Chinese Brushes: from Controllable Liquid Manipulation to Template-Free Printing Microlines

Nano Research 1 DOINano 10.1007/s12274Res -014-0699-1

Chinese brushes: from controllable liquid manipulation to template-free printing microlines

Qianbin Wang1, Qingan Meng1, Huan Liu1 (), and Lei Jiang1,2

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0699-1 http://www.thenanoresearch.com on December 16 2014

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Chinese Brushes: From Controllable Liquid Manipulation to Template-Free Printing Microlines

Qianbin Wang1, Qingan Meng1, Huan Liu1*, and Lei Jiang1,2

1 Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry and Environment, Beihang University, Beijing, 100191, P. R. China. 2 Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China.

In this mini-review, we discussed that the essence of the Chinese brush

in controllable liquid transfer lie in the anisotropic multi-scale Page Numbers. structural feature of the fresh emergent hairs. Drawing inspirations, its applications in controllable liquid pumping, highly efficient liquid transfer and template-free printing microlines were addressed respectively.

1 Nano Res DOI (automatically inserted by the publisher) Review Article

Chinese Brushes: From Controllable Liquid Manipulation to Template-Free Printing Microlines

Qianbin Wang1, Qingan Meng1, Huan Liu1 (), and Lei Jiang1,2

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT As a traditional writing instrument for calligraphy and painting, the Chinese brush enjoys a high reputation over the last 5000 years due to its distinguishable ability in controllable handling ink liquid, which therefore be widely used to deposit ink into certain characters or figures during culture communication. In this mini-review, we firstly discussed that the essence of the Chinese brush in controllable liquid transfer lie in the anisotropic multi-scale structural feature of the fresh emergent hairs. Then, drawing inspirations, its applications in controllable liquid pumping, highly efficient liquid transfer and template-free printing microlines were addressed respectively. We envision that the fundamental of Chinese brushes and its applications in liquid manipulation mentioned in this review may also be extended to other liquid phase functional material systems.

KEYWORDS Chinese brushes, dynamic wetting, multi-scale structures, liquid transfer, direct printing

1. Introduction especially those in microscopic scale. So far, attempts on the topic of wetting on fibers have been mostly As an important writing tool, the Chinese brush focused in the area of efficient water collection, the plays a crucial role in the process of the oriental directional water transport, and self-assembly into civilization development over the past five thousand micro-patterns on both natural and synthetic fibers, years [1]. The distinguishable feature of the Chinese as has been reviewed recently [4-5]. However, brush over other writing tools is that it enables manipulating liquid in more sophisticated and controllable transfer of low viscosity ink liquid onto well-controlled way by structured conical fibers papers or other substrates [2-3]. Therefore, remains challenge, and the Chinese brush offers a understanding the mechanism of liquid transfer of solution in this aspect. the Chinese brush is not only scientifically important This mini-review is focused on the fundamental for the deep knowing the fundamental of wetting in of controllable liquid transfer of the Chinese brush, open fibrous systems, but also practically and its applications in liquid manipulation ranging meaningful for manipulate liquid in versatile fields, from the liquid pumping, highly efficient liquid ———————————— Address correspondence to Huan Liu, [email protected]

2 transfer to template-free printing of microlines. was described, followed by the mechanism analysis. There are four main sections. First, the importancy of The third section presents the applications of the Chinese brush as a writing tool was briefly Chinese brushes inspired devices in liquid introduced, as well as its basic physicochemical manipulation and template-free printing. In the last nature. Then, the representative behavior of the section, we make a conclusion and present our own Chinese Brush in controllable liquid manipulation thoughts about the future development of this area.

Figure 1 The fundamental of the controllable liquid transfer of the Chinese brush and its applications in liquid manipulation and template-free printing. (a) The Chinese brush is a traditional writing tool for calligraphy and painting, and often used to deposit ink onto certain substrates as various characters and figures, which then as a carrier to deliver emotions or convey knowledge from one generation to another. Recreation from internet

3 raw materials, and the horse was painted by Xu Beihong. (b) The optical picture of the freshly emergent hair (FE-hair) and its schematic illustration reveal the unique anisotropic microstructures featured by tapered architecture with oriented squamae. (c) When drop was released on the surface of FE-hairs, driving force Fl arising from Laplace pressure difference, gravitational force Fg, and the asymmetric retention force Fa arising from anisotropic arranged squamae would working on it. And the conical feature of FE-hairs can induce a Laplace pressure difference to propel the droplet move to the low curvature area. (d) The oriented squamae may generate a retention force to control the liquid unidirectional spreading or moving along the oriented squamae [2]. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e-g) the applications of the Chinese brush inspired device in various fields: (e) Controllable liquid pumping. Left: Silicon oil droplet would present self-propelled movement behavior and be balanced at the same position for a fixed volume no matter where it was released on the surface of FE-hairs array. Right: The balance position show linearly inversely proportional to Ω1/3 (Ω is the drop volume) [2]; (f) Highly efficient liquid transfer. Left: The design of the bio-inspired highly efficient liquid transfer system. When a drop was hold at the tip region, the bio-inspired multi-scale structured conical fiber exhibit highest efficiency in liquid manipulation. Right: Change the apex angles and tilt angles could significantly alter the maximum liquid capacity of the bio-inspired conical fibers, and the maximum liquid capacity is 428 times greater than its own volume was realized [6]. Copyright 2014 American Chemical Society. (g) Template-free printing of microlines. Left: Schematic illustration of template-free printing device with two parallel FE-hairs and an artificial liquid reservoir. Right: the microlines generated by direct writing ink liquid (surface tension of 45 mN/m) on glass surface by using the model bio-inspired device, exhibiting tunable width range from 10 μm to ca. 130 μm [2].

2. General knowledge of Chinese brushes hairs of rabbit, under the inspiration that the tail of an injured rabbit could leave a long and continuous Among all the writing tools, the Chinese brush, alias blood trace behind. Since then, Chinese brushes have ink brush or writing brush, is a unique writing been used as a main tool to record people’s thinking instrument that is peculiar to China. It is titled the and feeling by various characters and figures, which most ancient one among Four Treasures of Chinese facilitate the culture spreading and transmission traditional Study [1]. As an important writing from one generation to another. instrument for calligraphy and painting, the Chinese Normally, qualified Chinese brushes are made of brush have played a distinctive and irreplaceable a bundle of quasi-parallel freshly emergent hairs role in the culture exchange and heritage for its (FE-hairs) of animals such as hare, goat, horse, rabbit, ability in deposit liquid ink onto papers in various mouse, wolf, fox, gorilla, swam, pig, badger, or even patterns and characters [3]. By using brush in cursive the virgin hairs of human babies. Each type of hair and fast way with varying posture, speed and has a different elasticity or even ink capacity, which pressure, eastern artists renders each ink stroke in a in turn results in a different appearance of ink stroke. continuous and harmonious rhythm to deliver their The most notable feature of the Chinese brush is its emotions or convey knowledge [1, 7], as shown in ability in large-mass ink loading and the continuous Figure 1a. Thanks to the Chinese brush, Chinese and uniform ink delivery onto the paper with culture have spread to neighboring countries and well-controlled manner, despite different brushes are then to all over the world. used for different styles of calligraphy and writing. Been a traditional tool for calligraphy and Worth noting is that only the FE-hairs render the painting, the Chinese brush has been existed more Chinese brush the excellent ability in liquid than five thousand years. The archaeological manipulation, which will be addressed in detail in discoveries demonstrated that the inscriptions on this mini-review. painted pottery and shell-and-bone writing indicated the use of Chinese brushes in the Neolithic Age [7]. 3. Large-mass ink holding by Chinese Today’s brushes, as has been documented [8], were developed in Qin Dynasty (221-206 BC) by using the brushes

4 The first step of traditional Chinese calligraphy or Laplace pressure that acts on it is much larger than painting is dipping the Chinese brush into the that acts on the top region (low curvature site), low-viscosity ink, by which step the ink was because the local curvature variation at the tip region transferred into the brush. Upon this ink loading is much bigger than that at the top region [2, 13]. operation, the naturally dispersed FE-hairs in the Thus, the P can generate a large force to propel the brush would re-shape into a bundle with a tapered drop move upwards apart from the tip, until it was topological architecture under the interaction balanced by the anisotropic force that generated between the capillary forces and the elasticity of the from the oriented squamae and the gravity. hairs [9-10], and presents an obvious expanded Another possible force favorable for the liquid appearance on a certain position. We quantitatively transfer of the Chinese brush is assignable to the demonstrated that over 2 times greater than its own unique oriented squamae of the FE-hairs [14-15], weight of ink could be stored within the spaces which could generate anisotropic wetting behavior between FE-hairs in the Chinese brush [2], which [16-17]. For a drop on the asymmetric structures of renders the brush an adaptable “ink reservoir”. To oriented squamae,2 and 1, the anisotropic better visualize the ink loading process, we retention force arising from the oriented squamae constructed a two parallel hairs model system to will induce a special retention force differential explore the liquid transfer behavior of the Chinese (Figure 1d) could be express as [18], brush. When a silicon oil droplet (surface tension of 1 Fa  f d  f u 2 w sin[  r00   a ]  20 mN m–1) was released on the surface of two 2 parallel hairs, it could exhibit different self-propelled 11 (2) [sin(21   )  sin(     )] movement behaviors along the FE-hairs at different 22 regions. And for a fixed volume of droplet, it would where is the solid–liquid contact width, θa and θr is always stop and be balanced at the same position the apparent advancing and receding contact angles,  a0 r0 despite the place it was released. This behavior was and   ar00   (θ and θ are the true advancing also shared by other particles-based suspensions, for and receding contact angles). When 2 > 1, the example, ink liquid with surface tension = 45 mN retention force differential is Fa > 0, which can m–1. [2] therefore generate an anisotropic hysteresis, leading Why the silicon oil droplet can be dynamically to the liquid unidirectional spreading or moving balanced within the two parallel hairs system? The along the oriented squamae [19-20]. mechanical analysis revealed that the unique The cooperative effect of P that aroused from anisotropic multi-scale structures of the FE-hairs, the conical feature of the FE-hairs, the asymmetrical featured by tapered architecture with oriented retention force that generated by the oriented squamae (Figure 1b), is the key [2]. It is well known squamae and the gravity make large mass of liquid that for a droplet held at a conical fiber, the curvature dynamically balanced within the brush, empowering gradient of a conical fiber would arouse a Laplace the Chinese brush the unique liquid controlling and pressure difference (P), driving the drop move from holding capacity, and consequently its versatile the tip to the top of the conical fiber [2, 11-13], as applications in controllable manipulating functional presented in Figure 1c. The Force arise from the liquid phase materials. Laplace pressure difference can be obtained by integrating P times the area around the entire liquid 4 Applications of the Chinese brush and surface, express as, bio-inspired devices 4 tan (1) Fl ~ 2  ()rRf  0 Differ from the other reported fibers systems where where  is the surface tension of the liquid,  is the the driving force normally was one-directional, the half apex angle of the conical fiber, rf is the radius of Chinese brush offer a multiple driving forces to the fiber, R0 is maximum radius of the drop, and  is make possible the liquid could be continuously the volume of the liquid. For a droplet on the tip stabilized/hold within it. It thus provides solutions in region (high curvature site) of the conical fiber, the controllable manipulating liquid in various ways.

5 4.1 Controllable liquid pumping dependence on the volume of liquid, presenting a well-controlled manner. Although liquid pumping Directional driving of liquid droplets on solid by fibrous media have been recently suggested as a surfaces has attracted much attention in many fields, feasible strategy in applications as liquid transport such as microfluidic devices [21-23] and lab-on-chip [43], water collection [5, 11, 13, 44-45] and water–oil architectures [24-26]. Until now, great progress has separation [46], none of them proceed in a been made to control the motion of a liquid droplet controllable manner. The controllable liquid on a solid surface by introducing energy gradients pumping on the FE-hairs give our new insight in such as chemical [27-28], thermal [29-30], shape transporting liquid with controllable direction, [31-32] or wetability gradients on surfaces [33-34] or location and even speed. using external driving forces [35-36] such as gravity, electric or magnetic fields. Besides, surfaces with 4.2 Highly Efficient Liquid Transfer anisotropic chemical, mechanical, physical, Highly efficient liquid transfer is significantly morphological, and topological properties have been important in chemical reactions [47-48], printing or engineered for transporting drops [37-39], and may patterning [49-52] and biological assays [53-54]. So offer new approaches to propel liquid droplets in the far, many bio-inspired artificial liquid transfer demand direction. For example, we recently reported systems were developed, and could be divided into that spider silks with unique periodic knot/joints and three main categories. The first is the drop on the cacti with conical spines are capable of collecting tiny functional surface. For example, superhydrophobic water droplets from humid air. [11, 13] We also surfaces with high adhesion could provide particular demonstrated that water droplets could move in a advantages in lossless liquid droplet transporting. controllable demand direction under the action of [55]. The second is drop in the enclosed channel surface roughness gradient or stimuli-responsive system. Such as, a synthetic microfluidic system wettability. [40-42] could realize water transport at large negative However, the directional liquid motion via an pressures, which provide an approach for open fibrous media, especially those with anisotropic technological uses of water under tension [56]. The arranged microstructures, is rarely reported so far. last is the drop on the open fibrous system. For Chinese brushes here provide one alternative instance, bio-inspired thin fibers with unique spindle solution. We recently demonstrated that the FE-hair knot and the joint structures could hang a large enables liquid pumping, even in a controllable volume of water droplet between spindle knots [45]. manner. [2], as shown in Figure 1e. It is illustrated Although dynamic wetting in these systems offers that silicon oil droplets would present benefits in many practical applications, the efficiency position-dependent self-propelled movement when of liquid transfer system is far away from the released at the surface of parallel FE-hairs. For detail, requirements of engineering applications. the drop would spontaneously move from the tip to The great ink loading ability of the Chinese brush the region of larger radius when released at the tip of allows us to design and fabricate bio-inspired the hairs since the conicity of the tip region is the materials that could be applied in highly efficient most remarkable. While when released at the top of liquid transfer [2, 6], as presented in Figure 1f. the conical structure, the liquid would automatically Recently, drawing inspiration from the controllable move downward to the region of higher curvature, liquid transfer in ratchet conical hairs, a dynamic because the local conicity is too small to generate electrochemical method was develop to fabricate an enough driven force to overcome the retention force anisotropic multiscale structured conical copper wire that acted oppositely. Most importantly, all the (SCCW) with controllable conicity and surface droplets would stop and be balanced at certain morphology that mimic the structural feature of position where the P was balanced by the gravity FE-hairs in Chinese brushes. The conicity of the and anisotropic retention force. We also copper wire was controlled by length of the wire demonstrated that the balance position of droplets immersed into the electrolyte, while the surface on the surface of the FE-hairs would show significant morphology could be accurately controlled by

6 reciprocating the working electrode at a small reservoir was equipped and connected to the parallel velocity (v < 0.02 mm/s). The SCCW exhibit the FE-hairs. When such model device was contacted similar behavior with that of the Chinese brush: the with the high adhesive substrate, such as paper, glass water droplet with fixed volume could be balanced slide and silicon wafer, liquid materials in the at certain position dynamically under the reservoir could steadily, uniformly and continuously cooperative effect of P, the asymmetrical retention transferred onto the substrate, leaving a microline on force, and the gravity. Here, due to the physical the surface of substrate. This direct printing device rigidity of the SCCW, the effect of the way it interacts enables preparing 1D microlines with well-defined with liquid on the liquid amount could be profile on various smooth substrates through direct investigated. It is found that the highly efficient writing liquid phase materials. To be noticed, the liquid manipulation ability was realized by using the resolution could be down to 10 m. This bio-inspired boundary condition of the dynamic liquid balance direct printing device provides a new simple behavior at the tip region of the SCCW. When alternative for preparation microlines that could be cooperatively controlling the tilt angle and apex applied for easy printing micro-patterned optical, angle of the SCCW, a large mass of liquid can be electric, as well as sensing devices. We envision this manipulated in a well controllable manner, and over technique is applicable for versatile functional liquid 428 times greater than its own volume of liquid phase material with inherit or better performances. could be operated, presenting a highly efficient liquid transfer system. This bio-inspired SCCW 5. Conclusion and perspectives could provide new insight in designing novel materials and devices to manipulate liquid in a more As one of the most ancient writing tools, the Chinese controllable way and with high efficiency. brush provides a powerful and efficient approach to control liquid transfer. By analyzing the 4.3 Template-Free Printing Microlines structure-function relationship in the Chinese brushes, we conclude that the cooperative effect of As a maskless, non-lithographic route, direct-write P that aroused from conical architecture, the technologies offer the ability to rapidly fabricate asymmetrical retention force arising from oriented one-dimensional (1D) structured materials for micro-meter scaled squamae and the gravity is photonic [57-60], electrical [61-62] and biomaterial responsible for the controllable liquid manipulation. applications [63]. Different from traditional Understanding the mechanism of the hairs structures approaches, such as ink-jet printing, screen printing in ink holding of the Chinese brush system inspired or micro-contact printing, direct-writing technologies us to develop novel bio-inspired devices that could required neither costly and complicated equipment manipulate liquid materials, especially in microscale, nor complex fabrication processes [64-66], which as liquid pumping for transporting liquid with would therefore be beneficial for direct patterning controllable direction, location and speed; high 1D structured materials with a low cost and high efficiency liquid transfer for chemical reactions or yield. For example, Lewis et al used rollerball pens biological assays; and template-free printing for easy with conductive silver inks to direct print electrodes printing micro-patterned optical, electric, as well as on paper, offering a low-cost, portable fabrication sensing devices. route for printed electronic and optoelectronic Utilizing natural product of the animal hair to devices [61]. Mirkin et al fabricated a polymer pen record the thinking and living experiences onto some lithography, which could be employed to direct write medium, is always beneficial for the development of protein nanoarrays in a low-cost and civilization in the human history, although the high-throughput manner [67]. mechanism has far from being discussed. Realizing Recently, inspired by the fascinating ability of such ancient wisdom is of paramount importance FE-hairs in manipulating low viscosity liquid, a facile even to modern science and technology, since it direct printing device with two parallel FE-hairs was provides versatile promising applications in field of designed and fabricated, as presented in Figure 1g the modern science. Here, we envision that the [2]. To supply a continual liquid, a man-made liquid

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