Surface wettability effects on flow in rough wall nanochannels

A fluid flow in a nanochannel highly depends on the wettability of the channel surface to the fluid. The permeability of the nanochannel is usually very low, largely due to the adhesion of fluid at the solid interfaces. Using molecular dynamics (MD) simulations, we demonstrate that the flow of water in a nanochannel with rough hydrophilic surfaces can be significantly enhanced by the presence of a thin layer of supercritical carbon dioxide (scCO2) at the water-solid interfaces. The thin scCO2 layer acts like an atomistic lubricant that transforms a hydrophilic interface into a super-hydrophobic one and triggers a transition from a stick- to- a slip boundary condition for a nanoscale flow. This work provides an atomistic insight into multicomponent interactions in nanochannels and illustrates that such interactions can be manipulated, if needed, to increase the throughput and energy efficiency of nanofluidic systems.

Studies have been carried out on the adsorption properties of plasma fibronectin (FN) on hydrophobic and hydrophilic surfaces. The hydrophobic and hydrophilic surfaces used were nonwettable bacteriological dishes and wettable tissue culture dishes, respectively. At low plasma FN concentrations, there was more plasma FN binding to hydrophobic surfaces than to hydrophilic surfaces. Under these conditions, the binding of antifibronectin to adsorbed fibronectin was much higher on the hydrophilic surfaces than the hydrophobic ones suggesting that adsorption of plasma FN on the two surfaces occurred in different conformations. In the presence of small amounts of serum albumin, however, antibody binding to plasma FN adsorbed on hydrophobic surfaces increased markedly indicating that a more favorable conformation had been attained. The adsorption of plasma FN onto both hydrophobic and hydrophilic surfaces from serum-containing solutions was found to depend on the serum concentration. At low serum concentrations in the incubations (up to 0.1%), there was increased adsorption of plasma FN with increasing serum concentrations. The highest concentration of adsorbed fibronectin was about 12 ng/cm2 and was sufficient to promote complete cell spreading. Above 1.0% serum there was a marked decrease in fibronectin adsorption and at 10% serum very little adsorption occurred. This indicated that at high serum concentrations other serum proteins were able to compete with fibronectin for surface adsorption sites. This was ccrfirmed by determining adsorption isotherms for fibro1:ectin in the presence of 0.5 and 10% fibronectin-depleted serum. In long term cell spreading experiments (1 h to 2 days), cell spreading eventually occurred on hydrophilic surfaces but not on hydrophobic ones in the presence of 10% serum. This could not be accounted for by exchange on the surfaces of fibronectin for adsorbed serum proteins in the absence of the cells. In the presence of the cells, however, there was an increase in fibronectin associated with the hydrophilic surfaces and a decrease in fibronectin associated with hydrophobic surfaces that occurred concomitantly with increased or decreased cell spreading. This result suggested the possibility that in long term cultures, cells deposit endogenous spreading factors on top of or in place of the adsorbed non-fibronectin serum proteins.

We are reporting electroosmotic flows in nanochannels having different surface roughness. Molecular dynamics simulation technique has been applied to understand microscopic or molecular aspects of solid-liquid interactions. The surface roughness in this study was modeled as a succession of expanding and contracting steps along the flow direction, determined by the electric field direction. Water and sodium ion density profiles for the smooth wall show strong layering of water molecules near the solid wall. For rough walls, the density profiles are very similar for all cases except where the steps are located. We observed that this disturbance grows with the amplitude of the roughness and decreases with the period of the roughness. To further investigate strong layering of the water molecules, the net electric dipole moment was computed for the smooth and rough walls. It shows ordering of water molecules near the wall. The rough wall result show water molecules are ordered more in the expanded region. The velocity profiles and the flow rate were calculated for all cases. From these, electric double layers are found to overlap. The maximum velocities and the flow rate decreased for the lower period and the higher amplitude of surface roughness.

Droplet evaporation is widespread in natural and industrial application, and the rapid and efficient evaporation can significantly improve energy efficiency. However, the fundamental mechanism of contact line dynamics and the microscopic characteristics of evaporating nanodroplets are not well understood. Moreover, how to design a nanostructure surface to enhance nanodroplet evaporation remains unclear. Here, through molecular dynamics simulation, we investigated the evaporation dynamics of nanodroplets on various nanoring surfaces with different geometric parameters and wettability. By measuring the changes of contact radius and contact angle, the results showed that nanodroplets successively exhibit constant contact angle (CCA), constant contact radius (CCR), and mix mode during evaporation, and the evaporation-induced CCA-CCR transition, in essence, is a Cassie-Wenzel wetting transition, whose onset time is remarkably dependent on the surface roughness and wettability. We found that this evaporation-induced wetting transition is postponed on the surface with small nanostructure spacing and weak hydrophilicity, and the evaporation rate of nanodroplets improves accordingly. The dense and hydrophobic nanostructures can not only restrain the Cassie-Wenzel transition, but also enhance the evaporation rate of nanodroplets. Last, through the potential energy field analysis of nanoring substrates, we revealed that the Cassie-Wenzel wetting transition of nanodroplets is a process of molecule migration to low potential energy regions. Our work provides guidance for designing nanostructure surfaces to effectively control the droplet wetting state and enhance its mass transfer performance of phase change.

The present paper characterizes the attachment ability of males and females of Nezara viridula (Heteroptera: Pentatomidae) on artificial surfaces (smooth hydrophilic, smooth hydrophobic, different surface roughness) and on both leaf surfaces of the typical host plant species Vicia faba, using a centrifugal force tester and a traction force experiments set up. N. viridula is a serious crop pest in the world and shows attachment devices different from the so far investigated Heteroptera, with a tarsus characterized by distal smooth flexible pulvilli combined with claws and proximal ventral hairy pad. Notwithstanding the different body mass between the sexes, no difference was found between friction forces generated by females and males. Friction force was higher on hydrophilic surfaces than on hydrophobic ones and was lower on both sides of V. faba leaf compared with both hydrophilic and hydrophobic artificial smooth surfaces. On the surfaces with different roughness, the friction force values varied significantly, with the higher attachment ability on the surface with very high asperity size followed by the smooth surface. The lowest attachment was on the surfaces with intermediate asperity sizes. These results could be related to the specific combination of attachment devices of N. viridula.

Two-dimensional mesoscale simulations of saturated pool boiling from rough surfaces. Part II: Bubble interactions above multi-cavities

Effects of surface wettability and roughness of microchannel on flow behaviors of thermo-responsive microspheres therein during the phase transition

Development of nano-devices for various applications has drawn great attention recently driven by the need of miniaturizing the devices for the integration and automation of Biochips or Lab-on-a-Chip devices. Fundamental understanding of transport phenomena in nanofluidic channels is critical for systematic design and precise control of such devices. The goal of this study is to develop a theoretical model to study electroosmotic flow in nanochannels. Instead of using the Boltzmann distribution, the conservation condition of ion number and the Nernst equation are used in this new model to find the ionic concentration field in the nanochannels. A correct boundary condition for the concentration field at the wall of the channel is developed and the symmetry condition of the potential field at the center of the nanochannel is applied to this model. The ionic concentration field, electrical potential field and flow field are obtained by numerically solving this model. Comparisons of area-average velocity between the numerical simulations and experimental results reported in literature are provided.

Nonequilibrium molecular dynamics simulation is applied to investigate the effect of periodic wall roughness on the flow of liquid argon through krypton nanochannels. The effect of the length of a rectangular protrusion on the flow is investigated and compared to the case of nanochannels with flat walls. The results show a clear trapping of fluid atoms inside the rectangular cavities that are formed between successive protrusions. The size of the cavities affects the potential energy map and, consequently, fluid atom localization. This localization results in a reduction of velocity values inside the cavities, as well as a reduction of the slip length near the rough wall.

Surface wettability effect on the rising of a bubble attached to a vertical wall

The effect of surface wettability on frost melting

In this work, we use molecular dynamics simulations to study the transport of ions in electromechanical flows in slit-like graphene nanochannels. The variation of ionic currents indicates a nonlinear coupling between pressure-driven and electroosmotic flows, which enhances the ionic currents for electromechanical flows compared with the linear superposition of pressure-driven and electroosmotic flows. The nonlinear coupling is attributed to the reduction of the total potential energy barrier due to the density variations of ions and water molecules in the channel. The numerical results may offer molecular insights into the design of nanofluidic devices for energy conversion.

Intrinsic dependence of ice adhesion strength on surface roughness

Air assisted impact of drops: The effect of surface wettability

The surface hydrophilicity of nanoparticles has a major impact on their biological fates. Ascertaining the correlation between nanoparticle surface hydrophilicity and their biological behaviors is particularly instructive for future nanomedicine design and their antitumor efficacy optimization. Herein, we designed a series of polymeric nanoparticles based on polyphosphoesters with well-controlled surface hydrophilicity in the molecular level and systemically evaluated their biological behaviors. The results demonstrated that high surface hydrophilicity preferred lower protein absorption, better stability, longer blood circulation, and higher tumor accumulation but lower cellular uptake. Upon encapsulation of drugs, nanoparticles with high hydrophilicity showed an excellent antitumor therapeutic efficacy in both primary and metastatic tumors as compared to the relatively hydrophobic ones. Further analyses revealed that the superior antitumor outcome was attributed to the balance of tumor accumulation and cellular uptake, demonstrating the particular importance of nanoparticle surface hydrophilicity regulation on the antitumor efficacy. Our work provides a potent guideline for a rational designation on the surface hydrophilicity of nanoparticles for cancer treatment optimization.