Prediction of Contact Angles and Density Profiles of Sessile Droplets Using Classical Density Functional Theory Based on the PCP-SAFT Equation of State.

This study demonstrates the capability of the density functional theory (DFT) formalism to predict contact angles and density profiles of model fluids and of real substances in good quantitative agreement with molecular simulations and experimental data. The DFT problem is written in cylindrical coordinates, and the solid-fluid interactions are defined as external potentials toward the fluid phase. Monte Carlo (MC) molecular simulations are conducted in order to assess the density profiles resulting from the Helmholtz energy functional used in the DFT formalism. Good quantitative agreement between DFT predictions and MC results for Lennard-Jones and ethane nanodroplets is observed, both for density profiles and for contact angles. That comparison suggests, first, that the Helmholtz energy functional proposed in a previous study [ Sauer , E. ; Gross , J. Ind. Eng. Chem. Res. 56 , 2017 , 4119 - 4135 ] is suitable for three-phase contact lines and, second, that Lagrange multipliers can be used to constrain the number of molecules, similar to a canonical ensemble. Experiments of sessile droplets on solid surfaces are performed to assess whether a real solid with its microscopic roughness can be described through a simple model potential. Comparison of DFT results to experimental data is done for a Teflon surface because Teflon can be regarded as a substrate exhibiting only attractive interactions of van der Waals type. It is shown that the real solid can be described as a perfectly planar solid with effective solvent-to-solid interactions, defined through a single adjustable parameter for the solid. Subsequent predictions for the contact angle of eight solvents, including polar components such as water, are found in very good agreement to experimental data using simple Berthelot-Lorentz combining rules. For the eight investigated solvents, we find mean absolute deviations of 3.77°.

In Situ Tracking of Wetting-Front Transient Heat Release on a Surface-Mounted Metal-Organic Framework.

Transient heat generation during guest adsorption and host-guest interactions is a natural phenomenon in metal-organic framework (MOF) chemistry. However, in situ tracking of such MOF released heat is an insufficiently researched field due to the fast heat dissipation to the surroundings. Herein, a facile capillary-driven liquid-imbibition approach is developed for in situ tracking of transient heat release at the wetting front of surface-mounted MOFs (SURMOFs) on cellulosic fiber substrates. Spatiotemporal temperature distributions are obtained with infrared thermal imaging for a range of MOF-based substrates and imbibed liquids. Temperature rises at the wetting front of water and binary mixtures with organic solvents are found to be over 10 K with an ultrafast and distinguishable thermal signal response (<1 s) with a detectable concentration limit ≤1 wt%. As an advancement to the state-of-the-art in trace-solvent detection technologies, this study shows great prospects for the integration of SURMOFs in future sensor devices. Inspired by this prototypal study, SURMOF-based transient heat signal transduction is likely to be extended to an ever-expanding library of SURMOFs and other classes of surface-grafted porous materials, translating into a wide range of convenient, portable, and ubiquitous sensor devices.

Simultaneous optical and infrared thermal imaging of isotachophoresis.

Joule heating in isotachophoresis (ITP) can limit minimum assay times and efforts to scale up processed sample volumes. Despite its significance, the dynamics of Joule heating on spatiotemporal temperature fields in ITP systems have not been investigated. We here present novel measurements of spatiotemporal temperature and electromigration fields in ITP. To achieve this, we obtain simultaneous and registered optical and infrared thermal images of the ITP process. We conduct a series of experiments at constant current operation and vary the leading electrolyte concentration to study and highlight the importance of buffer-dependent ionic conductivity on the resulted temperature rise. The measurements demonstrate a substantial increase of temperature in the adjusted trailing electrolyte region, and the propagation of a thermal wave in the ITP channel with a velocity equal to that of the electromigration front. We present scaling of the experimental data that indicates the dependence of front velocity and temperature rise on current density and ionic conductivity. The current study has direct application to the design and optimization of scaled-up ITP systems and the validation of numerical models of Joule heating.

Occurrence of temperature spikes at a wetting front during spontaneous imbibition.

It is reported that temperature rises at wetting front during water infiltration into soil. The temperature goes back to the background value after passage of water front. Different explanations have been provided for source of energy causing temperature spike. Some have contributed it to heat of condensation released due to condensation of vapor on "dry" solid surface. Some other stated that the heat of wetting or heat of adsorption is responsible for the temperature rise. In this research, we revisited this issue. First, we provide a comprehensive review about occurrence of temperature spike at a wetting front. Then, we report about experiments we performed on the rise of water in dry paper. Using infrared and optical imaging techniques, we could monitor temperature changes in time and space. For all samples maximum temperature rise occurred at the wetting front. The magnitude of temperature spike depended on paper material, thickness, and liquid composition. It was larger for cellulose-fiber-based paper than for plastic-based paper. For a given paper type, thicker samples showed a larger temperature spike. Adding salt to the water caused reduction of temperature spike. It was concluded that replacement of air-solid interface with water-solid interface releases energy, which causes temperature rise.