Photopatterning via Photofluidization of Azobenzene Polymers

In current photo-based patterning techniques, an image is projected onto a photosensitive material to generate a pattern in the area where the light is focused. Thus, the size, shape, and periodicity of the pattern are determined by the features on the photomask or projected images, and the materials themselves generally do not play an active role in changing the features. In contrast, azobenzene polymers offer a unique type of photopatterning platform, where photoisomerization of the azobenzene groups can induce substantial material movements at the molecular, micro-, and macroscales. Stable surface relief patterns can be generated by exposure to interference light beams. Thus, periodic nanoand microstructures can be fabricated with both twoand three-dimensional spatial control over a large area in a remarkably simple way. Polarized light can be used to guide the flow of solid azobenzene polymers along the direction of light polarization via an unusual solid-to-liquid transition, allowing for the fabrication of complex structures using light. This review summarizes the recent progress in advanced manufacturing using azobenzene polymers. This includes a brief introduction of the intriguing optical behaviors of azobenzene polymers, followed by discussions of the recent developments and successful applications of azobenzene polymers, especially in microand nanofabrication.


Introduction
The manufacturing of micro-and nanoscale patterns has broad applications in optics, 1 biology, 2 and microelectronics. 3 These patterns can be achieved using top-down approaches, such as photolithography 4 and imprinting 5 or bottom-up approaches, such as the growth of nanowires 6 or self-assembly of block copolymers. 7 Laser-based techniques have recently emerged owing ACCEPTED ARTICLE PREVIEW to their flexibility for the rapid manufacturing of arbitrary structures with precise control of the laser beam on photoresponsive materials, which comprise macromolecules that undergo physical and chemical changes upon light irradiation at a specific wavelength. These changes lead to large-scale motion or macroscopic movements within the material system. 8,9 Photoresponsive materials have been widely studied in many fields, including photo-optical media, 9 photoswitches, 10 photomechanical systems, 11 micro-and nano-patterning, 12 and nonlinear optical media. 13,14 Photoresponsive materials are typically prepared by incorporating photoresponsive chromophores, such as azobenzene, 15 stilbene, 16 spiropyran, 17 spirooxazines, 18 and fulgides, 19 into the main chain or side chains of the polymer.
Photoresponsive materials based on azobenzene and its derivatives are broadly termed azobenzene polymers, and these materials have attracted considerable attention owing to their unique optical responses. 20 Specifically, azobenzene polymers exhibit photo-isomerization involving molecular structure changes under specific wavelengths, thereby altering the intrinsic physical and chemical properties of the material. The most prominent change is anisotropic fluid flow, in which the polymer chains move in a direction parallel to the polarization direction of the irradiated light. 21 Once the light irradiation is removed, the mobile azobenzene polymers immediately solidify. Thus, photofluidization has been exploited in micro-and nano-patterning, 22 nanophotonics, 23 electrics and electronics, 24 and environmental and energy applications. 25 Nevertheless, the exact mechanism of this unique photofluidic movement remains unclear. 10,11,14,[21][22][23] This review provides a brief overview of azobenzene polymers with a focus on the effects of photofluidic movement. Recent advances in this field are discussed, and a few examples of applications of azobenzene polymers in photonics, micro-and nano-structuring, and surface engineering are presented.

Molecular motion of azobenzene groups in response to light
Azobenzene groups undergo reversible photoisomerization from the stable trans-form to the metastable cis-form when irradiated with visible or UV light (Fig. 1a). Subsequent irradiation of the cis-form by visible light or the use of thermal relaxation leads to isomerization back to the trans-form. 26 Notably, the trans-form selectively absorbs polarized light along the polarization direction, and the absorbance depends on the angle between the transition moments and the light polarization direction. The trans-isomer with transition moments perpendicular to the polarization direction is not activated. In contrast, the cis-isomer does not exhibit selective absorbance to the polarization direction of light. Therefore, when exposed to visible or UV light, transto cisand cisto transisomerization occur simultaneously in azobenzene molecules owing to the overlap in their optical absorption spectra. 27 Combining the polarization-selective transto cisisomerization with the unselective cisto transreversible isomerization, the azobenzene groups will gradually reorient to a plane orthogonal to the polarization direction, which is known as photo-induced alignment (Weigert effect), as shown in Fig. 1b. 28 Isomerization involves substantial changes in the geometric size and dipole moment of the molecules. For example, the distances between the para-carbons in transand cis-azobenzene are 9 and 5.5 Å (Fig. 1a), 26,29 respectively, while the dipole moments are 0 and 3.1 D, respectively. 30 At the molecular scale, these differences are significant. The molecular motion resulting from the gradual increase in the dichroic ratio (i.e., the ratio of the absorbance in the parallel direction to that in the perpendicular direction) 31 leads to rearrangement of the randomly arranged groups within the polymer matrix such that they become perpendicular to the polarization direction of the light. 24 This can be attributed to the fact that azobenzene groups preferentially absorb polarized light in the parallel direction; as the groups rearrange in the parallel direction, those perpendicular to the polarization direction begin to accumulate (Fig.   1b). 24

Photofluidic movement of azobenzene polymers
Pseudo-stilbene azobenzene polymers are characterized by swapped electron-donating and electron-withdrawing groups within the azobenzene ring. This leads to an overlap in the ranges of wavelengths that are absorbed by the transand cisisomers. 32 Therefore, transto cisisomerization and cisto transisomerization can occur simultaneously, resulting in continuous trans-cis-trans isomerization cycling during irradiation. These azobenzene polymers liquefy and soften owing to the vibrations caused by cyclic isomerization. This mechanism is similar to how heat energy causes rapid and random vibrations in solid-state polymers, which decreases the long-range order and structural symmetry to ultimately liquefy the solid-state polymer. 33 Several models have been proposed to describe this phenomenon, including a thermal model, 34 pressure gradient force model, 35 mean-field model, 36 optical field gradient force model, 37,38 and asymmetric diffusion model. 39,40 However, none of these models can completely explain the various liquefying phenomena, particularly those related to the large increase in free volume due to the cisand transisomer size difference (Fig. 1a) and the ACCEPTED ARTICLE PREVIEW rapidly repeating isomerization (on the order of picoseconds) (Fig. 2a). 41 Light irradiation leads to the disappearance of the elastic region in the load-penetration curves of azobenzene polymer films in the absence of irradiation (Fig. 2b), 21 while angular azobenzene polymer pillars become rounded. 42 Both changes demonstrate that azobenzene polymers fluidize under light irradiation (Fig. 2c). We note that this fluidization differs from liquefaction due to heating, as photofluidization occurs near room temperature, which is below the glass transition temperature (Tg) of the polymer. 43 Photofluidization is based on the bulk isomerization of azobenzene groups; thus, it is often referred to as athermal photofluidization. 43 (pDR1m) (Fig. 3a). 44 Novel azobenzene-containing materials have been developed to extend the range of photofluidization, impart new functions, and widen their applications, including (i) azobenzene molecular glasses (Fig. 3b), 52 (ii) a supramolecular approach in which the pendent groups of azobenzene molecules are non-covalently attached to an inorganic precursor ( Fig. 3c), 53 and (iii) grafting of azobenzene onto protein-based materials (i.e., azobenzenefunctionalized native protein) (Fig. 3d). 54 Unlike typical heat-induced fluidization, the photofluidic movement of azobenzene polymers is characterized by movement parallel to the light polarization direction (Fig. 4a).
For example, an azobenzene polymer film marked with the letter 'X' and subjected to irradiation under linearly polarized light would begin to flow parallel to the polarization direction. Thus, only the sections of the 'X' mark perpendicular to the polarization direction would be filled (Fig. 4a). 21 This is the most prominent characteristic of the photofluidic movement of azobenzene polymers, and it can be used to control the movement of azobenzene polymers based on the polarization direction. For example, the irradiation of azobenzene polymer pillars with light polarized in various linear or circular directions allows for control over the direction and form of the azobenzene polymer movement (Fig. 4b). 24 This unique fluidization phenomenon can be attributed to the rapid transto cisisomerization, which introduces a large free volume in the polymer 41 ( Fig. 1a and Fig. 2a) and an anisotropic arrangement of the azobenzene groups only in the direction parallel to the polarized light 21 (Fig.   1b).
As azobenzene polymers undergo an immediate phase transition upon light irradiation, both ACCEPTED ARTICLE PREVIEW the magnitude and duration of their movement can be readily controlled by adjusting the light irradiation time and intensity. As shown in Fig. 4c, azobenzene polymer micropillars with a circular cross-section (2 μm in diameter) become ellipsoidal when exposed to linearly polarized light in a specific direction; the aspect ratio of the ellipsoidal cross-section continues to increase with the increase in irradiation time. The fluidization movement speed also increases with increasing irradiation intensity (Fig. 4d).

Micro/nano-structuring and recent applications
This section presents applications of the photofluidic phenomena in both fundamental and technologically oriented studies considering the general rules governing the macroscopic photofluidic movement of azobenzene polymers.

Surface relief gratings on azobenzene polymer films
The irradiation of a thin azobenzene polymer film (typically less than 1 μm thick) with a sinusoidal intensity light pattern leads to the formation of sinusoidally modulated surface relief gratings (SRGs) (Fig. 5a). 42,55 SRGs with large modulation amplitudes are unique to azobenzene polymer systems. This process differs from other conventional light-based microand nanopatterning techniques, such as laser ablation and chemical etching. Specifically, lightinduced SRG formation is a facile one-step procedure, and the modulation amplitude can be precisely controlled by adjusting the irradiation time and polarization states of the irradiating beams. SRGs can be formed at low light intensities (several mW•cm -2 ), which indicates that they are not formed by ablative or destructive processes. Instead, SRGs are known to result from the fluidic movement of azobenzene polymers owing to their high sensitivity to the polarization distribution of the interference light. 51,[56][57][58] The spatial distribution of the polarization state along the grating vector of interference light is presented in Fig. 5b, 56 while the topological evolution owing to various types of polarization combinations can be visualized using atomic force microscopy (AFM), as shown in Fig. 5c. 56 The surface relief maxima and minima of the interference pattern are clearly observed at positions with vertically (↕) and horizontally (↔) oscillating polarizations, respectively. This confirms that the formation of SRGs occurs during the polarization movement of azobenzene polymers.
A typical experimental setup for the preparation of SRGs is presented in Fig. 5d and 5e. 51,59 Laser beams with wavelengths of either 488 or 532 nm are spatially filtered, collimated, and split into two beams of equal intensity. The two beams are reflected by mirrors and recombined

ACCEPTED ARTICLE PREVIEW
to form an interference pattern at the recording medium plane. The two interfering lights independently pass through half-wave (λ/2) plates to control the linear polarization state of the recording beams. Interference light comprising different polarization states can be achieved by controlling these half-wave plates, while circularly polarized beams can be obtained by replacing the half-wave plates with quarter-wave (λ/4) plates. SRGs can be formed by exposing an azobenzene polymer film to an interference pattern of appropriately polarized beams, where the angle (θ) between the interfering beams can be adjusted to obtain the desired grating periodicity (D, see Fig. 5f) 60 according to Bragg's law: Most studies use recording beam intensities of 5-100 mW•cm -2 to avoid sample heating and spurious photothermal effects. 42,51 SRGs with tetragonal, hexagonal, or even more complex symmetries can be generated based on two or multiple sequential inscriptions of simple SRGs in different directions. 61,62 For example, tetragonally arranged SRGs can be obtained by superimposing lines of SRGs by rotating the sample 90° between two sequential exposure steps under a fixed interference light ( Fig. 6a). Increasing the number of sequential exposures and controlling the sample rotation angle at each stage allows for the design of rotationally symmetric complex surface textures over large areas (Fig. 6b-c). Furthermore, amplitudes of several hundreds of nanometers and periodicities ranging from hundreds of nanometers to several micrometers can be achieved. 61 The formation of SRGs offers the potential for cost-effective all-optically driven surface structuration. The periodicity of SRGs can be controlled by adjusting the irradiation wavelength and incidence angle of the collimated beams ( Fig. 5f). 59 Thus, the inscribed surface modulation may be tuned. In addition, SRG patterns can be reprogrammed by erasing and rewriting the SRG with another structure, regardless of the degree of geometric complexity ( Fig. 6d). 21,55,59,62 This is possible owing to the reversible nature of the photofluidic motion of the azobenzene polymers. 63 This advantage is particularly relevant in the development of all-reversible optical-patterned surfaces, where the removal of inscribed superficial structures under irradiation with circularly polarized or unpolarized light is required. Thus, unwanted patterns can be selectively erased by simply shining a beam with a spatially structured intensity profile onto the film surface, and a new grating vector can be generated within specified areas of the film, e.g., by using a transmission mask. 59

Fabrication and applications of surface relief gratings
The periodic structures of SRGs can be patterned and reconfigured at will over areas of a few square centimeters with amplitudes up to 1 μm and periods ranging from hundreds of nanometers to several micrometers. 59 The patterned surfaces maintain their quality for at least several years under ordinary storage conditions. 64 These features of azobenzene polymers offer significant potential for use in micro-and nanopatterning 23, 59, 65-67 as well as for the fabrication of various photonic elements, such as diffraction gratings, 35, 68-70 microlens arrays, 71 photonic crystals, 72 distributed feedback lasers (DFB), 55 perfect absorbers, 73 plasmonic nanostructures, 74 and recently proposed optical metasurfaces and metamaterials. 59 Similar to photoresists, azobenzene polymers can also be used to make etching masks to create patterns, e.g., on a Si wafer via reactive ion etching (RIE) ( Fig. 7a-b). 65 Because SRGs are produced under ambient room light, they offer better tolerance to overexposure than traditional photoresists. The etching contrast of the Si wafer versus the photoresist can be increased by introducing a 5 nm-thick layer of Al2O3 and a 20 nm-thick amorphous Si layer beneath the azobenzene polymer layer. The Al2O3 layer acts as a hard mask, and the amorphous Si ensures tight adhesion between the azobenzene polymer layer and the substrate. Fig. 7a shows a multi-layered approach for pattern transfer, where the azobenzene polymer mask pattern is first transferred onto the amorphous Si via RIE, followed by wet etching of the Al2O3 layer and dry etching of the underlying Si substrate. An example of the resulting silicon nanocones is shown in Fig. 7b, which are promising for applications such as self-cleaning superhydrophobic or superoleophobic surfaces 75 and anti-reflective coatings. 76 Azobenzene polymer SRGs can also be used as etching templates (Fig. 7c-d). 77 A thin gold layer is electron-beam deposited on tetragonally aligned SRGs. Subsequently, the crests of the gold-coated grating are etched using ion-beam milling to form plasmonic arrays. The symmetry of the resulting structure can be controlled by adjusting the irradiation time of the first and second SRG inscriptions (Fig. 7d). The size of the structures is determined by the ion-beam milling time: a short milling time produces dielectric openings in the gold film, and a long milling time leads to the formation of discrete nano-sized gold islands.
Direct applications of azobenzene polymer SGRs in optical devices can be difficult, because problems such as photo-bleaching or undesired photofluidic movement can occur during device operation. Furthermore, azobenzene polymers are typically opaque in the ultraviolet (UV)/visible spectral regions. These limitations can be overcome by transferring textured SRGs onto an elastomeric polymer such as poly(dimethylsiloxane) (PDMS) via replica ACCEPTED ARTICLE PREVIEW molding. 78 PDMS-based replica molding of line-and dome-array SRGs has been reported to enhance the performance of photovoltaic devices (Fig. 8a-d). 79 Fig. 8a shows a technique for fabricating such an organic photovoltaic cell. An azobenzene polymer-based SRG is used as the master to create a PDMS stamp; an AFM image of the stamp is shown in Fig. 8b. Fig. 8c shows the fabrication process, which includes soft-contact imprinting of the SRG onto the active layer and subsequent thermal evaporation of calcium and aluminum onto the patterned surface. The absorption efficiency of photons propagating in the textured active layer is enhanced by imprinting the SRG texture onto the active layer, and the performance of the organic photovoltaic cells is improved owing to the substantial diffraction of incident sunlight by the textured surface (Fig. 8d). According to the literature, such structuring can increase the cell efficiency by approximately 15%. 79 Similarly, the performance of a solid-state DFB laser, 55 the light extraction efficiency of light emitting diodes (LED), 80 and other optoelectronic devices can be improved using SRG textures. 81,82 For example, Fig. 8e-g demonstrate that a simple 2-D hexagonally patterned SRG structure can be used to improve the performance of OLEDs. 80 Fig. 8e illustrates the fabrication of a hexagonally patterned SRG structure using sequential light irradiation. The AFM image in Fig. 8f confirms the successful formation of a hexagonal nanopattern with a pitch of 500 nm and a depth of 50 nm. For photovoltaic applications, the patterned azobenzene polymer film is replicated to create a master made from UV-transparent silicon rubber, which is further replicated with UV-curable epoxy onto the glass substrate, as shown in Fig. 8g. In Fig. 8h, hexagonal diffraction light patterns can be clearly observed in the azobenzene polymer/glass, free-standing silicon rubber, and UV-cured epoxy/glass during irradiation with a He-Cd laser beam (325 nm), indicating a high fidelity of replication. Finally, the extracted current efficiency of an electro-luminescent (EL) device with hexagonal patterns is measured and compared with that of an EL device without the 2-D grating, as shown in Fig. 8i. The EL device with the patterned SRG exhibits an extracted current efficiency that is almost twice that of the nonpatterned device. This is because the 2-D grating modifies the emitted light through Bragg diffraction into free space instead of trapping it in substrate-guided modes.
Line-shaped SRGs exhibit structural anisotropy, which leads to anisotropic surface properties such as wettability and adhesion (Fig. 9). 83,84 These properties are highly dependent on the spatial configuration of the superficial roughness. As shown in Fig. 9a, water droplets propagate parallel to the direction of the textured surface owing to the asymmetric elongation of the liquid contact line. Hence, the deposition of a liquid droplet on such an anisotropic surface results in asymmetric elongation of the liquid contact line, which is quantified by the anisotropy in the liquid contact angles measured along different directions of the substrate. Fig.   9b shows the side view of the droplet seen from the directions parallel (top) and perpendicular (bottom) to the SRG texture. D. Wu et al. investigated wetting anisotropy and its relation to the periodicity and height of the pattern (Fig. 9c-d). 84 In that study, no obvious differences were observed in the anisotropic wetting behavior with varying the periodicity of the pattern from 0.9 to 2.5 μm. Interestingly, the increase in the pattern height resulted in an increase in the water contact angle in the perpendicular direction, but no significant change was observed in the parallel direction. It is notable that the height and periodicity of SRGs prepared from azobenzene polymers can be easily tuned by varying the irradiation parameters (e.g., the incident angle, light irradiation time, or light intensity), 84-86 which is extremely difficult to achieve using other fabrication techniques. 87,88 Line-shaped SRGs can also be used in tissue engineering as extracellular matrices (ECM) to modulate living cells; 89,90 in ECM, the cell attachment, spreading, and differentiation are highly dependent on the surface topography. 91 The mass production potential of controlled large-scale micro-textured SRGs is particularly attractive for investigating cell responses to dynamically varying surface textures in response to external stimuli. The interactions between living cells and a flat surface (Fig. 10a), a line-shaped SRG surface (Fig. 10b), and an optically erased SRG surface (Fig. 10c) (Fig. 10e). Therefore, by writing and rewriting SRGs from azobenzene polymers, the cell alignment on the surface can be fine-  , Fig. 11a). This prospective 3D photoengraving strategy offers significant advantages over other strategies. Specifically, it allows for the deterministic inscription of distinct micro-and nanotextures with modulated heights (e.g., lines, domes, and complex quasi-textures) on every facet of the parent surface. This level of deterministic control over the morphology allows for the preparation of increasingly precise and complex hierarchical architectures. In addition, this strategy remains relatively simple and easy to access, despite offering a high degree of control and flexibility. This is due to the elimination of the wet etching and mechanical contact steps, the capability for large-area fabrication (approximately several square centimeters at a time), and the minimal incidence of defects. As shown in Fig. 11b-d, the 3D photoengraving process is applied to a square array of PDO 3 micropillars (height = 40 µm, aspect ratio (height/width) = 1, spacing = 100 µm), achieving heterogeneous integration of different types of micro-and nanotextured surfaces with different grating vectors and dimensionalities on each single pillar. This outcome is difficult to achieve using other nanoand microfabrication methods, such as irreversible chemical etching and self-assembly. Fig.   11b shows the low-magnification view; Fig. 11c and 11d show high-magnification views in the directions opposite to a single micropillar (viewing directions are marked in Fig. 11b). The benefits of this technique are further demonstrated by preparing a multiplexing structure-color filter with 2D metasurfaces of different periodicities (i.e., different grating colors) in a compact micropillar array (Fig. 11e-k). Reflected structural colors offer significant advantages over conventional pigment-based color filters in terms of brightness, robustness, and environmental friendliness. 92 However, the integration of diverse color filters with discrete wavelength selectivities in a single compact device is complex when traditional fabrication methods are used. This is demonstrated by performing 3D photoengraving of secondary line patterns with different periodicities (D = 588, 686, and 833 nm) on three facets of an inverse pyramid, followed by physical vapor deposition (PVD) of a 10-nm-thick surface layer of copper to enhance the reflectivity (Fig. 11e-f). The engraved specimen is illuminated with oblique white light at an incident angle of 54.7° to the target surface and normal to the engravings, leading to the appearance of vivid structural colors, namely red, green, and blue, at individual facets of the inverse pyramids (Fig. 11k). This multiplexing structured color filter with pixelated diffraction colors integrated into a single composite color offers application potential for structure-color displays, light-tracking sensors, high-capacity optical data storage, and secure product labeling.

Photo-driven reconfiguration of pre-patterned structures
Isolated azobenzene polymer structures can be subjected to deterministic reshaping of the original micro-and nanostructural features (e.g., size, shape, and geometry) via single-light irradiation. 23,[93][94][95][96] As discussed previously, the movement of azobenzene polymers is polarization-dependent (Fig. 4). Thus, the type of light polarization, polarization direction, or irradiation period can be adjusted to precisely reshape the pristine pattern of the azobenzene polymer, which is typically fabricated via various top-down and bottom-up methods, to form more intricate architectures. 22,[97][98][99] For example, a hexagonal array of a hemispherical pattern of (poly(4-vinylpyridine)-bpoly(6-[4-(4-butyloxyphenylazo)phenoxy]hexylmethacrylate) (P4VP-b-PAzoMA) copolymer ( Fig. 12a) is manufactured using the breath-figure technique, and it can be reconfigured into different patterns using single-light irradiation. 100 The polarization direction of a single laser beam, whether Sor V-polarized (see definition in Owing to the elastomeric nature of PDMS, a uniform conformal contact between the arrays and the PDMS film could be obtained. Subsequently, the azobenzene polymers were irradiated through the transparent PDMS overlay, during which the azobenzene polymers behaved as a viscoelastic fluid and adhered to the PDMS surface, resulting in a gradual rectangularization of the round wall, as shown in Fig. 12c. For a line pattern, a polarized light normal to the grating vector (Fig. 12d) was irradiated, and the irradiated azobenzene polymers flowed along the direction parallel to the light polarization but remained as a solid in other directions. Owing to the intermolecular force between the fluidized azopolymers and PDMS, an interface between the azobenzene polymers and PDMS was created to dissipate the internal stress, transforming the round edge to a rectangular edge with an increase in the light irradiation dose. The structural evolution during the rectangularization process is shown in Fig. 12e. Notably, during the early stage of irradiation, only the middle-top portion of the rounded strip in direct contact with the PDMS is flattened, whereas the sidewall angle remains unchanged, as indicated by the red triangles in Fig. 12f. As the light irradiation dose increases, the sidewall angle begins to decrease gradually with the gradual flattening of the top surface and eventually reaches 90° (i.e., with sharp edges). During the reduction of the sidewall angle, the gap size remains unchanged, as shown by the blue circles in Fig. 12f, because the light polarization is perpendicular to the grating vector. As shown in Fig. 12g, line patterns with a rectangular wall and different gap sizes (1 and 0.5 µm) can be successfully obtained using the rectangularization process.
A light-induced reconfiguration strategy can also be effectively used to achieve an excellent liquid-repellent surface. As shown in Fig. 12h-l, the 3D architecture of the microstructures can be designed to sustain a strong repellence to the wetting of almost any liquid, a property commonly referred to as omniphobicity, which is relevant for applications that require liquidrepellency and self-cleaning. 102 The realization of such omniphobic surfaces typically requires the fabrication of a specific and highly engineered re-entrant geometry, 103, 104 which involves multi-step lithographic processes that are time-consuming, low-yield, and expensive. However, the realization of such surfaces can be made considerably easier and more cost-effective with the use of a single-step light-induced reconfiguration of an array of cylindrical microposts irradiated with circularly polarized light, as shown in Fig. 12h-j. The resulting structure can repel different types of liquids, including water, glycerol, olive oil, and ethanol (see Fig. 12kl).

Light-driven self-healing and pore size control for water/oil separation
Polymers with azobenzene-type chromophores on the side chains, referred to as P1, have been shown to exhibit light-enabled self-healing ( Fig. 13a-b). 105  Azobenzene polymers can also be coated on a rigid plastic (e.g., PET) to repair cracks on the substrate via irradiation with polarized light (Figure 13c-d). 24 The azobenzene polymers move parallel to the direction of polarization, resulting in gradual self-healing of the damage (Fig. 13c). This light-assisted behavior can be extended to repair electric conductors (Fig. 13d) consisting of silver nanowires covered by an electrically conductive azobenzene polymer film.
When the coated film is scratched, a short circuit is formed; thus, LEDs arranged in a 'K'shape are dimmed. Light irradiation-induced self-healing movement in the azobenzene polymers drags the silver nanowires near the crack to reconnect, and the LEDs light up again (Fig. 13d). This self-healing of electrically conductive and flexible films has potential applications in flexible electronic devices, where simple light irradiation can fix a short circuit without the need to change the parts. This will allow for longer product lives, thereby minimizing electronic waste.
Azobenzene polymers can be electrospun into fibers with controllable fiber and pore sizes.
Membranes with hierarchical architectures, where pores of different sizes are integrated in tandem, are of great interest for water/oil emulsion separation owing to their potential for achieving a high flux, high selectivity, and mechanical durability. Such hierarchical membranes can be obtained by utilizing the photofluidic movement of PDO 3, as shown in Fig. 14. 25 Here, PDO 3 is electrospun (Fig. 14a) onto an optically inactive polycaprolactone (PCL) fibrous supporting layer. Upon light irradiation, the PDO 3 fibers undergo polarizationdependent mass transport due to the photofluidization effect despite the fact that PDO 3 has a Tg of 120 °C. The electrospun PDO 3 fibrous layer partially melts, thus shrinking the pore size, while the PCL supporting layer remains intact, forming a hierarchical membrane with modified nanopores in the PDO 3 layer and micropores in the PCL layer. (Fig. 14b) The membrane performance is evaluated based on the separation of oil/water emulsions (see the experimental apparatus and procedure in Fig. 14c). The results show that the fabricated hierarchical membranes can selectively and effectively separate (>99.96 %) various droplets from both surfactant-free and surfactant-stabilized water-in-oil emulsions (sizes as small as 50 nm) solely driven by gravity with a high flux (ca. 15,000 L m -2 h -1 bar -1 for surfactant-free emulsions and ca. 1000 L m -2 h -1 bar -1 for surfactant-stabilized emulsions) (Fig. 14d-e).

Conclusions
Herein, we briefly review the origin of the unique photofluidic movement of azobenzene polymers and their applications. The photofluidization of azobenzene groups occurs below Tg within localized regions subjected to light irradiation and can be directed anisotropically using