Molecular simulation of thermal transport across hydrophilic interfaces

Role of Underlying Substrates on the Interfacial Thermal Transport in Supported Graphene Nanochannels: Implications of Thermal Translucency

We study the role of underlying substrates on interfacial heat transfer within supported graphene nanochannels. Due to graphene's translucency, the underlying substrate, apart from its known hydrodynamic impact on fluid flow, also influences heat transport. We introduce the term "thermal translucency" to describe this phenomenon in the context of interfacial heat transfer. Our findings reveal that the Kapitza resistance, RK, is dependent on the specific underlying substrate. The specific underlying substrate alters the water-graphene interface potential landscape due to graphene's translucency, leading to a breakdown in the inverse relationship between interfacial water density peaks and RK values, typically observed at water-graphene and water-graphite interfaces. Remarkably, higher interfacial water density peaks correlate with more ordered energy patterns, not necessarily tied to more hydrophilic substrates as the literature commonly suggests for lower RK values. The insights provided have implications for controlling and tuning thermal transport and heat storage in nanofluidic devices.

Curvature and temperature-dependent thermal interface conductance between nanoscale gold and water

Plasmonic gold nanoparticles (AuNPs) can convert laser irradiation into thermal energy for a variety of applications. Although heat transfer through the AuNP-water interface is considered an essential part of the plasmonic heating process, there is a lack of mechanistic understanding of how interface curvature and the heating itself impact interfacial heat transfer. Here, we report atomistic molecular dynamics simulations that investigate heat transfer through nanoscale gold-water interfaces. We simulated four nanoscale gold structures under various applied heat flux values to evaluate how gold-water interface curvature and temperature affect the interfacial heat transfer. We also considered a case in which we artificially reduced wetting at the gold surfaces by tuning the gold-water interactions to determine if such a perturbation alters the curvature and temperature dependence of the gold-water interfacial heat transfer. We first confirmed that interfacial heat transfer is particularly important for small particles (diameter ≤10 nm). We found that the thermal interface conductance increases linearly with interface curvature regardless of the gold wettability, while it increases nonlinearly with the applied heat flux under normal wetting and remains constant under reduced wetting. Our analysis suggests the curvature dependence of the interface conductance coincides with changes in interfacial water adsorption, while the temperature dependence may arise from temperature-induced shifts in the distribution of water vibrational states. Our study advances the current understanding of interface thermal conductance for a broad range of applications.

Development of mesoporous silica-based nanoprobes for optical bioimaging applications

A mesoporous silica nanoparticle (MSN)-based nanoplatform has attracted growing attention in the biomedical field due to the unique characteristics of MSNs including a high surface area, tunable pore sizes, colloidal stability, ease of functionalization, and desirable biocompatibility. Typically, MSNs are designed as nanocarriers for the incorporation of a variety of contrast agents for bioimaging, which can address the intrinsic drawbacks of contrast agents, including poor solubility in water, rapid photobleaching, and low stability. This review summarizes the recent advances in the field of MSN-based nanoprobes for fluorescence imaging and photoacoustic (PA) imaging applications. The approaches for the incorporation of contrast agents into MSN-based nanoplatforms including encapsulating contrast agents within MSNs, covalently conjugating contrast agents on the surface or pores of MSNs, physically absorbing contrast agents in the pores of MSNs, and doping contrast agents in the framework of MSNs are introduced. MSN-based nanoprobes for fluorescence imaging and PA imaging are discussed. The enhanced fluorescence imaging and PA imaging performances of MSN-based nanoprobes relative to the bare contrast agents are introduced and the underlying mechanisms are discussed in detail. Finally, current challenges and perspectives of MSN-based nanoprobes in the bioimaging field are discussed.

Photoacoustic Imaging and Photothermal Therapy of Semiconducting Polymer Nanoparticles: Signal Amplification and Second Near-Infrared Construction

Photoacoustic (PA) imaging and photothermal therapy (PTT) have attracted extensive attention in disease diagnosis and treatment. Although many exogenous contrast agents have been developed for PA imaging and PTT, the design guidelines to amplify their imaging and therapy performances remain challenging and are highly demanded. Semiconducting polymer nanoparticles (SPNs) composed of polymers with π-electron delocalized backbones can be designed to amplify their PA imaging and PTT performance, because of their clear structure-property relation and versatility in modifying their molecular structures to tune their photophysical properties. This review summarizes the recent advances in the photoacoustic imaging and photothermal therapy applications of semiconducting polymer nanoparticles with a focus on signal amplification and second near-infrared (NIR-II, 1000-1700 nm) construction. The strategies such as structure-property screening, fluorescence quenching, accelerated heat dissipation, and size-dependent heat dissipation are first discussed to amplify the PA brightness of SPNs for in vivo PA. The molecular approaches to shifting the absorption of SPNs for NIR-II PA imaging and PTT are then introduced so as to improve the tissue penetration depth for diagnosis and therapy. At last, current challenges and perspectives of SPNs in the field of imaging and therapy are discussed.

Atomic structure causing an obvious difference in thermal conductance at the Pd-H2O interface: a molecular dynamics simulation

Thermal transfer across solid-liquid interfaces is influenced by multiple factors such as surface wettability, interfacial water layer density, molecular structure, and mass density depletion length. However, the dominant factors in interfacial heat transport are yet to be investigated. In this work, we explore the contributions from these factors by employing the Pd-water model for water molecules forming ordered, partially ordered, and disordered structures on Pd (100), (110) and (111) surfaces, respectively. The results revealed that the ordered water layer on the (100) surface can introduce a "phonon bridge" at the solid-liquid interface to improve thermal transfer, while the partially ordered water layer on the (110) surface can further promote thermal transfer due to the enhanced interfacial friction. On the other hand, the decreased density depletion length also makes dominant contributions to the enhancement of interfacial thermal transfer. The results are explained by the interfacial friction coefficient, surface potential energy distribution and density depletion length. We also introduce an efficient technique by tuning the vacancy defects on the solid surface to tune the atomic structure as well as the thermal transfer. Our study reveals the complex relationship between the atomic structure of the crystal face, the water layer structure and the thermal boundary conductance, which will inspire more experimental and theoretical studies toward the improvement of interfacial thermal transport by tuning the structure of the water layer.

Organic Semiconducting Agents for Deep-Tissue Molecular Imaging: Second Near-Infrared Fluorescence, Self-Luminescence, and Photoacoustics

Optical imaging has played a pivotal role in biology and medicine, but it faces challenges of relatively low tissue penetration and poor signal-to-background ratio due to light scattering and tissue autofluorescence. To overcome these issues, second near-infrared fluorescence, self-luminescence, and photoacoustic imaging have recently emerged, which utilize an optical region with reduced light-tissue interactions, eliminate real-time light excitation, and detect acoustic signals with negligible attenuation, respectively. Because there are only a few endogenous molecules absorbing or emitting above the visible region, development of contrast agents is essential for those deep-tissue optical imaging modalities. Organic semiconducting agents with π-conjugated frameworks can be synthesized to meet different optical imaging requirements due to their easy chemical modification and legible structure-property relation. Herein, the deep-tissue optical imaging applications of organic semiconducting agents including small-molecule agents and nanoparticle derivatives are summarized. In particular, the molecular engineering and nanoformulation approaches to further improve the tissue penetration and detection sensitivity of these optical imaging modalities are highlighted. Finally, current challenges and potential opportunities in this emerging subfield of biomedical imaging are discussed.

Solid-Liquid Thermal Transport and Its Relationship with Wettability and the Interfacial Liquid Structure

Experiments and atomistic simulations have suggested the existence of a direct correlation between the wetting properties of a surface and heat transfer across it. In this investigation, molecular dynamics simulations of surface wettability and solid-liquid thermal transport were conducted in order to better understand the relationship between the surface chemistry and thermal transport. The wettability transparency of graphene-coated surfaces was considered in order to investigate heat transfer across a complex interface with similar wettability as a bare surface. The results indicate that the relationship between the interfacial heat transfer and wettability is not universal. The density depletion length was found to reconcile the thermal boundary conductance calculations for different bare and graphene-coated silicon surfaces.

Local thermal transport of liquid alkanes in the vicinity of α-quartz solid surfaces and thermal resistance over the interfaces: A molecular dynamics study

Thermal transport in liquid n-alkanes in the vicinity of α-quartz substrates and thermal boundary resistance between the liquid n-alkanes and the α-quartz substrates have been investigated using nonequilibrium molecular dynamics simulations. The study considers two liquid alkanes, methane and decane, and three crystal orientations of α-quartz substrate terminated with -H or -OH groups. The local thermal conductivity (LTC), defined in the same manner as with macroscopic thermal conductivity, is used to measure the efficiency of thermal energy transport of the liquids in the vicinity of the solid surface. The variations in the LTC of the liquid alkanes in the layered region next to the surface of the substrate were examined. The modeled LTC values of the alkanes were found to oscillate in the solid-liquid interface region. These fluctuations were typically proportional to the oscillations in the local density profile. The correlation between the thermal conductivity and density was linear in the bulk liquid region. The correlation between LTC and local density in the first adsorption layer is not a straightforward extension of that of the bulk liquid, which is mostly due to the specific molecular-scale ordering structure that occurs in the liquids formed by the proximity of the solid substrate. Thermal boundary resistance between the liquid alkanes and the quartz substrate was also evaluated. It was observed that thermal boundary resistance is relatively large when the in-plane molecular-scale structure in the first adsorption layer is sparse, and is lower when the liquid structure is dense in the adsorption layer.

Water nanoconfinement induced thermal enhancement at hydrophilic quartz interfaces

We report the effect of water nanoconfinement on the thermal transport properties of two neighbor hydrophilic quartz interfaces. A significant increase and a nonintuitive, nonmonotonic dependence of the overall interfacial thermal conductance between the quartz surfaces on the water layer thickness were found. By probing the interfacial structure and vibrational properties of the connected components, we demonstrated that the mechanism of the peak occurring at submonolayer water originates from the freezing of water molecules at extremely confined conditions and the excellent match of vibrational states between trapped water and hydrophilic headgroups on the two contact surfaces. Our results show that incorporation of polar molecules into hydrophilic interfaces is very promising to enhance the thermal transport through thermally smooth connection of these interfaces.