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The solid surfaces used in evaporation studies of nanoparticle sessile droplets usually exhibit significant surface roughness, causing significant pinning of the three-phase contact lines and producing different types of nanoparticle deposits, from single and multiple coffee rings (formed at the initial pining of triple contact lines) to central bumps. Here we used nanometer-scale smooth hydrophobic surfaces to investigate the evaporation of sessile water droplets containing silica nanoparticles and organic pigment nanoparticles. We observed a new type of coffee ring deposits which were not formed at the initial pinning but at the later pinning. We referred them to as the inner coffee ring deposits (ICRDs). The radius of ICRDs was smaller than the radius of the initially pinned contact area and increased with increasing concentration of added salts and nanoparticles and with increasing contact angle hysteresis of hydrophobic surfaces. We also observed different dendrite deposit patterns inside ICRDs. We argue that all the deposit patterns are due to the second pinning of the three-phase contact lines, which occur when the forces on particles are balanced. The hypothesis is further supported by the transient changes of the dynamic contact angles and contact base area radius. The contact angle hysteresis, the particle concentration, and the colloidal interaction forces such as the electrical double-layer forces play a vital role in determining the size and patterns of ICRDs and the evaporation kinetics of nanoparticle sessile droplets.

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The unexpected stability and anomalous contact angle of gaseous nanobubbles at the hydrophobic solid-liquid interface has been an issue of debate for almost two decades. In this work silicon-nitride tipped AFM cantilevers are used to probe the highly ordered pyrolytic graphite (HOPG)-water interface with and without solvent-exchange (a common nanobubble production method). Without solvent-exchange the force obtained by the single force and force mapping techniques is consistent over the HOPG atomic layers and described by DLVO theory (strong EDL repulsion). With solvent-exchange the force is non-DLVO (no EDL repulsion) and the range of the attractive jump-in (>10 nm) over the surface is grouped into circular areas of longer range, consistent with nanobubbles, and the area of shorter range. The non-DLVO nature of the area between nanobubbles suggests that the interaction is no longer between a silicon-nitride tip and HOPG. Interfacial gas enrichment (IGE) covering the entire area between nanobubbles is suggested to be responsible for the non-DLVO forces. The absence of EDL repulsion suggests that both IGE and nanobubbles are not charged. The coexistence of nanobubbles and IGE provides further evidence of nanobubble stability by dynamic equilibrium. The IGE cannot be removed by contact mode scanning of a cantilever tip in pure water, but in a surfactant (SDS) solution the mechanical action of the tip and the chemical action of the surfactant molecules can successfully remove the enrichment. Strong EDL repulsion between the tip and nanobubbles/IGE in surfactant solutions is due to the polar heads of the adsorbed surfactant molecules.

We have investigated the control preparation and aqueous stability of a potential suspension fertilizer: zinc hydroxide nitrate. We have observed that this compound can be synthesized by quick precipitation of zinc nitrate in sodium hydroxide solution under various conditions, whereas it can also be readily transformed to more stable Zn(OH) 2 or ZnO at pH >6.5 and aged at 50 °C. The transformation from zinc hydroxide nitrate to Zn(OH) 2 and ZnO has been examined with XRD/FTIR/SEM techniques and discussed in detail, presumably involving the formation and dissociation of the intermediate solution species [Zn(OH) 4 ] 2− /[Zn(OH) 3 ] − . We have also found that as-prepared zinc hydroxide nitrate crystals are very stable when they are isolated and then dispersed in aqueous solution with pH 5−9 while slightly dissolved to give zinc ion concentration of 30−50 mg/L. Such aqueous stability and solubility have thus suggested that this compound can be used as a long-term zinc foliar fertilizer of various crops.

We report the effects of ions on rupture and lifetime of aqueous foam films formed from sodium chloride (NaCl), lithium chloride (LiCl), sodium acetate (NaAc), and sodium chlorate (NaClO 3) using microinterferometry. In the case of NaCl and LiCl, the foam films prepared from the salt solutions below 0.1 M were unstable they thinned until rupturing. The film lifetime measured from the first interferogram (appearing at a film thickness on the order of 500 nm) until the film rupture was only a second or so. However, relatively long lasting and nondraining films prepared from salt solutions above 0.1 M were observed. The film lifetime was significantly longer by 1 to 2 orders of magnitude, i.e., from 10 to 100 s. Importantly, both the film lifetime and the (average) thickness of the nondraining films increased with increasing salt concentration. This effect has not been observed with foam films stabilized by surfactants. The film lifetime and thickness also increased with increasing film radius. The films exhibited significant surface corrugations. The films with large radii often contained standing dimples. There was a critical film radius below which the films thinned until rupturing. In the cases of NaAc and NaClO 3, the films were unstable at all radii and salt concentrations they thinned until rupturing, ruling out the effect of solution viscosity on stabilizing the films.