Machine learning coarse-grained models of dissolutive wetting: a droplet on soluble surfaces.

Dissolutive wetting is not only a key problem in application fields such as energy, medicine, micro-devices and etc., but also a frontier issue of academic research. As an important tool for exploring the micro-mechanisms of dissolutive wetting, molecular dynamics simulations are limited by simulation scale and force field parameters. Thus, artificial intelligence is introduced into the multi-scale simulation framework to tackle such challenges. By combining density functional theory, molecular dynamics simulations and experiments, we obtain a coarse-grained model of the glucose-water dissolution pair. Furthermore, the structure of the solid molecules and the hydration shell near the solute particles are calculated by quantum mechanics/molecular mechanics to verify the accuracy of the model. Finally, the applicability of the coarse-grained model in dissolutive wetting is proven by experimental results. We believe our machine learning method not only lays a foundation for exploring the micro-mechanisms of dissolutive wetting, but also provides a general approach for obtaining the force field parameters of different systems.

Formation of Deposition Patterns Induced by the Evaporation of the Restricted Liquid.

Evaporation-induced self-assembly of colloids or suspensions has received increasing attention. Given its critical applications in many fields of science and industry, we report deposition patterns constructed by the evaporation of the restricted aqueous suspension with polystyrene particles at different substrate temperatures and geometric container dimensions. With the temperature increases, the deposition patterns transition from honeycomb to multiring to island, which is attributed to the competition between the particle deposition rate UP and the contact line velocity UCL, and the dimension of the geometric container has an effect on the characteristics of patterns. In this paper, the formation of an ordered multiring pattern is mainly focused on as a result of UP keeping up with UCL such that the entire contact line can be pinned, that is, the periodic stick-slip motion of the contact line and the particle sedimentation. Moreover, based on the Onsager principle, we develop a theoretical model to reveal the physical mechanisms behind the multiring phenomena. The position and spacing of rings are measured, which shows that the theoretical prediction agrees well with experiments. We also find that the ring spacing decays exponentially from center to edge experimentally and theoretically. This may not only help us to understand the formation of the deposition patterns but also assist future design and control in practical applications.

Topography-induced symmetry transition of droplets on quasi-periodically patterned surfaces.

Quasi-periodic structures of quasicrystals yield novel effects in diverse systems. However, there is little investigation on employing quasi-periodic structures in morphology control. Here, we show the use of quasi-periodic surface structures in controlling the transition of liquid droplets. Although surface structures seem random-like, we find that on these surfaces, droplets spread to well-defined 5-fold symmetric shapes and the symmetry of droplet shapes spontaneously restores during spreading, hitherto unreported in the morphology control of droplets. To obtain physical insights into these symmetry transitions, we conduct energy analysis and perform systematic experiments by varying the properties of both liquid droplets and patterned surfaces. The results show the dominant factors in determining droplet shapes to be surface topography and the self-similarity of the surface structure. Quantified results of the droplet spreading process show distinct dynamics from the spreading experiments on periodically micropatterned surfaces. Our findings significantly advance the control capability of the droplet morphology. Such a quasi-periodic patterning strategy can offer a new method to achieve complex patterns, and may be used to model patterns in the study of rough surfaces.

Dynamics of Dissolutive Wetting: A Molecular Dynamics Study.

Dissolutive wetting, i.e., dynamic wetting of a liquid droplet on dissolvable substrates, has been studied by molecular dynamics simulations. In dissolutive wetting, the geometry and properties of the solid-liquid interface evolve with the solid dissolving into the droplet; meanwhile, the droplet spreads on the receding solid surfaces. The droplets advance on the dissolvable substrate following different dynamic laws, compared with spreading on nondissolutive substrate. On the basis of molecular kinetic theory, we develop a theoretical model to reveal physical mechanisms behind the dissolutive wetting phenomena. We also find that solid particles are pulled by their hydration shells to dissolve into liquid, changing the flow field, the atomic structure, and the hydrogen bond network in the droplet. Our findings may help to comprehend the dynamics of dissolutive wetting and assist future design in practical applications.

Molecular dynamics simulations of the enhanced recovery of confined methane with carbon dioxide.

For the first time, the enhanced recovery of confined methane (CH4) with carbon dioxide (CO2) is investigated through molecular dynamics simulations. The adsorption energy and configuration of CH4 and CO2 on the carbon surface were compared, which shows that CO2 is a good candidate in displacing confined CH4. The energy barrier required for displacing CH4 by CO2 injection was found to depend on the displacement angle. When CO2 approached vertically to the carbon surface, the displacement of CH4 occurred most easily. The curvature and size effects of the carbon nanopores on CH4 recovery were revealed and indicated that there exists an optimum pore size making the displacement occur most efficiently. The underlying mechanisms of these phenomena were uncovered. Our findings and related analyses may help to understand CO2 enhanced gas recovery from the atomic level and assist the future design in engineering.

Tuning structural and mechanical properties of two-dimensional molecular crystals: the roles of carbon side chains.

A key requirement for the future applicability of molecular electronics devices is a resilience of their properties to mechanical deformation. At present, however, there is no fundamental understanding of the origins of mechanical properties of molecular films. Here we use quinacridone, which possesses flexible carbon side chains, as a model molecular system to address this issue. Eight molecular configurations with different molecular coverage are identified by scanning tunneling microscopy. Theoretical calculations reveal quantitatively the roles of different molecule-molecule and molecule-substrate interactions and predict the observed sequence of configurations. Remarkably, we find that a single Young's modulus applies for all configurations, the magnitude of which is controlled by side chain length, suggesting a versatile avenue for tuning not only the physical and chemical properties of molecular films but also their elastic properties.

Evaporation-induced fractal patterns: A bridge between uniform pattern and coffee ring.

HYPOTHESIS: The rich variety of patterns induced by evaporating drops containing particles has significant guidance for coating processes, inkjet printing, and nanosemiconductors. However, most existing works construct a uniform pattern by suppressing the coffee ring effect, and establishing the connection between them is still an academic challenge. EXPERIMENTS: We report uniform, polygonal, and coffee ring patterns obtained by adjusting the solute concentration that sets in when an ethanol drop with dissolved ibuprofen is deposited on a silicon wafer. FINDINGS: Pattern formation involves rich hydrodynamic events: spreading, evaporative instability, dewetting, film formation, and particle deposition. Based on the distinct multiscale properties, this series of patterns is directly connected from the perspective of fractal geometry, which allows us to name them "fractal deposition patterns". A theoretical model considering film stability is established to explain the mechanism behind pattern formation, which is well verified by experiments. This work has presented a unique strategy that can directly connect uniform, polygonal, and coffee ring patterns under the same physics, hoping to provide instructive guidance for practical applications.

Shape optimization of a meniscus-adherent nanotip.

A soluble tip can dissolve into a tip with curvature when partially immersed in a liquid. This process has been used in the manufacture of sophisticated tips. However, it is difficult to observe the dissolution process in the laboratory, and the dissolution mechanisms at the nanoscale still need to be better understood. Here we utilize molecular dynamics simulations to study the dissolution process of a meniscus-adherent nanotip. The tip apex curvature radius reaches its minimum in the intermediate state. The shape of this state is defined as the optimized shape, which can be used as the termination criterion in applications. In addition, the shape of one optimized tip can be well-fitted to a double-Boltzmann function. The upper Boltzmann curve of this function forms via the competition between the chemical potential influence and the intermolecular forces, while the formation of the lower Boltzmann curve is controlled by the chemical potential influence. The parameters of the double-Boltzmann function are strongly correlated with the nanotip's initial configuration and dissolubility. A shape factor ξ is proposed to characterize the sharpness of optimized tips. Theory and simulations show that optimized tips possess a greater ability to shield the capillary effect than common tips. Our findings elucidate the meniscus-adherent nanotip's dissolution process and provide theoretical support for nano-instrument manufacture.

Direct observation of spreading precursor liquids in a corner.

Precursor liquid is a nanoscale liquid creeping ahead of the macroscopic edge of spreading liquids, whose behaviors tightly correlate with the three-phase reaction efficiency and patterning accuracy. However, the important spatial-temporal characteristic of the precursor liquid still remains obscure because its real-time spreading process has not been directly observed. Here, we report that the spreading ionic liquid precursors in a silicon corner can be directly captured on video using in situ scanning electron microscopy. In situ spreading videos show that the precursor liquid spreads linearly over time ([Formula: see text]) rather than obeying the classic Lucas-Washburn law ([Formula: see text]) and possesses a characteristic width of ∼250-310 nm. Theoretical analyses and molecular dynamics simulations demonstrate that the unique behaviors of precursor liquids originate from the competing effect of van der Waals force and surface energy. These findings provide avenues for directly observing liquid/solid interfacial phenomena on a microscopic level.

Hierarchical self-assembly of achiral amino acid derivatives into dendritic chiral nanotwists.

The organogel formation and self-assembly of a glycine-based achiral molecule were investigated. It has been found that the compound could gel organic solvents either at a lower temperature with lower concentration or at room temperature with higher concentration, which showed different self-assembled nanostructures. At a low temperature of -15 °C, the compound self-assembled into fibrous structures, whereas it formed distinctive flat microbelts at room temperature. When the organogel with nanofibers formed at -15 °C was brought into an ambient condition, chiral twist nanostructures were immediately evolved, which subsequently transferred to a giant microbelt through a hierarchical dendritic twist with the time. Although the compound is achiral, it formed chiral twist with both left- and right-handed twist structures simultaneously. When a trace analogical chiral trigger, L-alanine or D-alanine derivative, was added, a complete homochiral dendritic twist was obtained. Interestingly, a reverse process, i.e. the transformation of the microbelts into twists, could occur upon dilution of the organogel with microbelt structure. During the dilution, both left- and right-handed chiral twists could be formed again. Interestingly, the same branch from the microbelt formed the twist with the same handedness. A combination of the density functional theory (DFT), molecular mechanics (MM), and molecular dynamics (MD) simulations demonstrates that the temperature-induced twisting of the bilayer is responsible for the morphological transformation and evolution of the dendrite twist. This research sheds new light on the hierarchical transformation of the chiral structures from achiral molecules via controlled self-assembly.

Measurement of the rate of water translocation through carbon nanotubes.

We present an approach for measuring the water flow rate through individual ultralong carbon nanotubes (CNTs) using field effect transistors array defined on individual tubes. Our work exhibits a rate enhancement of 882-51 and a slip length of 53-8 nm for CNTs with diameters of 0.81-1.59 nm. We also found that the enhancement factor does not increase monotonically with shrinking tube diameter and there exists a discontinuous region around 0.98-1.10 nm. We believe that these single-tube level results would help understand the intrinsic nanofluidics of water in CNTs.

Wall-Confined Spreading Dynamics on the Surface of Surfactant Solution.

A liquid spreading over another is a universal physical process in the nature, which was investigated by the scaling law to reveal the underlying mechanical mechanism over the decades. However, scaling laws are restricted to piecewise physical stages, respectively. It is a challenge to present a full physical picture for a dynamic spreading process covering a wide-spectrum speed. We propose a general wall-confined spreading dynamics (WCSD) model originating from molecular kinetic theory (MKT). It creatively illustrates the order and domination between driving energy and energy dissipation (or transfer) using a phase diagram according to theory and experiments. This work reveals the deep mechanical mechanism of WCSD which provides an indirect guidance on the solution processing methods of two-dimensional molecular crystals (2DMCs) growth.

Precursor film in dynamic wetting, electrowetting, and electro-elasto-capillarity.

Dynamic wetting and electrowetting are explored using molecular dynamics simulations. The propagation of the precursor film (PF) is fast and obeys the power law with respect to time. Against the former studies, we find the PF is no slip and solidlike. As an important application of the PF, the electro-elasto-capillarity, which is a good candidate for drug delivery at the micro- or nanoscale, is simulated and realized for the first time. Our findings may be one of the answers to the Huh-Scriven paradox and expand our knowledge of dynamic wetting and electrowetting.

Hydroelectric voltage generation based on water-filled single-walled carbon nanotubes.

A DFT/MD mutual iterative method was employed to give insights into the mechanism of voltage generation based on water-filled single-walled carbon nanotubes (SWCNTs). Our calculations showed that a constant voltage difference of several mV would generate between the two ends of a carbon nanotube, due to interactions between the water dipole chains and charge carriers in the tube. Our work validates this structure of a water-filled SWCNT as a promising candidate for a synthetic nanoscale power cell, as well as a practical nanopower harvesting device at the atomic level.

Statics and dynamics of electrowetting on pillar-arrayed surfaces at the nanoscale.

The statics and dynamics of electrowetting on pillar-arrayed surfaces at the nanoscale are studied using molecular dynamics simulations. Under a gradually increased electric field, a droplet is pushed by the electromechanical force to spread, and goes through the Cassie state, the Cassie-to-Wenzel wetting transition and the Wenzel state, which can be characterized by the electrowetting number at the microscale ηm. The expansion of the liquid is direction-dependent and influenced by the surface topology. A positive voltage is induced in the bulk droplet, while a negative one is induced in the liquid confined among the pillars, which makes the liquid hard to spread and further polarize. Based on the molecular kinetic theory and the wetting states, theoretical models have been proposed to comprehend the physical mechanisms in the statics and dynamics of electrowetting, and are validated by our simulations. Our findings may help to understand the electrowetting on microtextured surfaces and assist the future design of engineered surfaces in practical applications.

Phase transitions of a water overlayer on charged graphene: from electromelting to electrofreezing.

We show by using molecular dynamics simulations that a water overlayer on charged graphene experiences first-order ice-to-liquid (electromelting), and then liquid-to-ice (electrofreezing) phase transitions with the increase of the charge value. Corresponding to the ice-liquid-ice transition, the variations of the order parameters indicate an order-disorder-order transition. The key to this novel phenomenon is the surface charge induced change of the orientations of water dipoles, which leads to the change of the water-water interactions from being attractive to repulsive at a critical charge value qc. To further uncover how the orientations of water dipoles influence the interaction strength between water molecules, a theoretical model considering both the Coulomb and van der Waals interactions is established. The results show that with the increase of the charge value, the interaction strength between water molecules decreases below qc, then increases above qc. These two inverse processes lead to electromelting and electrofreezing, respectively. Combining this model with the Eyring equation, the diffusion coefficient is obtained, the variation of which is in qualitative agreement with the simulation results. Our findings not only expand our knowledge of the graphene-water interface, but related analyses could also help recognize the controversial role of the surface charge or electric field in promoting phase transitions of water.

Wetting on flexible hydrophilic pillar-arrays.

Dynamic wetting on the flexible hydrophilic pillar-arrays is studied using large scale molecular dynamics simulations. For the first time, the combined effect of the surface topology, the intrinsic wettability and the elasticity of a solid on the wetting process is taken into consideration. The direction-dependent dynamics of both liquid and pillars, especially at the moving contact line (MCL), is revealed at atomic level. The flexible pillars accelerate the liquid when the liquid approaches, and pin the liquid when the liquid passes. The liquid deforms the pillars, resulting energy dissipation at the MCL. Scaling analysis is performed based on molecular kinetic theory and validated by our simulations. Our results may expand our knowledge of wetting on pillars and assisting the future design of active control of wetting in practical applications.

Capillary wave propagation during the delamination of graphene by the precursor films in electro-elasto-capillarity.

Molecular dynamics simulations were carried out to explore the capillary wave propagation induced by the competition between one upper precursor film (PF) on the graphene and one lower PF on the substrate in electro-elasto-capillarity (EEC). During the wave propagation, the graphene was gradually delaminated from the substrate by the lower PF. The physics of the capillary wave was explored by the molecular kinetic theory. Besides, the dispersion relation of the wave was obtained theoretically. The theory showed that the wave was controlled by the driving work difference of the two PFs. Simulating the EEC process under different electric field intensities (E), the wave velocity was found insensitive to E. We hope this research could expand our knowledge on the wetting, electrowetting and EEC. As a potential application, the electrowetting of the PF between the graphene and the substrate is a promising candidate for delaminating graphene from substrate.

Transport properties and induced voltage in the structure of water-filled single-walled boron-nitrogen nanotubes.

Density functional theorymolecular dynamics simulations were employed to give insights into the mechanism of voltage generation based on a water-filled single-walled boron-nitrogen nanotube (SWBNNT). Our calculations showed that (1) the transport properties of confined water in a SWBNNT are different from those of bulk water in view of configuration, the diffusion coefficient, the dipole orientation, and the density distribution, and (2) a voltage difference of several millivolts would generate between the two ends of a SWBNNT due to interactions between the water dipole chains and charge carriers in the tube. Therefore, this structure of a water-filled SWBNNT can be a promising candidate for a synthetic nanoscale power cell as well as a practical nanopower harvesting device.