Net Unidirectional Fluid Transport in Locally Heated Nanochannel by Thermo-osmosis.

Thermo-osmosis driven by temperature gradients generally requires two liquid reservoirs at different temperatures connected by porous bodies or capillaries. We demonstrate, by molecular dynamics simulation, a new phenomenon toward nanoscale thermo-osmosis. Upon heating at a certain region of a nanochannel, multiple nanoscale convective layers are formed and can be manipulated to generate a net fluid transport from one reservoir to another, even without a temperature difference between them. A net unidirectional fluid transport with different rates can be achieved by precisely controlling location of the heated region. The net fluid transport can be enhanced further by tuning liquid-wall interactions. The demonstrated phenomenon provides a strategy for enhancing fluid mixing, which is often inefficient in nanoscale flows. Our finding is promising for chip-level cooling. The heat generated by chips can be employed to produce asymmetric temperature gradients in channels through proper configuration. Coolant liquids can thus be circulated without extra pumps.

Understanding and Reducing Photothermal Forces for the Fabrication of Au Nanoparticle Dimers by Optical Printing.

Optical printing holds great potential to enable the use of the vast variety of colloidal nanoparticles (NPs) in nano- and microdevices and circuits. By means of optical forces, it enables the direct assembly of NPs, one by one, onto specific positions of solid surfaces with great flexibility of pattern design and no need of previous surface patterning. However, for unclear causes it was not possible to print identical NPs closer to each other than 300 nm. Here, we show that the repulsion restricting the optical printing of close by NPs arises from light absorption by the printed NPs and subsequent local heating. By optimizing heat dissipation, it is possible to reduce the minimum separation between NPs. Using a reduced graphene oxide layer on a sapphire substrate, we demonstrate for the first time the optical printing of Au-Au NP dimers. Modeling the experiments considering optical, thermophoretic, and thermo-osmotic forces we obtain a detailed understanding and a clear pathway for the optical printing fabrication of complex nano structures and circuits based on connected colloidal NPs.

Highly-Adaptable Optothermal Nanotweezers for Trapping, Sorting, and Assembling across Diverse Nanoparticles.

Optical manipulation of various kinds of nanoparticles is vital in biomedical engineering. However, classical optical approaches demand higher laser power and are constrained by diffraction limits, necessitating tailored trapping schemes for specific nanoparticles. They lack a universal and biocompatible tool to manipulate nanoparticles of diverse sizes, charges, and materials. Through precise modulation of diffusiophoresis and thermo-osmotic flows in the boundary layer of an optothermal-responsive gold film, highly adaptable optothermal nanotweezers (HAONTs) capable of manipulating a single nanoparticle as small as sub-10 nm are designed. Additionally, a novel optothermal doughnut-shaped vortex (DSV) trapping strategy is introduced, enabling a new mode of physical interaction between cells and nanoparticles. Furthermore, this versatile approach allows for the manipulation of nanoparticles in organic, inorganic, and biological forms. It also offers versatile function modes such as trapping, sorting, and assembling of nanoparticles. It is believed that this approach holds the potential to be a valuable tool in fields such as synthetic biology, optofluidics, nanophotonics, and colloidal science.

Surface hydrophilicity-mediated migration of nano/microparticles under temperature gradient in a confined space.

HYPOTHESIS: Particle transport by a temperature gradient is prospective in many biomedical applications. However, the prevalence of boundary confinement in practical use introduces synergistic effects of thermophoresis and thermo-osmosis, causing controversial phenomena and great difficulty in understanding the mechanisms. EXPERIMENTS: We developed a microfluidic chip with a uniform temperature gradient and switchable substrate hydrophilicity to measure the migrations of various particles (d = 200 nm - 2 µm), through which the effects of particle thermophoresis and thermo-osmotic flow from the substrate surface were decoupled. The contribution of substrate hydrophilicity on thermo-osmosis was examined. Thermophoresis was measured to clarify its dependence on particle size and hydrophilicity. FINDINGS: This paper reports the first experimental evidence of a large enthalpy-dependent thermo-osmotic mobility χ âˆ¼ ΔH on a hydrophobic polymer surface, which is 1-2 orders of magnitude larger than that on hydrophilic surfaces. The normalized Soret coefficient for polystyrene particles, ST/d = 18.0 K-1µm-1, is confirmed to be constant, which helps clarify the controversy of the size dependence. Besides, the Soret coefficient of hydrophobic proteins is approximately-four times larger than that of hydrophilic extracellular vesicles. These findings suggest that the intrinsic slip on the hydrophobic surface could enhance both surface thermo-osmosis and particle thermophoresis.

Enhancing Thermo-Osmotic Low-Grade Heat Recovery by Applying a Negative Pressure to the Feed.

A newly developed technology, thermo-osmotic energy conversion (TOEC), is supposed to convert low-grade heat into power. However, the performance of existing TOEC experiments is deficient. This paper discusses the feasibility of strengthening TOEC by applying negative pressure to the feed liquid, which can reduce air pressure in the membrane pores and molecular diffusion resistance. Theoretical calculation shows that when the cooling and heating temperatures are 40 and 80 °C, respectively, and the transmembrane pressure difference is 5.0 MPa, the TOEC system with a negative pressure of 0.5 bar at the feed side can approach an efficiency of 3.01% and a power density of 16.85 W m-2, which increases by 20% and 27% compared with no negative pressure, respectively. Given the nonuniformity in the real system, computational fluid dynamics simulation is used to obtain the correction factor, which is then used to revise the theory prediction results for the first time. Moreover, a lab-scale experiment also proves that a negative pressure at the feed benefits the performance of the TOEC device. Overall, this research presents a feasible method to enhance a TOEC system, which may promote the development of a more-efficiently TOEC system for low-grade heat utilization.

Stack Thermo-Osmotic System for Low-Grade Thermal Energy Conversion.

Thermo-osmotic energy conversion (TOEC) technology, developed from membrane distillation, is an emerging method that has the potential of obtaining electricity efficiently from a low-grade heat source but faces the difficult problem of pump power loss. In this study, we build a novel TOEC system with a multistage architecture that can work without pump assistance. The experiment system, made of cheap commercial materials, can obtain a power density of 1.39 ± 0.25 W/m2, with a heating temperature of 80 °C, and its efficiency increased linearly with the total stage number. A theory calculation shows that a 30-stage system with a specific membrane and a working pressure of 5.0 MPa can obtain an efficiency of 2.72% with a power density of 14.0 W/m2. By a molecular dynamics simulation, it is shown that a high-performance membrane has the potential to work at 40 MPa. This study proves that TOEC technology is a practical and competitive approach to covert low-grade thermal energy into power efficiently.

Thermo-osmosis-Coupled Thermally Regenerative Electrochemical Cycle for Efficient Lithium Extraction.

Lithium (Li) production based on the soda evaporation process is time-consuming and unsustainable. The emerging electrochemical Li extraction is time-efficient but requires high-concentration Li sources and significant electrical energy input. Here, we demonstrate a fast, energy-saving, and environment-friendly Li production process by coupling a thermally regenerative electrochemical cycle (TREC) using lithium manganese oxide (LMO) and nickel hexacyanoferrate (NiHCF) electrodes with poly(vinylidene fluoride) membrane-based thermo-osmosis (denoted as TO-TREC). The characterization of LMO and NiHCF electrodes confirmed that the relatively high temperature of TO-TREC has negligible adverse effects on the ion intercalation in LMO and NiHCF electrodes. The LMO/NiHCF pair has a positive temperature coefficient of 0.843 mV K-1. In the TO-TREC process, Li ions are selectively extracted from a Li-containing brine warmed by low-grade heat and then released into a room-temperature recovery solution such as LiCl with a production rate of 50-60 mmol Li+ m-2 h-1. Li source solutions are concentrated by thermo-osmosis simultaneously, making it possible to utilize previously unusable Li-containing sources, such as concentrated brines from desalination plants and industrial effluents. Besides, the TREC harvests thermal energy from the heated brine, saving >20% of electrical energy compared to conventional electrochemical methods. The new process shows the potential to meet the growing global Li demands for many applications.

Photothermoelectric Response of Ti3C2Tx MXene Confined Ion Channels.

With recent growing interest in biomimetic smart nanochannels, a biological sensory transduction in response to external stimuli has been of particular interest in the development of biomimetic nanofluidic systems. Here we demonstrate the MXene-based subnanometer ion channels that convert external temperature changes to electric signals via preferential diffusion of cations under a thermal gradient. In particular, coupled with a photothermal conversion feature of MXenes, an array of the nanoconfined Ti3C2Tx ion channels can capture trans-nanochannel diffusion potentials under a light-driven axial temperature gradient. The nonisothermal open-circuit potential across channels is enhanced with increasing cationic permselectivity of confined channels, associated with the ionic concentration or pH of permeant fluids. The photothermoelectric ionic response (evaluated from the ionic Seebeck coefficient) reached up to 1 mV·K-1, which is comparable to biological thermosensory channels, and demonstrated stability and reproducibility in the absence and presence of an ionic concentration gradient. With advantages of physicochemical tunability and easy fabrication process, the lamellar ion conductors may be an important nanofluidic thermosensation platform possibly for biomimetic sensory systems.