 565−586. (33) Li, M.; Xu, Q.; Wu, X.; Li, W.; Lan, W.; Heng, L.; Street, J.; Xia, Z., Tough Reversible Adhesion Properties of a Dry Self-Cleaning Biomimetic Surface. ACS Appl Mater Interfaces 2018, 10 (31), 26787-26794. (34) Xu, Q.; Li, M.; Niu, J.; Xia, Z., Dynamic enhancement in adhesion forces of microparticles on substrates. Langmuir 2013, 29 (45), 13743-13749. (35) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W., Environmental Applications of Semiconductor Photocatalysis. Chem Rev 1995, 95 (1), 69-96. (36) Kozbial, A.; Li, Z.; Conaway, C.; McGinley, R.; Dhingra, S.; Vahdat, V.; Zhou, F.; D'Urso, B.;

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2500 2500

Adhesion force (nN)

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6% PDMS-CDs 6% PDMS-CDs without UV under 365nm uv radiation 6% PDMS-CDs with UV

2000 2000

50 µm

25.9 3℃

1500 1500 1000 1000

30 µm

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00

39.8 7℃

0.001

0.01 0.01

0.1 0.1

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Retraction velocity (μm/s)

The TOC graphic (For Table of Contents Use Only)


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