Xiaobao Cao

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Cao

First Name

Xiaobao

ORCID

0000-0003-2211-2823

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Publications 1 - 10 of 19

Microfluid Switching-Induced Transient Refractive Interface
Tang, Jiukai; Qiu, Guangyu; Cao, Xiaobao; et al. (2022)
ACS Sensors
The laminar flow interface (LFI) developed at low Reynolds numbers is one of the most prominent features of microscale flows and has been employed in a diverse range of optofluidic applications. The formation of LFIs usually requires the manipulation of multiple streams within a microchannel using a complex hydrodynamic pumping system. Herein, we present a new type of LFI that is generated by fluid switching within a three-dimensional (3D) microlens-incorporating microfluidic chip (3D-MIMC). Since Poiseuille flows exhibit a parabolic velocity profile, the LFI is cone-like in shape and acts as a transient refractive interface (TRI), which is sensitive to the refractive index (RI) and the Péclet number (Pe) of the switching fluids. In response to the TRI, the intensity of the transmitted light can be intensified or attenuated depending on the sequence of fluid switching operations. By incorporating three-dimensional (3D) microlenses and increasing the Pe values, the profile and amplitude of the intensity peak are both significantly improved. The limit of detection (LoD) for a sodium chloride (NaCl) solution at Pe = 1363 is as low as 0.001% (w/w), representing an improvement of 1–2 orders of magnitude when compared to existing optofluidic concentration sensors based on intensity modulation. Fluid switching of a variety of inorganic and organic sample fluids confirms that the specific optical response (Kor) correlates positively with both Pe and the specific RI (Knc), obeying a linear relationship. This model is further verified through cross-validations and used to estimate the molecular diffusion coefficient (D) of a range of species. Furthermore, by virtue of the TRI, we achieve a sensitive measurement of optical-equivalent total dissolved solids (OE-TDS) for environmental samples.
Programmable Control of Multiscale Droplets using V-Valves
Xue, Tian; Jain, Ankit; Cao, Xiaobao; et al. (2023)
Advanced Materials Technologies
Contemporary droplet-based microfluidic platforms generate large numbers of sub-nanoliter (<10−9 L) volume droplets on short timescales. The controllable generation of femtoliter (10−15 L) volume droplets is however a far more challenging task. Herein, the design, fabrication, and testing of a valve-based droplet-on-demand generator that enables the robust and controllable production of femtoliter-volume water-in-oil droplets is described, where droplet size is determined by on-chip pressure. This generator enables the sequential supply of femtoliter droplets to much larger “mother” droplets stored on-chip, which in turn allows for high-precision droplet dilution and programmable droplet pairing/merging operations. Such an approach can generate in excess of 700 fL-volume droplets of variable content and with dilution factors of almost three orders of magnitude. Using the same principle, the microfluidic platform can also perform rapid, low-volume consumption titrations. Accordingly, the valve-based droplet-on-demand generator allows for a range of automated and multi-step droplet-based manipulations.
Replicating the Cynandra opis Butterfly's Structural Color for Bioinspired Bigrating Color Filters
Cao, Xiaobao; Du, Ying; Guo, Yujia; et al. (2022)
Advanced Materials
Multilayer grating structures, such as those found on the wings of the butterfly Cynandra opis, are able to interact with light to generate structural coloration. When illuminated and viewed at defined angles, such structural color is characterized by exceptional purity and brightness. To provide further insight into the mechanism of structural coloration, two-photon laser lithography is used to fabricate bioinspired bigrating nanostructures, whose optical properties may be controlled by variation of the height and period of the grating features. Through the use of both spectral measurements and finite-element method simulations, herein specific feature dimensions are identified that due to the combined effects of multilayer interference and diffraction generate excellent spectral characteristics and high color purity over the entire visible range. Additionally, it is demonstrated that variation of feature period and/or height plays a central role in controlling both hue and purity. Importantly, such tuneable bigrating structures are of significant utility in color filtering applications.
Laminar Flow-Based Fiber Fabrication and Encoding via Two-Photon Lithography
Cao, Xiaobao; Gao, Quan; Li, Shangkun; et al. (2020)
ACS Applied Materials & Interfaces
In recent years, flow photolithography (FL) has emerged as a powerful synthetic tool for the creation of barcoded microparticles with complex morphologies and chemical compositions which have been shown to be useful in a range of multiplexed bioassay applications. More specifically, FL has been highly successful in producing micron-sized, encoded particles of bespoke shape, size, and color. That said, to date, FL has been restricted to generating barcoded microparticles and has lacked the ability to produce hybrid fibers which are structurally and spectrally encoded. To this end, we herein present a method that combines a continuous flow microfluidic system with two-photon polymerization (2PP) to fabricate microscale-encoded fibers and Janus strips in a high-throughput manner. Specifically, two co-flow liquid streams containing a monomer and initiator are introduced through a Y-shape channel to form a stable interface in the center of a microfluidic channel. The flow containing the (fluorescently labeled) monomer is then patterned by scanning the voxel of the 2PP laser across the interface to selectively polymerize different regions of the forming fiber/particle. Such a process allows for rapid spectral encoding at the single fiber level, with the resulting structurally coded fibers having obvious application in the fields of security identification and anticounterfeiting.
Liquid repellency enhancement through flexible microstructures
Hu, Songtao; Cao, Xiaobao; Reddyhoff, Tom; et al. (2020)
Science Advances
Artificial liquid-repellent surfaces have attracted substantial scientific and industrial attention with a focus on creating functional topological features; however, the role of the underlying structures has been overlooked. Recent developments in micro-nanofabrication allow us now to construct a skin-muscle type system combining interfacial liquid repellence atop a mechanically functional structure. Specifically, we design surfaces comprising bioinspired, mushroom-like repelling heads and spring-like flexible supports, which are realized by three-dimensional direct laser lithography. The flexible supports elevate liquid repellency by resisting droplet impalement and reducing contact time. This, previously unknown, use of spring-like flexible supports to enhance liquid repellency provides an excellent level of control over droplet manipulation. Moreover, this extends repellent microstructure research from statics to dynamics and is envisioned to yield functionalities and possibilities by linking functional surfaces and mechanical metamaterials.
Towards an active droplet-based microfluidic platform for programmable fluid handling
Cao, Xiaobao; Buryska, Tomas; Yang, Tianjin; et al. (2023)
Lab on a Chip
Droplet-based microfluidic systems have emerged as powerful alternatives to conventional high throughput screening platforms, due to their operational flexibility, high-throughput nature and ability to efficiently process small fluid volumes. However, the challenges associated with performing bespoke operations on user-defined droplets often limit their utility in screening applications that involve complex workflows. To this end, the marriage of droplet- and valve-based microfluidic technologies offers the prospect of balancing the controllability of droplet manipulations and analytical throughput. In this spirit, we present a microfluidic platform that combines the capabilities of integrated microvalve technology with droplet-based sample compartmentalization to realize a highly adaptable programmable fluid handling functionality. The microfluidic device consists of a programmable formulator linked to an automated droplet generation device and storage array. The formulator leverages multiple inputs coupled to a mixing ring to produce combinatorial solution mixtures, with a peristaltic pump enabling titration of reagents into the ring with picoliter resolution. The platform allows for the execution of user-defined reaction protocols within an array of storage chambers by consecutively merging programmable sequences of pL-volume droplets containing specified reagents. The precision in formulating solutions with small differences in concentration is perfectly suited for the accurate estimation of kinetic parameters. The utility of our platform is showcased through the performance of enzymatic kinetic measurements of beta-galactosidase and horseradish peroxidase with fluorogenic substrates. The presented platform provides for a range of automated manipulations and paves the way for a more diverse range of droplet-based biological experiments.
Self-aligned 3D microlenses in a chip fabricated with two-photon stereolithography for highly sensitive absorbance measurement
Tang, Jiukai; Qiu, Guangyu; Cao, Xiaobao; et al. (2020)
Lab on a Chip
Absorbance measurement is a widely used method to quantify the concentration of an analyte. The integration of absorbance analysis in microfluidic chips could significantly reduce the sample consumption and contribute to the system miniaturization. However, the sensitivity and limit of detection (LoD) of analysis in microfluidic chips with conventional configuration need improvements due to the limited optical pathway and unregulated light propagation. In this work, a 3D-microlens-incorporating microfluidic chip (3D-MIMC) with a greatly extended detection channel was innovatively fabricated using two-photon stereolithography. The fabrication was optimized with a proposed hierarchical modular printing strategy. Due to the incorporation of 3D microlenses, the light coupling efficiency and the signal-to-noise ratio (SNR) were respectively improved approximately 9 and 4 times. An equivalent optical path length (EOL) of 62.9 mm was achieved in a 3.7 μl detection channel for testing tartrazine samples. As a result, the sensitivity and LoD of the 3D-MIMC assay were correspondingly improved by one order of magnitude, compared with those of the 96-well plate assay. Notably, the 3D-MIMC has the potential to be integrated into a general microanalysis platform for multiple applications.
Rigid-flexible hybrid surfaces for water-repelling and abrasion-resisting
Hu, Songtao; Huang, Weifeng; Li, Jinbang; et al. (2023)
Friction
Droplets impacting solid superhydrophobic surfaces is appealing not only because of scientific interests but also for technological applications such as water-repelling. Recent studies have designed artificial surfaces in a rigid-flexible hybrid mode to combine asymmetric redistribution and structural oscillation water-repelling principles, resolving strict impacting positioning; however, this is limited by weak mechanical durability. Here we propose a rigid-flexible hybrid surface (RFS) design as a matrix of concave flexible trampolines barred by convex rigid stripes. Such a surface exhibits a 20.1% contact time reduction via the structural oscillation of flexible trampolines, and even to break through the theoretical inertial-capillary limit via the asymmetric redistribution induced by rigid stripes. Moreover, the surface is shown to retain the above water-repelling after 1,000 abrasion cycles against oilstones under a normal load as high as 0.2 N.mm(-1). This is the first demonstration of RFSs for synchronous waterproof and wearproof, approaching real-world applications of liquid-repelling.
In Situ Visualization of the Structural Evolution and Alignment of Lyotropic Liquid Crystals in Confined Flow
Rodriguez-Palomo, Adrian; Lutz-Bueno, Viviane; Cao, Xiaobao; et al. (2021)
Small
Self-assembled materials such as lyotropic liquid crystals offer a wide variety of structures and applications by tuning the composition. Understanding materials behavior under flow and the induced alignment is wanted in order to tailor structure related properties. A method to visualize the structure and anisotropy of ordered systems in situ under dynamic conditions is presented where flow-induced nanostructural alignment in microfluidic channels is observed by scanning small angle X-ray scattering in hexagonal and lamellar self-assembled phases. In the hexagonal phase, the material in regions with high extensional flow exhibits orientation perpendicular to the flow and is oriented in the flow direction only in regions with a high enough shear rate. For the lamellar phase, a flow-induced morphological transition occurs from aligned lamellae toward multilamellar vesicles. However, the vesicles do not withstand the mechanical forces and break in extended lamellae in regions with high shear rates. This evolution of nanostructure with different shear rates can be correlated with a shear thinning viscosity curve with different slopes. The results demonstrate new fundamental knowledge about the structuring of liquid crystals under flow. The methodology widens the quantitative investigation of complex structures and identifies important mechanisms of reorientation and structural changes.
Integrated microfluidics for high-content biological analysis
Cao, Xiaobao (2019)

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