Photonic crystals are periodic nanostructures that can affect and manipulate photons' behaviors in a similar fashion to how semiconductors could precisely tune the behaviors of electrons [1]. Photonic phenomena can be traced back to a long story. In 1887, Lord Rayleigh studied a type of one -dimensional photonic crystal and predicted the existence of high reflectivity of light of a specific wavelength range, which later became known as a stopband [2]. In the 1960s, Luck and Hiltner et al. respectively reported ordered structures from uniform-sized latex showing refraction phenomena and studied their refraction properties [3,4]. In 1979, Clark and Hurd et al. reported the crystals' formation and destruction of single colloidal crystal structures under external fields (such as shearing force, etc.) [5]. A few years later, the concept of photonic crystal was formally brought up by Yablonovitch and John et al. when they extended the study to solid ordered three-dimensional photonic structure [1,6]. Since then, research articles about the construction of photonic structures with diverse applications have exponentially increased [[7], [8], [9], [10], [11], [12], [13], [14]].

Due to their wide applications in a number of realms, such as photonic computers, sensing, biomimetics, diagnosis, novel pigments, and optical/electron devices, etc., photonic structures have attracted a large amount of research attention [7,9,10,[15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]]. In general, the construction of photonic crystal structures was mainly realized through two routes: one is the so-called top-down method, which can create periodic nanostructures on substrates coated with photoresists using precise processing tools and techniques, such as e-beam, nano imprint lithography, electrochemical etching, and film deposition [15,26,27]. This approach normally has some drawbacks, such as requiring expensive instruments and skillful instrument operators, or it can only construct limited duplicated layers [28]; another is the bottom-up approach, namely, the periodic ordered structures of photonic crystals were obtained through the self-assembly of amphiphilic block copolymers, colloids, and cholesteric liquid crystals (CLCs) [[28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]]. Compared to the top-down approach, photonic structures formed from the bottom-up assembly are much more facile to achieve and considerably less expensive, making it much more affordable for an ordinary laboratory [40]. In terms of the building blocks for self-assembly, photonic crystal from self-assembly of block copolymers requires the formation of microdomains of block copolymers with enough repeating units to form microdomains with thickness up to 100 nm. Despite the fact that the self-assembly of ultrahigh-molecular-weight block copolymers can be beneficial for generating sophisticated engineering of crystal lattices and resulting in better control for photonic waves, e.g., topologically protected photon propagation including Weyl photonic crystals, it requires block copolymers with precision architectures and high molecular weight, which in turn makes it difficult to be self-assembled into ordered structures due to the increased viscosity of the polymer melts or polymer solution. As a result, this would significantly extend the annealing cycle [12,[41], [42], [43]]. Moreover, the synthesis of such amphiphilic block copolymers with precise structures is much more complicated than that of the synthesis of colloidal particles (CPs) [29,35,36,44]. Despite homopolymer-based microobjects also displaying structural colors even though they don't have any periodic nanostructures, it requires the microobjects with curved interfaces to leverage the synergistic effect of total internal reflection and optical interference [[45], [46], [47], [48]]. Although photonic microobjects formed from the assembly of cholesteric liquid crystals (CLCs) have also been reported, the color saturations of these photonic microobjects remain to be improved [49,50]. Therefore, owing to their facile designable merits and convenient synthesis process, different colloids have been the most widely and intensively investigated building blocks for the construction of photonic structures via the self-assembly approach [9,10,28,34,[51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62]]. Since the end of last decade, there has been an enormous amount of research papers focusing on the fundamental theories and experimental characterizations of photonic structures formed from colloidal self-assembly [10,16,[63], [64], [65], [66], [67], [68], [69]]. In terms of the ‘hardness’ of colloidal building blocks for the construction of photonic structures, there are generally two categories of CPs employed: one is the nondeformable ‘rigid’ CPs under normal conditions, such as monodispersed SiO2, metallic oxide colloids (i.e., TiO2, γ-Fe2O3, Fe3O4, or the relevant compound colloids), metallic colloids (such as Au) with high refractive index, polymer colloids (i.e., polystyrene (PS), poly(methyl acrylate) (PMMA), ligin colloids, or polymer-based high refractive index colloids, such as fluorinated polymethacrylate poly(1H,1H-heptafluorobutyl methacrylate) (PFBMA)) [9,16,[69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81]]. For example, Pusey and Megen reported the phase behaviors of ‘hard’ or ‘near hard’ colloidal suspensions and studied the phase behaviors of the relationship between the states of fluid, crystal, and glassy and the concentrations of CPs [82,83]; the other is the deformable ‘soft’ colloids, such as thermoresponsive poly(N-isopropyl acrylamide) (pNIPAAm)-based or poly(ethylene glycol) (PEG) and the derivatives-based gel particles, or colloids with partial deformability under ordinary conditions (i.e., room temperature), such as PS-pNIPAAm or Au-pNIPAAm core-shell CPs with a rigid core [[84], [85], [86], [87], [88]]. In particular, photonic structures based on soft colloids have gained wide attention [10,[89], [90], [91], [92], [93], [94]]. Despite numerous articles about the construction of bulk photonic structures through diverse conventional methods that have been reported, colloid-based photonic microobjects are also significantly impacting this field [[95], [96], [97], [98], [99]]. Various methods have been used to manufacture photonic microobjects, among which producing colloid-based photonic microobjects through microfluidics in a precise manner is crucially important in many fields [72,95,97,98,[100], [101], [102], [103], [104], [105], [106], [107], [108]]. However, a systematicreview regarding the construction and tuning of the properties and shapes of photonic microobjects is still missing. In light of this, this review is particularly focused on the construction, shaping, and tuning of colloid-based photonic microobjects through droplet-based microfluidics, given that this technique allows researchers to control the resultant photonic microobjects in a precise and on-demand fashion [[109], [110], [111], [112]]. In this review, we summarize the development of colloidal photonic microobjects that are shaped and regulated by a combination of droplet-based microfluidics and a variety of other approaches, specifically elaborating on the advances in the aspects of photonic microspheres with isotropic properties, spherical photonic microobjects with anisotropic photonic properties, structurally anisotropic photonic microobjects, microcapsules, and fully inverse opal microparticles. In addition, future research directions about microfluidics-assisted preparation of colloid-based photonic microobjects and the relevant scientific questions regarding the structural and property regulation of these microobjects are also discussed.

As a powerful tool, microfluidics has been comprehensively developed in the past two decades and can be used to precisely handle very tiny amounts of liquid (10−9–10−18 L) to construct microobjects with different kinds of structures and functionalities [[113], [114], [115], [116], [117]]. Droplet microfluidics and the corresponding microobjects can lead to the resulting microobjects with very narrow size distributions (coefficient of variation (C.V.) < 5 %), facile tunable size, and precisely controllable droplet structure [115,116,[118], [119], [120], [121]]. This technique typically allows researchers to produce photonic microobjects with sizes ranging from a few (> 2) microns to hundreds of microns depending on several factors, including the types of building blocks used, the formation mechanisms, and also the flow rates of different phases used [[122], [123], [124]]. This technique could also impart the microobjects with unique advantages in terms of conducting research from a statistical point of view [125]. In the past two decades, the preparation of photonic microobjects by combining microfluidic techniques and colloidal assembly has attracted intensive research attention [[126], [127], [128], [129]]. Since the preparation of colloidal photonic structures using a co-flow microfluidic chip was firstly reported by Yang's group, droplet microfluidics has been widely used for the construction, shaping, and assisted regulating of colloidal photonic microstructures and their relevant properties [125,130]. A number of research groups, including Yang and Kim, Weitz, Kanai, Gu, Zhao, Chen, Zhu et al., all have respectively made considerable contributions in the realms of design and generation of photonic microstructures based on microfluidic platforms [109,110,[131], [132], [133]]. Regarding the packing densities of the CPs, there are two types of photonic structures: close-packed and non-close-packed colloidal photonic structures. The close-packed opal structure from monodispersed spherical CPs has been the most intensively studied architecture due to the fact that it is the simplest form of colloidal crystal [134,135]. So, we start our discussion with the opal-structured photonic microspheres in this review and roll out the summary of photonic microobjects in diverse forms. However, one common point of these photonic microobjects is that all of them involve the utilization of a microfluidics platform in the construction steps. We then elaborate on our discussions from the point of view of the colloid-based microobjects with different structures and how the structures and the corresponding photonic properties of the microobjects were regulated by different means.