Bulk Liquids Research Articles

Search for a Grotthuss mechanism through the observation of proton transfer

The transport of protons is critical in a variety of bio- and electro-chemical processes and technologies. The Grotthuss mechanism is considered to be the most efficient proton transport mechanism, generally implying a transfer of protons between ‘chains’ of host molecules via elementary reactions within the hydrogen bonds. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has been observed. Here we report the first experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid. Employing dielectric spectroscopy, quasielastic neutron, and light scattering, and ab initio molecular dynamic simulations we determined that protons move by surprisingly short jumps of only ~0.5–0.7 Å, much smaller than the typical ion jump length in ionic liquids. Our analysis confirms the existence of correlations in these proton jumps. However, these correlations actually reduce the conductivity, in contrast to a desirable enhancement, as is usually assumed by a Grotthuss mechanism. Furthermore, our analysis suggests that the expected Grotthuss-like enhancement of conductivity cannot be realized in bulk liquids where ionic correlations always decrease conductivity.

Amoeba-Inspired Magnetic Venom Microrobots.

Nature provides a successful evolutionary direction for single-celled organisms to solve complex problems and complete survival tasks - pseudopodium. Amoeba, a unicellular protozoan, can produce temporary pseudopods in any direction by controlling the directional flow of protoplasm to perform important life activities such as environmental sensing, motility, predation, and excretion. However, creating robotic systems with pseudopodia to emulate environmental adaptability and tasking capabilities of natural amoeba or amoeboid cells remains challenging. Here, this work presents a strategy that uses alternating magnetic fields to reconfigure magnetic droplet into Amoeba-like microrobot, and the mechanisms of pseudopodia generation and locomotion are analyzed. By simply adjusting the field direction, microrobots switch in monopodia, bipodia, and locomotion modes, performing all pseudopod operations such as active contraction, extension, bending, and amoeboid movement. The pseudopodia endow droplet robots with excellent maneuverability to adapt to environmental variations, including spanning 3D terrains and swimming in bulk liquids. Inspired by the "Venom," the phagocytosis and parasitic behaviors have also been investigated. Parasitic droplets inherit all the capabilities of amoeboid robot, expanding their applicable scenarios such as reagent analysis, microchemical reactions, calculi removal, and drug-mediated thrombolysis. This microrobot may provide fundamental understanding of single-celled livings, and potential applications in biotechnology and biomedicine.

How Does Electronic Polarizability or Scaled-Charge Affect the Interfacial Properties of Room Temperature Ionic Liquids?

The room temperature ionic liquid (RTIL) air-liquid interface plays an important role in many applications. Herein, we present molecular dynamics simulation results for the air-liquid interface of a common RTIL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [C4mim][NTf2]. To elucidate the effects of electronic polarizability and scaled-charge ions on the properties of the RTIL air-liquid interface, we employ three different kinds of force fields: a nonpolarizable force field (FF) with united ion charges (FixQ), a nonpolarizable FF with scaled-charge by 0.8 (ScaleQ), and a polarizable FF (Drude). To identify whether the ions reside at the interface or not, the method of identification of the truly interfacial molecules is used. The structural and dynamical properties in the interfacial, subinterfacial, and central layers are evaluated. In general for bulk liquids, the FixQ model predicts too-ordered structures and too-sluggish dynamics, while the ScaleQ model can serve as a simple cure. However, the ScaleQ model cannot reproduce the results of the Drude model at the interface, due to an inappropriate scaled-down charge near the interface.

AC/DC Thermal Nano-Analyzer Compatible with Bulk Liquid Measurements

Nanocalorimetry, or thermal nano-analysis, is a powerful tool for fast thermal processing and thermodynamic analysis of materials at the nanoscale. Despite multiple reports of successful applications in the material sciences to study phase transitions in metals and polymers, thermodynamic analysis of biological systems in their natural microenvironment has not been achieved yet. Simply scaling down traditional calorimetric techniques, although beneficial for material sciences, is not always appropriate for biological objects, which cannot be removed out of their native biological environment or be miniaturized to suit instrument limitations. Thermal analysis at micro- or nano-scale immersed in bulk liquid media has not yet been possible. Here, we report an AC/DC modulated thermal nano-analyzer capable of detecting nanogram quantities of material in bulk liquids. The detection principle used in our custom-build instrument utilizes localized heat waves, which under certain conditions confine the measurement area to the surface layer of the sample in the close vicinity of the sensing element. To illustrate the sensitivity and quantitative capabilities of the instrument we used model materials with detectable phase transitions. Here, we report ca. 106 improvement in the thermal analysis sensitivity over a traditional DSC instrument. Interestingly, fundamental thermal properties of the material can be determined independently from heat flow in DC (direct current) mode, by using the AC (alternating current) component of the modulated heat in AC/DC mode. The thermal high-frequency AC modulation mode might be especially useful for investigating thermal transitions on the surface of material, because of the ability to control the depth of penetration of AC-modulated heat and hence the depth of thermal sensing. The high-frequency AC mode might potentially expand the range of applications to the surface analysis of bulk materials or liquid-solid interfaces.

Mechanistic Insights into Proton Transport in Pure and Aqueous Phosphoric Acid

The transport of protons plays a key role in a variety of electrochemical processes and technologies. There are two major mechanisms underlying proton transport: the vehicular mechanism where a protonic defect moves with the aid of a molecular entity; and the ‘Grotthuss mechanism’ where a proton diffuses by being transferred between ‘chains’ of host molecules via elementary reactions within the hydrogen bond networks. The latter is regarded as the most efficient proton conductivity mechanism. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has thus far been reported. Recently, we observed the first direct experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid by employing dielectric spectroscopy, and quasi-elastic neutron scattering (QENS). Complementary to these experiments, ab initio molecular dynamics (AIMD) simulations were performed to gain molecular insight into the underlying mechanisms of proton transport allowing for the breaking and forming of covalent bonds since it calculates the forces from electronic structure methods ‘on the fly’.We calculated the self-intermediate scattering function of the protons, indicating three processes at high Q range consisting of two fast Q-independent processes and one slow Q-dependent process, which are in good agreement with QENS results. Also, the self-part of van Hove function of the protons for the fast characteristic time shows surprisingly short proton jumps of only ~0.5 - 0.7 Å confirmed by analysis of an individual proton trajectory. Furthermore, AIMD demonstrates that proton hopping plays an essential role in the long-range proton diffusion through the breaking and forming of hydrogen bonds. Our analysis confirmed the strong proton-proton correlations in these proton jumps. However, this correlated proton transport unexpectedly leads to a decrease of conductivity in these systems. Based on these results, we propose that the expected Grotthuss-like enhancement mechanism of conductivity cannot be realized in bulk liquids where ionic correlations always reduce conductivity, most likely because of the requirement for the momentum conservation. Our findings will propel the design of electrochemical materials with proton transfers involved in charge transport for achieving high proton conductivity in electrochemical applications including fuel cells.

Ultrafast Dynamics of Solute Molecules Probed by Resonant Optical Kerr Effect Spectroscopy.

Ultrafast molecular dynamics in fluids is of great importance in many biological and chemical systems. Although such dynamics in bulk liquids has been explored by various methods, experimental tools that unveil the dynamics of solvated solutes are limited. In this work, we have developed resonant optical Kerr effect spectroscopy (ROKE), which is an analogue of optical Kerr effect spectroscopy that measures the reorientational relaxation of a dilute solute in solution. By adjusting the pump and probe wavelengths at the resonant absorption band of a solute, the time response of the solute was distinguished easily from the negligible signal of the solvent. The heterodyne detection of ROKE enables the determination of reorientational relaxation time constants with an accuracy of 2.6%. The signal-to-noise ratio was high enough (average ∼26.7) to obtain an adequate signal from even a 10 μM solution. Thus, ROKE is a powerful tool to study solute dynamics with high sensitivity in a broad range of applications.

Broad chemical transferability in structure-based coarse-graining.

Compared to top-down coarse-grained (CG) models, bottom-up approaches are capable of offering higher structural fidelity. This fidelity results from the tight link to a higher resolution reference, making the CG model chemically specific. Unfortunately, chemical specificity can be at odds with compound-screening strategies, which call for transferable parameterizations. Here, we present an approach to reconcile bottom-up, structure-preserving CG models with chemical transferability. We consider the bottom-up CG parameterization of 3441 C7O2 small-molecule isomers. Our approach combines atomic representations, unsupervised learning, and a large-scale extended-ensemble force-matching parameterization. We first identify a subset of 19 representative molecules, which maximally encode the local environment of all gas-phase conformers. Reference interactions between the 19 representative molecules were obtained from both homogeneous bulk liquids and various binary mixtures. An extended-ensemble parameterization over all 703 state points leads to a CG model that is both structure-based and chemically transferable. Remarkably, the resulting force field is on average more structurally accurate than single-state-point equivalents. Averaging over the extended ensemble acts as a mean-force regularizer, smoothing out both force and structural correlations that are overly specific to a single-state point. Our approach aims at transferability through a set of CG bead types that can be used to easily construct new molecules while retaining the benefits of a structure-based parameterization.

Real-Time Offshore Supply Vessel Routing Problem with Random Service Requests

The routing of supply vessels to offshore platforms is one of the major tasks in upstream logistics planning in the oil and gas industry. This task involves the formulation and solution of a real-time variant of the stochastic dynamic vehicle routing problem, which reflects the operational environment of offshore platforms in practice. In this paper, a novel mixed integer linear programming-based heuristic is presented with a view to determining the optimal route of a single supply vessel allocated to provide service to a group of offshore platforms through the supply of spare parts, liquid bulk liquids, and other materials, allowing for random service requests by the offshore platform group. The proposed heuristic is applied to a real-world case study of vessel routing to offshore platforms in the South Atlantic Ocean Shelf. The computational performance of the heuristic, as assessed by the degree of dynamism, is found to be very good.

Orientational wetting and dynamical correlations toward glass transition on the surface of imidazolium-based ionic liquids

Surface induces many fascinating physical phenomena, such as dynamic acceleration, surface anchoring, and orientational wetting, and, thus, is of great interest to study. Here, we report classic molecular dynamics simulations on the free-standing surface of imidazolium-based ionic liquids (ILs) [C4mim][PF6] and [C10mim][PF6]. On [C10mim][PF6] surface, a significant orientational wetting is observed, with the wetting strength showing a diverging tendency. Depth of the wetting was captured from the density and orientational order profile by a static length, which remarkably increases below the temperature Tstat upon cooling down. The dynamical correlation length that measures the distance of surface-dynamics acceleration into the bulk was characterized via the spatial-dependent mobility. The translational correlation exhibits a similar drastic increment at Tstat, while the rotational correlation drastically increases at a lower temperature Trot. We connect these results to the dynamics in bulk liquids, by finding Tstat and Trot that correspond to the onset temperatures where the liquids become cooperative for translational and rotational relaxation, respectively. This signifies the importance of collective dynamics in the bulk on the orientational wetting and surface dynamics in the ILs.

Linking intermolecular interactions and rheological behaviour in capillary suspensions

HypothesisCapillary suspensions feature networks of particles connected by liquid bridges, which are obtained by adding a small amount of a second immiscible liquid to a suspension. It is possible to link the network formation as well as the rheological behaviour of capillary suspensions to the intermolecular interactions of their constituents. Experiments and simulationsThrough a combination of experimental and numerical methods, we present a novel approach, based on Hansen solubility parameters computed from Molecular Dynamics (MD) simulations, to rationalize and predict the rheological behaviour of capillary suspensions. We investigated the formation of capillary suspensions for various combinations of bulk and secondary liquids mixed with hydrophilic silica particles. The predictions were confirmed experimentally by rheological analysis, interfacial tension measurements and microscopy (CLSM) imaging. FindingsNumerical and experimental results show that the Hansen solubility parameters theory allows to predict the formation of capillary suspensions, whose strength exponentially decays with decreasing intermolecular interactions between the secondary liquids and the dispersed particles. High immiscibility between the bulk and secondary liquid strengthens the gel up to a critical immiscibility point, above which the strength of the gel remains mostly affected by the affinity between the secondary liquids and the dispersed particles. Furthermore, we find that hydrogen-bonding and polar interactions control the formation of capillary suspensions. This simple approach can guide the selection of adequate solvents and immiscible secondary liquids, allowing an easy formulation of new particulate-based gels.

The fate and dynamics of iron during the transformation of activated sludge into oxygenic photogranules (OPGs) under hydrodynamic batch conditions for environmental applications

Oxygenic photogranules (OPGs) are dense, spherical structures containing filamentous cyanobacteria, microalgae, and non-phototrophic bacteria. Besides hydrostatic batch conditions, OPGs can be produced directly from activated sludge under illuminated hydrodynamic batch conditions. The role of Fe in these hydrodynamic batch conditions is still unknown. In this study, four replicate hydrodynamic batches with two times diluted Amherst and Hadley activated sludges were operated under continuous illumination (126 ± 9 µmol/m2-s) and mixing (20 rpm) for a period of 19 days. Illumination and initial anaerobic condition development led to the rapid release of Fe, especially Fe (II), into the bulk liquids. However, this Fe pool quickly declined and remained at overall steady values. The Fe linked with biomass-bound extracellular polymeric substances (bEPS-Fe) continued to decline during batches and eventually reached stable levels. Cyanobacterial growth exhibited moderate to strong negative correlations to both bEPS and bulk-liquid Fe pools whereas microalgal growth showed more dependence on bulk-liquid Fe compared to bEPS-Fe. The Fe distribution analysis revealed higher Fe content in the biomass pellet fraction, followed by bEPS extract and bulk liquid fraction. Increases in the level of pelletized Fe, over the course of photogranulation, are likely due to increases in intracellular Fe content. Overall, the study demonstrates that the limitation of available Fe in bEPS and bulk liquid fractions encouraged the formation of photogranules in hydrodynamic batch conditions. These observations are similar to earlier reported batch hydrostatic cultivation, which further supports the shared photogranulation phenomena in the two batch conditions.

Ab Initio Cluster Approach for High Harmonic Generation in Liquids.

High harmonic generation (HHG) takes place in all phases of matter. In gaseous atomic and molecular media, it has been extensively studied and is very well understood. In solids, research is ongoing, but a consensus is forming for the dominant microscopic HHG mechanisms. In liquids, on the other hand, no established theory yet exists, and approaches developed for gases and solids are generally inapplicable, hindering our current understanding. We develop here a powerful and reliable ab initio cluster-based approach for describing the nonlinear interactions between isotropic bulk liquids and intense laser pulses. The scheme is based on time-dependent density functional theory and utilizes several approximations that make it feasible yet accurate in realistic systems. We demonstrate our approach with HHG calculations in water, ammonia, and methane liquids and compare the characteristic response of polar and nonpolar liquids. We identify unique features in the HHG spectra of liquid methane that could be utilized for ultrafast spectroscopy of its chemical and physical properties, including a structural minimum at 15–17 eV that is associated solely with the liquid phase. Our results pave the way to accessible calculations of HHG in liquids and illustrate the unique nonlinear nature of liquid systems.

Molecular Origin of Driving-Dependent Friction in Fluids.

The friction coefficient of fluids may become a function of the velocity at increased external driving. This non-Newtonian behavior is of general theoretical interest and of great practical importance, for example, for the design of lubricants. Although the effect has been observed in large-scale atomistic simulations of bulk liquids, its theoretical formulation and microscopic origin are not well understood. Here, we use dissipation-corrected targeted molecular dynamics, which pulls apart two tagged liquid molecules in the presence of surrounding molecules, and analyze this nonequilibrium process via a generalized Langevin equation. The approach is based on a second-order cumulant expansion of Jarzynski's identity, which is shown to be valid for fluids and therefore allows for an exact computation of the friction profile as well of the underlying memory kernel. We show that velocity-dependent friction in fluids results from an intricate interplay of near-order structural effects and the non-Markovian behavior of the friction memory kernel. For complex fluids such as the model lubricant C40H82, the memory kernel exhibits a stretched-exponential long-time decay, which reflects the multitude of timescales of the system.

Environmental Exploration of Ultra-Dense Nanobubbles: Rethinking Sustainability

Nanobubbles are nanoscopic gaseous domains than can exist on solid surfaces or in bulk liquids. They have attracted significant attention in the last decade due to their long-time (meta)stability and ready potential for real-world applications, especially in environmental engineering and more sustainable ecosystems, water treatment, irrigation, and crop growth. After reviewing important nano-bubble science and activity, with some of the latest promising results in agriculture, we point out important directions in applications of nano-bubble phenomena for boosting sustainability, with viewpoints on how to revolutionise best-practice environmental and green sustainability, taking into account economic drivers and impacts. More specifically, it is pointed out how nanobubbles may be used as delivery vehicles, or “nano-carriers”, for nutrients or other agents to specific targets in a variety of ecosystems of environmental relevance, and how core this is to realising a vision of ultra-dense NBs in shaping a positive and lasting impact on ecosystems and our natural environment.

Русская и украинская историческая метрология и её отражение в паремиях

The present article is devoted to the historically metrology of Russian and Ukrainian people, the emergence and use of different units for measuring length, width, area, weight and volume of bulk solids and liquids, etc. and their reflection in the proverbs of these nations.

Spectral and intensity control of high energy terahertz radiation from bulk liquids

High power, broadband terahertz (THz) radiation from liquids, excited by intense, femtosecond 800 nm laser pulses has been demonstrated recently, overturning the long held belief that liquids would not give such emission due to their absorption in the THz region. Given the widespread interest in the use of THz radiation for several applications and the energy and bandwidth limitations of existing sources, liquids are expected to attract great attention in the future. While the emission at tens of microjoules from liquids is very promising, it is important to explore whether control of the THz flux and spectrum is achievable by manipulating the laser or liquid parameters. In this paper we present results on manipulating the spectrum of THz radiation from liquids by chirping the input laser pulse and optimizing the THz output energy by laser chirp as well as optimizing the focal position. We demonstrate tunability by varying the chirp of the laser pulse and show that the THz emission predominantly comes from the region near the end of the liquid path, consistent with the absorption of THz radiation by liquids. This control gives us an opportunity to tune the THz radiation to suit experimental needs. We present simulations that support the results.

Spinnoise gradient echoes.

Nuclear spinnoise spectroscopy in the absence of radio frequency pulses was studied under the influence of pulsed field gradients (PFGs) on pure and mixed liquids. Under conditions where the radiation-damping-induced line broadening is smaller than the gradient-dependent inhomogeneous broadening, echo responses can be observed in difference spectra between experiments employing pulsed field gradient pairs of the same and opposite signs. These observed spinnoise gradient echoes (SNGEs) were analyzed through a simple model to describe the effects of transient phenomena. Experiments performed on high-resolution nuclear magnetic resonance (NMR) probes demonstrate how refocused spin noise behaves and how it can be exploited to determine sample properties. In bulk liquids and their mixtures, transverse relaxation times and translational diffusion constants can be determined from SNGE spectra recorded following tailored sequences of magnetic field gradient pulses.

Rheological behaviour of IoNanofluids based on [emim][DCA] and [emim][TCM

The viscosity of ionanofluids consisting of the two ionic liquids (ILs), 1-Ethyl-3-methylimidazolium dicyanamide ([emim][DCA]) and 1-Ethyl-3-methylimidazolium tricyanomethanide ([emim][TCM]), and aluminium oxide and magnesium oxide nanoparticles has been measured. The pure ionic liquids were also measured, and a simple Andrade equation and a Vogel-Tammann-Fulcher equation were presented to predict the temperature dependency of the ILs’ viscosity with a good accuracy for both fluids (R2 > 0.99). The measurements for both the pure ILs as well as the ionanofluids were performed over the temperature range of 295.15–353.15 K. The rheological behaviour was also examined for ILs as well as for the ionanofluids. For the ionanofluids two different particle sizes were investigated to see if a general trend could be found which was applied to both the bulk liquids. Yet this was not the case. The pure ILs were found to be perfectly Newtonian which supports the research data of other research groups. The ionanofluids were found to consist of a mixture of Newtonian and non-Newtonian fluids. All the non-Newtonian fluids displayed pseudoplastic behaviour which was determined through the Power Law Index as well as visual examination of graphs displaying the viscosity along various shear rates. Both theoretical and empirical models were applied to the ionanofluids to examine if the prediction would match the measured data, though the predictions were not accurate. The effect of time was also investigated using hysteresis-loops at various temperatures as well as extended measurements where the samples were heated and cooled. Time dependency was not found in the classical form of thixotropy or rheopexy but a trend of increasing viscosity at sufficiently low shear rates and high temperatures was detected. These are believed to be due to changes of the internal structure that the nanoparticles assemble within the fluid.

Time-Variable Chiroptical Vibrational Sum-Frequency Generation Spectroscopy of Chiral Chemical Solution.

Vibrational sum-frequency generation (VSFG) spectroscopy, a surface-specific technique, was shown to be useful even for characterizing the vibrational optical activity of chiral molecules in isotropic bulk liquids. However, accurately determining the spectroscopic parameters is still challenging because of the spectral congestion of chiroptical VSFG peaks with different amplitudes and phases. Here, we show that a time-variable infrared-visible chiroptical three-wave-mixing technique can be used to determine the spectroscopic parameters of second-order vibrational response signals from chiral chemical liquids. For varying the delay time between infrared and temporally asymmetric visible laser pulses, we measure the chiral VSFG, achiral VSFG, and their interference spectra of bulk R-(+)-limonene liquid and perform a global fitting analysis for those time-variable spectra to determine their spectroscopic parameters accurately. We anticipate that this time-variable VSFG approach will be useful for developing nearly background-free chiroptical characterization techniques with enhanced spectral resolution.

Spray characteristics of an ultrasonic microdroplet generator with a continuously variable operating frequency.

Droplet spraying is utilized in diverse industrial processes and biomedical applications, including nanomaterial synthesis, biomaterial handling, and inhalation drug delivery. Ultrasonic droplet generators transfer energy into bulk liquids using acoustic waves to disrupt the free liquid surface into fine microdroplets. We previously established a method combining ultrasonic actuation, resonant operation, and acoustic wave focusing for efficient spraying of various liquids (e.g., low surface tension fuels, high viscosity inks, and suspensions of biological cells). The microfabricated device comprises a piezoelectric transducer, sample reservoir, and an array of acoustic horn structures terminated by microscale orifices. Orifice size roughly dictates droplet diameter, and a fixed reservoir height prescribes specific device resonant frequencies of operation. Here, we incorporate a continuously variable liquid reservoir height for dynamic adjustment of operating parameters to improve spray efficiency in real-time and potentially tune the droplet size. Computational modeling predicts the system harmonic response for a range of reservoir heights from 0.5 to 3 mm (corresponding to operating frequencies from ∼500 kHz to 2.5 MHz). Nozzle arrays with 10, 20, and 40 μm orifices are evaluated for spray uniformity and stability of the active nozzles, using model predictions to explain the experimental observations.