Accelerated microrockets with a biomimetic hydrophobic surface

Abstract

In order to increase the velocity and propulsion efficiency, an accelerated microrocket with a biomimetic lotus-leaf-like surface is proposed through an electrodeposition technique along with a self-assembly technique. The microrocket is propelled by the thrust of hydrogen bubbles generated from a redox reaction in a strong acidic solution. A low-surface-energy (LSE) layer along with rough structures is constructed at the outer surface of the microrocket to reduce the drag force resulting from the environmental fluid. Physical insights on the drag force reduction and the corresponding acceleration are identified. The decrease of drag reduction is achieved. A comparison of the average velocity of the microrockets with and without a LSE hydrophobic layer is performed. As we found, the average velocity of the microrocket is increased after being self-assembled with a biomimetic hydrophobic surface.

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1. Introduction

Similar to other actuators, microrockets can convert the energy of light and electricity, as well as chemical energy into mechanical energy in terms of propulsion velocity or force.¹ Substantial research efforts^2–4 have been devoted to fabricating micro/nanorockets owing to their considerable promise for diverse potential applications, ranging from targeted drug^5,6 and cargo⁷ delivery to collective behavior^8,9 and environmental remediation.^10,11 Various propulsion mechanisms were proposed in the past few years, such as catalytic propelled,^12–14 magnetically powered,^15,16 acoustically propelled,^17,18 self-electrophoresis,¹⁹ self-diffusiophoresis,²⁰ interfacial tension,²¹ and light-powered.²² In order to improve the propulsion efficiency of microrocket, it is vital to increase the velocity and loading capability. It is found that the drag force resulting from environmental solutions dominates the resistance and thereby has a significant effect on the velocity and loading capability of microrockets during the propulsion process in microfluidic systems.^23,24 With the aim of increasing the velocity and loading capability of microrocket, decreasing the drag force caused by environment is crucial.

Plants with complex hydrophobic structures leading to low drag force are pretty common to be found in nature. For example, the leaves of lotus and rice are of hierarchical rough structures, and the surface of rough structures is coated with a low-surface-energy (LSE) wax epicuticula leading to low drag force.^25,34 Fig. 1 presents a typical example of water droplets on a lotus leaf with rough microstructures. It shows that water droplets easily roll down from the surface of lotus leaf as the leaf is titled even slightly. Therefore, the rough structure and the LSE material were widely applied in self-cleaning materials and marine coatings to reduce surface drag.²⁶

Fig. 1 Photographic images and scanning electron microscope (SEM) image of the surface of lotus leaf: (A) water droplet roll down from the lotus leaf and (B) the micro surface structures of lotus leaf.

Inspired by the intrinsic natural morphology of lotus leaves, two methods can be used to fabricate hydrophobic structures to reduce the drag force of microrocket. The first method is to construct rough microstructures. Cottin-Bizonne et al. proposed that the surface friction can be reduced as a result of rough microstructures.²⁷ A significant drag reduction at the contact interface of gas and liquid was observed by Ou et al. through constructing a micro-sized rough surface on the microchannel.²⁸ The other method to reduce drag force is to coat a LSE material on the outer surface. Drag force reduction corresponding to large velocity were also illustrated by Dong et al.²⁹ for a ship coated with a super-hydrophobic layer, and by Zhou et al.³⁰ for a ball coated with a LSE film, respectively.

As can be seen from the above literature review, there are lots of studies working on the fabrication of various types of microrockets. Most of the work focuses on the propulsion mechanisms. Fabricating hydrophobic structures or coating a LSE material is an efficient way to reduce drag force of macroscopic objects, and therefore, is widely used in literature. However, no studies were found on the acceleration of microrockets using biomimetic hydrophobic structures or coating LSE material on the outer surface. In this work, accelerated microrockets with improved velocity are fabricated using membrane-template electrodeposition method along with self-assembly technique. The microstructure and biomimetic LSE material on the outer surface of the microrockets are characterized. Physical insights on the drag force reduction and the corresponding acceleration are investigated theoretically and experimentally. The surface adhesion force corresponding to surface energy and drag force is quantified using an Atomic Force Microscopy (AFM). Finally, the accelerated velocity of microrocket with biomimetic surface is identified experimentally.

2. Experimental methods and details

As shown in Fig. 1(B), the lotus leaf exhibits a rough surface consisting of evenly distributed microstructures and a LSE wax epicuticula. Motivated by the intrinsic natural morphology of lotus leaf, microrockets with biomimetic rough surface and LSE coatings are proposed to achieve low drag force and thereby to accelerate the microrockets.

2.1 Fabrication

The microrockets with biomimetic surface were fabricated through electrodeposition technique in a cyclopore polycarbonate template containing 10 μm length microconical pores. A LSE hydrophobic molecular layer, 1-dodecanethiol, was coated on the outer surface of the microrocket to serve as LSE material, subsequently. Fig. 2(A) shows a typical fabrication process. The rough structure of polyaniline layer was prepared by chronoamperometry at a voltage of 0.8 V and an electric charge of 0.06C approximately. Secondly, a zinc layer was deposited galvanostatically at −6 mA for 2000 s, as shown in step (a). After being electrodeposited, the microrockets with biomimetic surface were obtained by removing the polycarbonate template using methylene chloride and then were collected by centrifugation at 6000 rpm for 3 minutes, as shown in step (b). After that, the microrockets were washed repeatedly with methylene chloride followed by ethanol (three times of each). Finally, the microrockets with biomimetic surface assembled with LSE hydrophobic molecular layer were obtained by being immersed in a 1-dodecanethiol solution for more than half an hour, as shown in step (c). The resulting microrockets were collected by centrifugation at 6000 rpm for 3 minutes after being washed with ethanol. The resulting microrockets are approximately 10 μm long, with an outer diameter of about 3 μm and an inner opening diameter of about 800 nm. The microrocket is consisted of a PANI outer layer with a thickness of 300 nm and a Zn inner layer with a thickness of 800 nm.

Fig. 2 Fabrication and characterization of microrocket with biomimetic lotus-leaf-like surface: (A) fabrication of accelerated microrockets: (a) deposition of the polyaniline (PANI) microtubes, (b) deposition of the Zn microtubes and (c) self-assembly of 1-dodecanethiol hydrophobic molecular layer. (B) SEM images of the surface structures of microrocket with biomimetic surface. (C) Energy-dispersive X-ray (EDX) spectroscopy results of the PANI/Zn microrockets.

2.2 Characterization

Scanning electron microscope (SEM) along with energy dispersive X-ray spectroscopy (EDX) analysis are performed to measure the surface morphology and to identify the presence of the LSE material, respectively.

As discussed by Brunner et al., the adhesion force between the tip of an atomic force microscope (AFM) and a modified surface is proportional to the surface energy of that solid surface, i.e., the surface energy decreases with the decrease of adhesion force which can be characterized using an AFM.³¹ In this work, an atomic force microscope (Bruker, Germany) was used to measure the adhesion force and thereby to quantify the surface energy of the rough PANI surface self-assembled with LSE 1-dodecanethiol layer.

3. Results and discussion

3.1 Surface structure and composition

The accelerated microrockets are prepared by electrodeposition method and then are self-assembled with a LSE layer on the outer surface of PANI/Zn microrockets. Fig. 2(B) presents the SEM images of an accelerated microrocket with biomimetic surface. We note from the blow-up image of microrocket surface in Fig. 2(B) that a rough surface, which is analogous to the surface of lotus leaf shown in Fig. 1(B), is observed. Fig. 2(C) shows the EDX results of the biomimetic lotus-leaf-like surface of microrocket used to identify the presence of LSE material, 1-dodecanethiol. It was reported in ref. 32 that the molecular formula of 1-dodecanethiol is C₁₂H₂₅-SH, i.e., carbon element which can be analyzed using EDX is the main composition of 1-dodecanethiol besides hydrogen element. As can be seen from Fig. 2(C), the proportion of zinc atoms decreases sharply after self-assembly, changing from 62.91% to 41.65%, while the proportion of carbon atoms increases from 37.09% to 58.35%. The increase of carbon element indicates that the LSE 1-dodecanethiol molecular layer was successfully assembled on the outer layer of microrocket.

The surface morphology of the outer PANI layer before and after surface modification is characterized using AFM. As we found, the PANI layer after being self-assembled with 1-dodecanethiol molecular layer becomes rougher and more orderly than that before surface modification, as shown in Fig. S1 of ESI.† The average surface roughness of the PANI layer before being modified is approximately 17.4 nm while the average surface roughness of the PANI layer after being modified is 20.4 nm, i.e., the surface becomes rougher after self-assembly. For a rough surface, an air layer will be entrapped between asperities and liquid. Therefore, the air layer can decrease the proportion of wetted area due to the fact that liquid is unable to penetrate into the asperities. More discussion will be provided later.

3.2 Wettability and surface energy

As discussed in ref. 31, surface energy can be quantified by measuring the contact angle of a liquid drop placed on a solid surface. The static contact angle θ, which is defined as the angle between the outline of liquid–air interface and the contact line of liquid–solid interface,³³ is shown in Fig. 3(A-a). Since the diameter of microrocket is approximately 3 μm, a flat polyaniline film was electrodeposited to characterize the surface contact angle before and after self-assembly. Fig. 3(A-b) and (A-c) show the morphology of a water droplet on the rough surface of a PANI film before and after being self-assembled with LSE 1-dodecanethiol hydrophobic molecular layer, respectively. As can be seen, the morphology of water droplet on the PANI film after self-assembly is analogous to the morphology of water droplet on lotus leaf, as shown in Fig. 3(A-d). A contact angle of 145° is obtained for the droplet on the PANI film after self-assembly compared with the contact angle of 31° before self-assembly, i.e., the surface energy of PANI film with rough structures decreases after being self-assembled with LSE material. In addition, the water droplet is quickly spread out on the PANI film before surface modification as shown in Fig. 3(B-a), which suggests that the PANI film before surface modification demonstrates hydrophilic property. However, the water droplet can easily roll down from the modified PANI film when the film is slightly tilted as shown in Fig. 3(B-b) and therefore, the outer PANI layer after being self-assembled with LSE 1-dodecanethiol molecular layer not only has the similar surface morphology with lotus leaf, but also suggests self-cleaning. To investigate the wettability and surface self-cleaning ability of the modified PANI layer, water sliding angle is introduced, which is defined as the angle by which the surface is tilted till water droplet on the surface starts to roll down, as shown in Fig. 3(C).³⁴ The water sliding angle of the PANI film after being modified is characterized to identify the property of low-drag and self-cleaning. The average water sliding angle of the modified PANI film is 15.41° which indicates that the modified film presents hydrophobic property and low adhesion.

Fig. 3 Contact angle on the polyaniline film: (A) static contact angle: (a) schematic of the surface contact angle, the surface contact angle before self-assembly (b) and after self-assembly (c), and (d) the morphology of a water droplet on a lotus leaf. (B) Dynamic images of water droplet on polyaniline film: (a) water droplet is spread out on the polyaniline film before surface modification and (b) water droplet rolls down from the polyaniline film when the film is slightly tilted after surface modification. (C) Definition of sliding angle.

Fig. 4(A) shows the detailed procedures of adhesion measurement used in this work. Firstly, the AFM tip is brought to approach to the PANI surface along path 1 gradually. An attraction force is received for the tip as it approaches to the surface infinitely, resulting in the cantilever getting bent. The AFM tip “snaps” into contact with the sample surface at the moment of point 2. Further approach of the AFM tip towards the surface of PANI surface along 2–5 causes loading of the tip onto the PANI surface. The cantilever is slowly retracted from the surface in the processes of 5–6 while the tip is still in contact with the PANI surface until it completely separates from the PANI surface at the moment of point 6. The deflection of cantilever between point 6 and 7 can be used to measure the adhesion force of PANI layer. As we all know, the surface adhesion can be affected by both tip and the surface to be measured, and also be influenced by the medium. Therefore, in order to keep consistency, the force–distance curve are obtained using the same AFM tip which was washed and dried after each measurement. In addition, the medium is assumed to be air.

Fig. 4 (A) A typical AFM force–distance curve during approaching and retracting. (B) The force–distance curve obtained by AFM on the PANI layer before and after being self-assembled with LSE 1-dodecanethiol layer: (a) without hydrophobic molecular layer and (b) with hydrophobic molecular layer.

Fig. 4(B) shows the force–distance curve when an AFM tip approaches to and retracts from the PANI surface of microrocket before and after self-assembly, respectively. The negative force at the retract process of force–distance curve shown in Fig. 4(B) indicates adhesion force. It illustrates that the maximum adhesion force on the rough surface with LSE hydrophobic layer (Fig. 4(B-b)) is less than that on the surface without LSE hydrophobic layer (Fig. 4(B-a)). In addition, a comparison of the adhesion force between an AFM tip and the rough PANI layer before and after being self-assembled with 1-dodecanethiol layer is performed, respectively. Four random locations are selected in each cases to be investigated. We find that the average adhesion force is 46 nN on the treated PANI layer whereas it is 59 nN on the untreated PANI layer. As discussed by Brunner et al.³¹ and Cheng et al.,³⁵ the low adhesion indicates a lower surface energy and a better drag reduction performance. Therefore, microrockets coated with LSE hydrophobic molecular layer have a lower drag force and can accelerate microrocket.

3.3 Modelling and analysis

The fabricated microrocket is composed of triple layers, i.e. a zinc layer, a rough PANI layer and a LSE hydrophobic layer, as shown in the Fig. 5(a). A spontaneous redox reaction occurs at the inner surface of the zinc layer. The growth and ejection of hydrogen bubbles propel the microrocket to move in acidic medium.^24,36 After being electrodeposited and self-assembled with 1-dodecanethiol, an air film will exist at the interface of microrocket and acidic medium due to the presence of the microstructure and LSE layer, that are analogous to lotus leaf, as shown in Fig. 5(b).^37,38 Therefore, the microrocket is not completely direct in contact with the acidic medium. Fig. 5(c) shows the equivalent schematic contact model for microrocket and acidic medium. It shows that acidic medium can impregnate partly into the interstices of the asperities on the biomimetic surface of microrocket. Hence, two kinds of interfaces coexist between microrocket and acidic medium, i.e., liquid–air interface (interface 1) and liquid–asperity interface (interface 2), as shown in Fig. 5(c). Such heterogeneous wetting regime can be described using the Cassie contact model.³⁹ The air film discussed in Cassie contact model gives rise to a slip velocity at the vicinity of the interface 1, shown in Fig. 5(d). Navier presented that the slip boundary condition can be expressed as:⁴⁰where represents the fluid shear rate, b is the height of asperity, L_S refers to sliding coefficient or slip length, and u_c represents the slip velocity. Slip velocity is proportional to the fluid shear rate on the solid surface in accordance with eqn (1). The presence of slip length is ascribed to the dissolved air film at the interface of solid and liquid.⁴¹

Fig. 5 Mechanism of the drag reduction of the PANI/Zn microrockets with biomimetic lotus-leaf-like surface: (a) schematic of the motion of microrocket in an acidic media, (b) schematic of the contact model at the interface of microrocket and acid medium, (c) equivalent Cassie contact model for a microrocket moving in acid medium and (d) sliding velocity at the contact interface of microrocket and acid medium.

If the outer surface of microrocket is smooth, the contact interface is only constructed by solid (microrocket) surface and liquid (acidic medium), and hence, the slip length is zero (L_S = 0), as shown in Fig. 5(d). Thus, the slip velocity u_c is zero based on eqn (1). On the contrary, the slip length is a non-zero number (L_S ≠ 0) if there is an air film existed at the interface of solid (microrocket) and liquid (acidic medium). The presence of slip velocity leads to a lower velocity gradient at the interface of air film and liquid. As reported by Zhang et al.,⁴¹ velocity gradient is proportional to fluid drag force. This can be understood by Newton's model, which is expressed as:⁴²

where F is the drag force, λ represents the dynamic viscosity of the fluid, S is the wetting area of the substrate, and is the velocity gradient in the fluid.

As can be seen from eqn (2), the drag force on the surface of microrocket is proportional to both wetting area and velocity gradient. The solid/liquid interface of microrocket and acidic medium is replaced by the liquid/air/solid interface (shown in Fig. 5) because of the rougher microrocket surface after surface modification. The wetting area (real contact area) at the interface decreases apparently because of surface roughness after self-assembly. In addition, the presence of slip velocity produces a lower velocity gradient at the interface of air film and liquid in accordance with eqn (1). Therefore, the drag force on the surface of microrocket decreases with the decrease of both wetting area and velocity gradient at the interface of microrocket and acidic medium according to the Navier's and Newton's model.^41,42

In accordance with the aforementioned theoretical analysis, the drag-reduction and acceleration of microrocket with biomimetic surface can be described by Cassie contact model, Navier's slip boundary theory and Newton's model. As discussed in ref. 41, the existence of air film, which is caused by rough microstructure and LSE layer, contributes to the presence of slip length and slip velocity, thereby decreases the drag force at the interface of microrocket and acidic medium. Consequently, it causes the acceleration of microrocket with biomimetic surface.

3.4 Velocity characterization

To investigate the acceleration of microrockets with biomimetic lotus-leaf-like outer surface, the motion of microrocket is recorded using an optical microscope. Fig. 6(a) and (b) display the trajectory of microrocket moving in acidic medium before and after being self-assembled with LSE 1-dodecanethiol layer, respectively. The trajectory of microrocket is presented over one period of 1 s at an interval of 0.25 s. As shown in Fig. 6, the microrocket coated with LSE material shows a larger distance at the same interval. It illustrates that the microrocket moves faster after self-assembly. This can also be observed from the videos in ESI.†

Fig. 6 The time-lapse images of biomimetic PANI–Zn microrockets (3 μm) before and after being self-assembled with LSE 1-dodecanethiol layer moving in an acid medium, respectively, at 7.0 s, 7.25 s, 7.5 s, 7.75 s and 8.0 s.

Fig. 7 presents the average velocity of microrocket with biomimetic surface as a function of the concentration of H₂SO₄ before and after being self-assembled with LSE hydrophobic layer. As can be seen, a similar trend for the relationship of velocity and pH value is observed for both microrockets with and without LSE hydrophobic layer, i.e., the velocity of microrocket decreases with an decrease of the concentration of H₂SO₄ which is expressed in terms of pH value. However, the microrocket self-assembled with LSE 1-dodecanethiol molecular layer shows a larger velocity than that without hydrophobic layer for the same pH value. This is because the conditions of Cassie contact model are satisfied due to the presence of LSE 1-dodecanethiol layer along with rough surface, as shown in Fig. 5. The results in Fig. 6 and 7 can be used to experimentally explain the acceleration of microrocket with biomimetic lotus-leaf-like surface. Consequently, the rough structure along with the LSE coating offers a significant drag reduction and allows for the acceleration of microrockets. The performance of accelerated microrockets with biomimetic surface presented in this work is quite similar to the model for a ship moving in a sink, provided by Dong et al.,²⁹ in which the velocity has been improved through a super-hydrophobic coating.

Fig. 7 Dependence of velocity of microrocket upon the H₂SO₄ concentration before and after self-assembly. Black and red curves represent the bioinspired microrockets before and after being assembled with 1-dodecanethiol layer, respectively.

4. Conclusion

A biomimetic method was employed to accelerate the velocity and thereby to improve its propulsion efficiency of microrockets. The lotus-leaf-like rough microstructure and LSE layer were constructed at the outer surface of microrocket through electrodeposition method along with self-assembly technique. The accelerated microrocket with biomimetic lotus-leaf-like surface was investigated by analyzing the morphology, the elemental composition and the surface energy of out layer, and the velocity of microrockets. As we found, the microrocket prepared by electrodeposition show a rough microstructure similar to the surface of lotus leaf. The presence of LSE 1-dodecanethiol layer on the microrocket after self-assembly was identified. The increase of static contact angle, small sliding angle and the decrease of surface adhesion force illustrate that the microrocket with biomimetic outer surface shows a low surface energy which results in the decrease of drag force and thereby accelerates the microrocket. The Cassie contact model was found to be appropriate to describe the interface of microrocket and acidic medium in the context of the presence of rough microstructure and LSE 1-dodecanethiol layer. The average velocity of microrocket is increased after being self-assembled with lotus-leaf-like surface. Such accelerated microrockets hold great promise for improving the propulsion efficiency of microrockets in strongly acidic environments such as human gastric juice.⁴³ It should be noted that the microrocket with biomimetic hydrophobic surface in this work has not reached the superhydrophobic state of lotus since the contact angle of the PANI film after self-assembly is 145°. However, even the modified surface of microrocket has not reached to the superhydrophobic property of lotus leaf, the velocity improvement is apparently. If a microrocket with a superhydrophobic surface is fabricated, we believe that the velocity and propulsion efficiency will be further improved.

Acknowledgements

The work was financially supported by the Foundation for Innovative Research Groups of the Natural Science Foundation of China (51521003 and 51175129). We would like to thank the Special Financial Grant from the China Postdoctoral Science Foundation (2012T50339). We also gratefully acknowledge the financial support provided by the Program of Introducing Talents of Discipline to Universities (Grant No. B07018) and the Self-Planned Task (No. SKLRS201607C) of State Key Laboratory of Robotics and System (HIT).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17066h

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