Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

We demonstrate universal computation in an all-fluidic two-phase microfluidic system Nonlinearity is introduced into an otherwise linear, reversible, low-Reynolds number flow via bubble-to-bubble hydrodynamic interactions A bubble traveling in a channel represents a bit, providing us with the capability to simultaneously transport materials and perform logical control operations We demonstrate bubble logic AND/OR/NOT gates, a toggle flip-flop, a ripple counter, timing restoration, a ring oscillator, and an electro-bubble modulator These show the nonlinearity, gain, bistability, synchronization, cascadability, feedback, and programmability required for scalable universal computation With increasing complexity in large-scale microfluidic processors, bubble logic provides an on-chip process control mechanism integrating chemistry and computation

Microfluidic bubble logic

Despite enormous breakthroughs in lab-on-a-chip techniques, light-driven manipulation faces two long-standing challenges: the ability to achieve both multiform manipulation and tunable manipulation range and the means to avoid potential thermal damage to the targets. By harnessing the optical heating of hybrid photothermal waveguides (HPW), we develop a hybrid optofluidic technique involving buoyancy and thermocapillary convection to achieve fluid transport with controllable modes and tunable strength. Switching of the optofluidic mode from buoyancy to thermocapillary convection, namely, from vertical to horizontal vortices, is employed for three-dimensional manipulation. The strong confinement and torque in the vortices are capable of trapping and rotating/spinning particles at the vortex centers rather than the HPW. Buoyancy convection provides a trapping circle to achieve collective trapping and vertical rotation/spin, while thermocapillary convection offers a trapping lattice to achieve distributed trapping and horizontal rotation/spin. By integrating micro/nanoparticles with various properties and sizes, further investigations of the optofluidic arrangement, mixing, and synthesis will broaden the potential applications of the hybrid optofluidic technique in the fields of lab-on-a-chip, materials science, chemical synthesis and analysis, photonics, and nanoscience. A method for manipulating tiny particles both vertically and horizontally using lasers has been developed by scientists in China. Intense beams of light create vortices in a fluid that can trap nanoscale objects. This concept is harnessed using so-called ‘optical tweezers’ for accurately positioning or sorting cells. The ideal optical manipulation set-up can transfer the target particle in any direction. Sailing He, Xiaobo Xing, and their colleagues at South China Normal University, Guangzhou, realized just such a system using micrometer-scale channels in graphene oxide. Vertical manipulation was achieved by using laser light to heat the fluid-carrying channel and generate buoyancy. Similarly, heat-induced capillary action drove horizontal motion. This approach could be used in lab-on-a-chip applications: portable compact devices that can sensitively and accurately analyze chemical or biological samples. A hybrid optofluidic technique was developed to achieve fluid transport with controllable modes and tunable strength. The switch of the optofluidic mode from buoyancy to thermocapillary convection is employed for three-dimensional manipulation. The strong confinement and torque in the convection are capable of trapping and rotating/spinning particles. The buoyancy convection provides a trapping circle to achieve collective trapping and vertical rotation/spin, while the thermocapillary convection offers a trapping lattice to achieve distributed trapping and horizontal rotation/spin. Further investigations in optofluidic arrangement, mixing, and synthesis will broaden its potential applications in the fields of lab-on-a-chip.

Hybrid optofluidics and three-dimensional manipulation based on hybrid photothermal waveguides

Manipulating large numbers of a variety of particles/wires is essential for many lab-on-a-chip technologies. Here we generate a planar array of optofluidic vortices with photothermal gradients from an easy-fabricated graphene oxide (GO) heater to achieve high-throughput and multiform manipulation at low excitation power and low loss. As a tweezer, each vortex can rapidly capture and confine particles without restrictions on shapes and materials. The stiffness of the confinement is easily tuned by adjusting the vortex dimension. As a motor, it can actuate any traps to persistently rotate/spin in clockwise or anti-clockwise mode. As a high-performance ‘workshop’, this work lays the groundwork for various self-assembly ranging from colloid-based clusters, chains, capsules, shells and ultra-thin films, through particles’ surface modification and fusion, to nanowire-based architectures. Furthermore, we can create multiple vortex arrays through fabricating an array of heaters, which enables massively parallel manipulation and distributed operations all on a chip. A lab-on-a-chip device can capture, contain and self-assemble large numbers of microparticles with innovative liquid vortex technology. Sailing He from Sweden's Royal Institute of Technology and co-workers in China developed a platform that uses graphene oxide ‘heaters’, activated by lasers, to alter the surface tension of nearby liquid droplets. This effect, known as thermocapillary convection, produces whirlpool-like spinning forces with a trapping power determined by droplet geometry. To demonstrate the potential of this approach, the team fabricated a microfluidic device featuring parallel arrays of vortices. Examples include precise control over materials, such as silver nanowires, that are difficult to handle with optical tweezers; a spinning quad-core motor providing persistent torque; and a ‘workshop’ for self-assembly that combines tweezer and motor functions to organize polystyrene particles into linear-, block- and triangle-shaped complexes. Planar arrays of optofluidic vortices are generated with photothermal gradients from an array of graphene oxide heaters to achieve multiform manipulations. As a tweezer, each vortex can rapidly capture and confine particles without any restriction on shapes or materials. As a motor, it can actuate any trapped particle to persistently rotate/spin in clockwise or anti-clockwise mode. Such a high-performance ‘workshop’ can be used for various self-assembly ranging from colloid-based clusters, chains, capsules, shells, and ultra-thin films, through particles’ surface modification and fusion, to nanowires-based architectures.

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Optofluidic vortex arrays generated by graphene oxide for tweezers, motors and self-assembly

The dramatic temperature rise of energetic materials when using only millisecond pulses of light is very important for photothermic ignition. Here we demonstrate a free-standing laser energy converter based on energetic graphene oxide, which exhibited an ultrafast and large temperature rise from 21 to 670 °C within 8.7 ms during the irradiation of laser pulses, while the temperature of the graphene-based laser energy converter can only be raised from 21 to 170 °C under the same conditions. The reason for the unexpected and much larger temperature rise of energetic graphene oxide than graphene was studied by laser scanning microscopy and thermal analysis, which indicated that the temperature rise is strongly dependent on the content of oxygen-containing groups in energetic graphene oxide. The combination process of focused ion beam and transmission electron microscope characterization was utilized for the direct observation of the layer-stacked structure in free-standing graphene oxide and graphene membranes. Furthermore, the initiating energy of laser initiators was 40% decreased by adding a thin layer of the free-standing laser energy converter based on energetic graphene oxide, which shows great potential for developing photothermic ignition with low initiating energy.

A free-standing laser energy converter based on energetic graphene oxide for enhanced photothermic ignition

Thermal microbubbles generally grow directly from the heater and are spherical to minimize surface tension. We demonstrate a novel type of microbubble indirectly generated from a graphene oxide-microheater. Graphene oxide's photothermal properties allowed for efficient generation of a thermal gradient field on the microscale. A series of approximately ellipsoidal microbubbles were generated on the smooth microwire based on heterogeneous nucleation. Other dynamic behaviors induced by the microheater such as constant growth, directional transport and coalescence were also investigated experimentally and theoretically. The results are not only helpful for understanding the bubble dynamics but also useful for developing novel photothermal bubble-based devices.

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Xiang Cai

Papers

Dynamic behaviors of approximately ellipsoidal microbubbles photothermally generated by a graphene oxide-microheater