Silica nanoparticles without any surface modification are not surface active at the toluene−water interface due to their extreme hydrophilicity but can be surface activated in situ by adsorbing ...cationic surfactant from water. This work investigates the effects of the molecular structure of water-soluble cationic surfactant on the surface activation of the nanoparticles by emulsion characterization, adsorption and zeta potential measurements, dispersion stability experiments, and determination of relevant contact angles. The results show that an adsorbed cationic surfactant monolayer on particle surfaces is responsible for the wettability modification of the particles. In the presence of a trace amount of cationic surfactant, the hydrophobicity of the particles increases, leading to the formation of stable oil-in-water O/W(1) emulsions. At high surfactant concentration (>cmc) the particle surface is retransformed to hydrophilic due to double-layer or hemimicelle formation, and the concentration of the free surfactant in the aqueous phase is high enough to stabilize emulsions alone. O/W(2) emulsions, probably costabilized by free surfactant and particles, are then formed. The monolayer adsorption seems to be charge-site dependent. Thus, using single-chain trimethylammonium bromide surfactants or a double-head gemini cationic surfactant, the hydrophobicity of the particles achieved is not sufficient to stabilize water-in-oil (W/O) emulsions, and no phase inversion is induced. However, using a double-chain cationic surfactant, the chain density on the particle surfaces endows them with a hydrophobicity high enough to stabilize W/O emulsions, and double phase inversion, O/W(1) → W/O → O/W(2), can then be achieved by increasing the surfactant concentration.
The in situ surface activation of unmodified CaCO3 nanoparticles by interaction with surfactant in aqueous media has been studied, and the impact of this on the foamability and foam stability of ...aqueous dispersions was assessed. Using complementary experiments including measurement of particle zeta potentials, adsorption isotherms of surfactant, air−water surface tensions, and relevant contact angles, the mechanism of this activation was revealed. The results show that the non-surface-active CaCO3 nanoparticles cannot be surface activated by interaction with cationic or nonionic surfactants but can be surface activated by interaction with anionic surfactants such as SDS and AOT, leading to a synergistic effect in both foamability and foam stability. The electrostatic interaction between the positive charges on particle surfaces and the negative charges of anionic surfactant headgroups results in monolayer adsorption of the surfactant at the particle−water interface and transforms the particles from hydrophilic to partially hydrophobic such that particles become surface active and stabilize bubbles. SDS is a more efficient surfactant for this surface activation than AOT. Possible reasons for this difference are suggested.
The in situ surface activation of raw CaCO3 nanoparticles by interaction with a series of sodium carboxylates of chain length between 6 and 12 as well as sodium 2-ethylhexylsulfosuccinate (AOT) was ...studied, and the impact of this on the stabilization and phase inversion of toluene–water emulsions was assessed. By using complementary experiments including measurement of particle zeta potentials, adsorption isotherms of amphiphile, and relevant contact angles, the mechanism of this activation was revealed. The results show that hydrophilic CaCO3 nanoparticles can be surface activated by interaction with sodium carboxylates and AOT even if they are not surface-active themselves. Both the electrostatic interaction between the positive charges on particle surfaces and the negative charges of anionic amphiphile headgroups and the chain–chain interactions of the amphiphile result in monolayer adsorption of the amphiphile at the particle–water interface. This transforms the particles from hydrophilic to partially hydrophobic such that they become surface-active and stabilize oil-in-water O/W(1) emulsions and induce O/W(1) → water-in-oil W/O phase inversion, depending on the chain length of the carboxylate molecules. At high amphiphile concentration, bilayer or hemimicelle adsorption may occur at the particle–water surface, rendering particles hydrophilic again and causing their desorption from the oil–water interface. A second phase inversion, W/O → O/W(2), may occur depending on the surface activity of the amphiphile. CaCO3 nanoparticles can therefore be made good stabilizers of both O/W and W/O emulsions once surface activated by mixing with traces of suitable anionic amphiphile.
The natural resonance appears at 16
GHz for (Fe, Ni)/C nanocapsules with (Fe, Ni) alloys as cores and graphite as shells. Reflection loss (RL) exceeding −10
dB was obtained in the whole Ku-band ...(12.4–18
GHz) for an absorber thickness of 2.0
mm, while it exceeds −20
dB over the 13.6–16.6
GHz range. In addition, the bandwidth does not change dramatically for the thicknesses of 1.87–2.1
mm for the RL values exceeding −10
dB. The (Fe, Ni)/C nanocapsules with wide bandwidth absorption can be used as excellent electromagnetic-wave-absorption materials in the whole Ku-band.
Magnetic flux ropes (MFRs) are significant regions for the production of energetic electrons in space and astrophysical plasmas. However, the research on electron heating and acceleration driven by ...turbulence in MFRs is still quite rare. Utilizing in‐situ measurements from MMS satellite, we study electron heating and associated electrostatic waves in an ion‐scale MFR within terrestrial super‐Alfvén plasma flow. Lower‐hybrid drift waves, generated locally in this MFR, can contribute to the perpendicular heating through their electrostatic potential accelerating electrons. The parallel heating is attributed to antiparallel propagating electron beams. These beams excite the broadband electrostatic waves that can interact with electrons and thermalize electrons. Our study promotes understanding of electron energization driven by plasma waves and wave‐particle interaction in MFRs.
Plain Language Summary
Magnetic flux ropes (MFRs) are ubiquitous magnetic structures existing in space and astrophysical plasmas. They are produced by magnetic reconnection, and can in turn trigger small‐scale reconnection and modulate reconnection process. In addition, they are widely used to explain electron heating and acceleration, which is a long‐standing problem in space and astrophysical plasmas. Thus, they receive significant attention nowadays. However, there has been still lack of research on electron heating and acceleration driven by turbulence in MFRs. In our study, we find perpendicular electron heating is partly attributed to lower‐hybrid drift waves and parallel electron heating is closely related to antiparallel propagating electron beams, which excites strong broadband electrostatic waves that can result in thermalization of electrons.
Key Points
Electron heating is detected inside an ion‐scale magnetic flux rope
Lower‐hybrid drift waves can contribute to the perpendicular heating through their electrostatic potential accelerating electrons
The parallel heating is caused by electron beams, which excite the broadband electrostatic waves that can thermalize electrons
Abstract
As the plasma boundary between two distinct plasma populations, dipolarization fronts (DFs) host abundant kinetic-scale substructures that change their normal directions and thus cause their ...deformation. However, studies on such deformation caused by an electron vortex have been lacking. Here, we present novel observations of a subion-scale magnetic hump (MHu) associated with an oblique electron vortex at a DF through strengthening three components of the magnetic field. A radial electric field in the MHu, showing bipolar variation, is also associated with the electron vortex as it is mainly ascribed to the electron convection term. There is apparent energy conversion (
J
→
·
E
→
∼−0.3 nw m
−3
) from the particles to the electromagnetic field in the MHu’s leading part, which is accompanied by inflow and outflow of electromagnetic energy (nonzero
∇
·
S
→
). The other regions of the DF host opposite energy conversion (
J
→
·
E
→
> 0). Broadband parallel electrostatic waves are also observed in the MHu. Our study provides insights into the kinetic-scale processes at DFs.
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•pH-Responsive Pickering foams are prepared using silica nanoparticles.•The particles are in situ hydrophobized by trace amount of carboxyl betaine.•The foams are stable at pH < 4 and ...unstable at pH > 10.•Multi-cycles between stable and unstable of the foams can be realized.•The responsive foams are prepared without using functional particles.
pH-Responsive Pickering foams were prepared by using negatively charged silica nanoparticles in combination with trace amount of dodecyl dimethyl carboxyl betaine as stabilizer. The foams are stable at pH ≤ 4.3 but unstable at pH ≥ 10 and can then be cycled between stable and unstable for many times by alternating the pH of the aqueous phase. It is shown that in acidic aqueous media the carboxyl betaine molecules are turned to cationic form which can adsorb at surfaces of the negatively charged silica nanoparticles with head-on configuration via electrostatic interaction, rendering particles surface activity by in situ hydrophobization, and the particles can then adsorb at air/water interface to stabilize the Pickering foams; whereas in neutral and alkaline aqueous media, the carboxyl betaine molecules are in zwitterionic form, which tend to desorb from particle surface due to weakening or removing of electrostatic interaction, triggering de-hydrophobization of the particles and defoaming of the systems. This principle makes it possible to construct stimuli-responsive aqueous foaming systems using commercial inorganic nanoparticles in combination with trace amount of conventional surfactants avoiding synthesis or preparation of complicated stimuli-responsive colloid particles.
Electrospinning of collagen and chitosan blend solutions in a 1,1,1,3,3,3-hexafluoroisopropanol/trifluoroacetic acid (v/v, 90/10) mixture was investigated for the fabrication of a biocompatible and ...biomimetic nanostructure scaffold in tissue engineering. The morphology of the electrospun collagen–chitosan nanofibers was observed by scanning electron microscopy (SEM) and stabilized by glutaraldehyde (GTA) vapor via crosslinking. Fourier transform infrared spectra analysis showed that the collagen–chitosan nanofibers do not change significantly, except for enhanced stability after crosslinking by GTA vapor. X-ray diffraction analysis implied that both collagen and chitosan molecular chains could not be crystallized in the course of electrospinning and crosslinking, and gave an amorphous structure in the nanofibers. The thermal behavior and mechanical properties of electrospun collagen–chitosan fibers were also studied by differential scanning calorimetry and tensile testing, respectively. To assay the biocompatibility of electrospun fibers, cellular behavior on the nanofibrous scaffolds was also investigated by SEM and methylthiazol tetrazolium testing. The results show that both endothelial cells and smooth muscle cells proliferate well on or within the nanofiber. The results indicate that a collagen–chitosan nanofiber matrix may be a better candidate for tissue engineering in biomedical applications such as scaffolds.
Both the whistler waves and high‐frequency electrostatic waves (HFEWs), playing significant roles in energy conversion, have been widely studied behind dipolarization fronts (DFs). Nevertheless, ...interactions between them have been so far elusive. Utilizing high‐resolution MMS data, we present two cases that HFEWs were modulated by whistler waves behind a DF. The HFEWs with frequency at fce ∼ fpe, propagating along the ambient magnetic field, were detected near δE// minimum (maximum) of parallel‐propagating (antiparallel‐propagating) whistler waves. The amplitudes of the HFEWs are comparable to that of the whistler waves, while their energy densities are two orders of magnitude lower than that of the whistler waves. The analysis indicates that the whistler waves were locally excited by anisotropic electron distributions and the HFEWs were excited by electron beams produced by the whistler waves via Landau resonance. Our study advances the understanding of wave‐wave interactions and cross‐scale processes in space plasmas.
Plain Language Summary
Dipolarization fronts (DFs) are generated by the interaction between busty bulk flows and ambient plasmas in the magnetotail plasma sheet. There are various waves and structures around DFs. In this study, we present observations of whistler waves and high‐frequency electrostatic waves behind a DF. The whistler waves are locally excited by anisotropic electron distributions, and modulate the high‐frequency electrostatic waves.
Key Points
Whistler waves and high‐frequency electrostatic waves (HFEWs) are simultaneously observed behind a dipolarization front
Whistler waves are locally excited by anisotropic electron distributions
HFEWs are excited by electron beams produced by the whistler waves via Landau resonance
