Printing Double-Network Tough Hydrogels Using Temperature-Controlled Projection Stereolithography (TOPS).

We report a new method to shape double-network (DN) hydrogels into customized 3D structures that exhibit superior mechanical properties in both tension and compression. A one-pot prepolymer formulation containing photo-cross-linkable acrylamide and thermoreversible sol-gel κ-carrageenan with a suitable cross-linker and photoinitiators/absorbers is optimized. A new TOPS system is utilized to photopolymerize the primary acrylamide network into a 3D structure above the sol-gel transition of κ-carrageenan (80 °C), while cooling down generates the secondary physical κ-carrageenan network to realize tough DN hydrogel structures. 3D structures, printed with high lateral (37 µm) and vertical (180 µm) resolutions and superior 3D design freedoms (internal voids), exhibit ultimate stress and strain of 200 kPa and 2400%, respectively, under tension and simultaneously exhibit a high compression stress of 15 MPa with a strain of 95%, both with high recovery rates. The roles of swelling, necking, self-healing, cyclic loading, dehydration, and rehydration on the mechanical properties of printed structures are also investigated. To demonstrate the potential of this technology to make mechanically reconfigurable flexible devices, we print an axicon lens and show that a Bessel beam can be dynamically tuned via user-defined tensile stretching of the device. This technique can be broadly applied to other hydrogels to make novel smart multifunctional devices for a range of applications.

Disjoining pressure driven transpiration of water in a simulated tree.

HYPOTHESIS: Transpiration occurs in 100 m tall redwood trees where water is passively pulled against gravity requiring the evaporating liquid meniscus in stomata pores to be under absolute negative pressures of -10 atm or higher. Disjoining pressure can significantly reduce pressure at meniscus in nanopores due to strong surface-liquid molecular interaction. Hence, disjoining pressure should be able to solely govern the transpiration process. SIMULATIONS: Expression of disjoining pressure in a water film is first developed from prior experimental findings. The expression is then implemented in a commercial CFD solver and validated against experimental data for water wicking in nanochannels of height varying from 59 nm to 1 µm. Following the implementation, the transpiration process is simulated in a 3D domain comprising of a nanopore connected to a tube with ground-based water tank, thus mimicking the stomata-xylem-soil pathway in a 100 m tall tree. FINDINGS: Disjoining pressure is found to induce absolute negative pressures as high as -23.5 atm at the evaporating meniscus and can also sustain high evaporation fluxes in nanopore before the meniscus completely dewets. This is the first report to integrate disjoining pressure into continuum simulations and study the transpiration process in a 100 m tall tree using such simulations.

Disjoining Pressure of Water in Nanochannels.

The disjoining pressure of water was estimated from wicking experiments in 1D silicon dioxide nanochannels of heights of 59, 87, 124, and 1015 nm. The disjoining pressure was found to be as high as ∼1.5 MPa while exponentially decreasing with increasing channel height. Such a relation resulting from the curve fitting of experimentally derived data was implemented and validated in computational fluid dynamics. The implementation was then used to simulate bubble nucleation in a water-filled 59 nm nanochannel to determine the nucleation temperature. Simultaneously, experiments were conducted by nucleating a bubble in a similar 58 nm nanochannel by laser heating. The measured nucleation temperature was found to be in excellent agreement with the simulation, thus independently validating the disjoining pressure relation developed in this work. The methodology implemented here integrates experimental nanoscale physics into continuum simulations thus enabling numerical study of various phenomena where disjoining pressure plays an important role.

Droplet Evaporation on Porous Nanochannels for High Heat Flux Dissipation.

Droplet wicking and evaporation in porous nanochannels is experimentally studied on a heated surface at temperatures ranging from 35 to 90 °C. The fabricated geometry consists of cross-connected nanochannels of height 728 nm with micropores of diameter 2 µm present at every channel intersection; the pores allow water from a droplet placed on the top surface to wick into the channels. Droplet volume is also varied, and a total of 16 experimental cases are conducted. Wicking characteristics such as wicked distance, capillary pressure, viscous resistance, and propagation coefficients are obtained at all surface temperatures. Evaporation flux from the nanochannels/micropores is estimated from the droplet experiments but is also independently confirmed via a new set of experiments where water is continuously fed to the sample through a microtube so that it matches the evaporation rate. Heat flux as high as ∼294 W/cm2 is achieved from channels and pores. The experimental findings are applied to evaluate the use of porous nanochannel geometry in spray cooling application and is found to be capable of passively dissipating high heat fluxes upto ∼77 W/cm2 at temperatures below nucleation, thus highlighting the thermal management potential of the fabricated geometry.

Evaporation Dynamics in Buried Nanochannels with Micropores.

Cross-connected buried nanochannels of height ∼728 nm, with micropores of ∼2 µm diameter present at each intersection, are used in this work to numerically and experimentally study droplet-coupled evaporation dynamics at room temperature. The uniformly structured channels/pores, along with their well-defined porosity, allow for computational fluid dynamics simulations and experiments to be performed on the same geometry of samples. A water droplet is placed on top of the sample causing water to wick into the nanochannels through the micropores. After advancing, the meniscus front stabilizes when evaporation flux is balanced with the wicking flux, and it recedes once the water droplet is completely wicked in. Evaporation flux at the meniscus interface of channels/pores is estimated over time, while the flux at the water droplet interface is found to be negligible. When the meniscus recedes in the channels, local contact line regions are found to form underneath the pores, thus rapidly enhancing evaporation flux as a power-law function of time. Temporal variation of wicking flux velocity and pressure gradient in the nanochannels is also independently computed, from which the viscous resistance variation is estimated and compared to the theoretical prediction.

Pool Boiling Coupled with Nanoscale Evaporation Using Buried Nanochannels.

Pool boiling is explicitly coupled with nanoscale evaporation by using buried nanochannels of height ∼728 nm and ∼100 nm to enhance critical heat flux (CHF) by ∼105%. Additional menisci and contact line formation in nanochannels are found to be the dominant factors of CHF enhancement. Wicking assists in creating the additional contact line but does not serve as the primary measurable factor in predicting such enhancement based on CFD simulations and wicking experiments. This work provides clarity on the roles of contact line and wicking in boiling heat transfer.