Bubble Attachment to Cellulose and Silica Surfaces of Varied Surface Energies: Wetting Transition and Implications in Foam Forming.

To better understand the complex system of wet foams in the presence of cellulosic fibers, we investigate bubble-surface interactions by following the effects of surface hydrophobicity and surface tension on the contact angle of captive bubbles. Bubbles are brought into contact with model silica and cellulose surfaces immersed in solutions of a foaming surfactant (sodium dodecyl sulfate) of different concentrations. It is observed that bubble attachment is controlled by surface wetting, but a significant scatter in the behavior occurs near the transition from partial to complete wetting. For chemically homogeneous silica surfaces, this transition during bubble attachment is described by the balance between the energy changes of the immersed surface and the frictional surface tension of the moving three-phase contact line. The situation is more complex with chemically heterogeneous, hydrophobic trimethylsilyl cellulose (TMSC). TMSC regeneration, which yields hydrophilic cellulose, causes a dramatic drop in the bubble contact angle. Moreover, a high interfacial tension is required to overcome the friction caused by microscopic (hydrophilic) pinning sites of the three-phase contact line during bubble attachment. A simple theoretical framework is introduced to explain our experimental observations.

How Cellulose Nanofibrils Affect Bulk, Surface, and Foam Properties of Anionic Surfactant Solutions.

We study the generation and decay of aqueous foams stabilized by sodium dodecyl sulfate (SDS) in the presence of unmodified cellulose nanofibrils (CNF). Together with the rheology of aqueous suspensions containing CNF and SDS, the interfacial/colloidal interactions are determined by quartz crystal microgravimetry with dissipation monitoring, surface plasmon resonance, and isothermal titration calorimetry. The results are used to explain the properties of the air/water interface (interfacial activity and dilatational moduli determined from oscillating air bubbles) and of the bulk (steady-state flow, oscillatory shear, and capillary thinning). These properties are finally correlated to the foamability and to the foam stability. The latter was studied as a function of time by monitoring the foam volume, the liquid fraction, and the bubble size distribution. The shear-thinning effect of CNF is found to facilitate foam formation at SDS concentrations above the critical micelle concentration (cSDS ≥ cmc). Compared with foams stabilized by pure SDS, the presence of CNF enhances the viscosity and elasticity of the continuous phase as well as of the air/water interface. The CNF-containing foams have higher liquid fractions, larger initial bubble sizes, and better stability. Due to charge screening effects caused by sodium counter ions and depletion attraction caused by SDS micelles, especially at high SDS concentrations, CNF forms aggregates in the Plateau borders and nodes of the foam, thus slowing down liquid drainage and bubble growth and improving foam stability. Overall, our findings advance the understanding of the role of CNF in foam generation and stabilization.

High Internal Phase Oil-in-Water Pickering Emulsions Stabilized by Chitin Nanofibrils: 3D Structuring and Solid Foam .

Chitin nanofibrils (NCh, ∼10 nm lateral size) were produced under conditions that were less severe compared to those for other biomass-derived nanomaterials and used to formulate high internal phase Pickering emulsions (HIPPEs). Pre-emulsification followed by continuous oil feeding facilitated a "scaffold" with high elasticity, which arrested droplet mobility and coarsening, achieving edible oil-in-water emulsions with internal phase volume fraction as high as 88%. The high stabilization ability of rodlike NCh originated from the restricted coarsening, droplet breakage and coalescence upon emulsion formation. This was the result of (a) irreversible adsorption at the interface (wettability measurements by the captive bubble method) and (b) structuring in highly interconnected fibrillar networks in the continuous phase (rheology, cryo-SEM, and fluorescent microscopies). Because the surface energy of NCh can be tailored by pH (protonation of surface amino groups), emulsion formation was found to be pH-dependent. Emulsions produced at pH from 3 to 5 were most stable (at least for 3 weeks). Although at a higher pH NCh was dispersible and the three-phase contact angle indicated better interfacial wettability to the oil phase, the lower interdroplet repulsion caused coarsening at high oil loading. We further show the existence of a trade-off between NCh axial aspect and minimum NCh concentration to stabilize 88% oil-in-water HIPPEs: only 0.038 wt % (based on emulsion mass) NCh of high axial aspect was required compared to 0.064 wt % for the shorter one. The as-produced HIPPEs were easily textured by taking advantage of their elastic behavior and resilience to compositional changes. Hence, chitin-based HIPPEs were demonstrated as emulgel inks suitable for 3D printing (millimeter definition) via direct ink writing, e.g., for edible functional foods and ultralight solid foams displaying highly interconnected pores and for potential cell culturing applications.

Cellulose nanofibrils prepared by gentle drying methods reveal the limits of helium ion microscopy imaging.

TEMPO-oxidized cellulose nanofibrils (TCNFs) have unique properties, which can be utilised in many application fields from printed electronics to packaging. Visual characterisation of TCNFs has been commonly performed using Scanning Electron Microscopy (SEM). However, a novel imaging technique, Helium Ion Microscopy (HIM), offers benefits over SEM, including higher resolution and the possibility of imaging non-conductive samples uncoated. HIM has not been widely utilized so far, and in this study the capability of HIM for imaging of TCNFs was evaluated. Freeze drying and critical point drying (CPD) techniques were applied to preserve the open fibril structure of the gel-like TCNFs. Both drying methods worked well, but CPD performed better resulting in the specific surface area of 386 m2 g-1 when compared to 172 m2 g-1 and 42 m2 g-1 of freeze dried samples frozen in propane and nitrogen, respectively. HIM imaging of TCNFs was successful but high magnification imaging was challenging because the ion beam tended to degrade the TCNFs. The effect of the imaging parameters on the degradation was studied and an ion dose as low as 0.9 ion per nm2 was required to prevent the damage. This study points out the differences between the gentle drying methods of TCNFs and demonstrates beam damage during imaging like none previously reported with HIM. The results can be utilized in future studies of cellulose or other biological materials as there is a growing interest for both the HIM technique and bio-based materials.

Expression of the Alternative Oxidase Influences Jun N-Terminal Kinase Signaling and Cell Migration.

Downregulation of Jun N-terminal kinase (JNK) signaling inhibits cell migration in diverse model systems. In Drosophila pupal development, attenuated JNK signaling in the thoracic dorsal epithelium leads to defective midline closure, resulting in cleft thorax. Here we report that concomitant expression of the Ciona intestinalis alternative oxidase (AOX) was able to compensate for JNK pathway downregulation, substantially correcting the cleft thorax phenotype. AOX expression also promoted wound-healing behavior and single-cell migration in immortalized mouse embryonic fibroblasts (iMEFs), counteracting the effect of JNK pathway inhibition. However, AOX was not able to rescue developmental phenotypes resulting from knockdown of the AP-1 transcription factor, the canonical target of JNK, nor its targets and had no effect on AP-1-dependent transcription. The migration of AOX-expressing iMEFs in the wound-healing assay was differentially stimulated by antimycin A, which redirects respiratory electron flow through AOX, altering the balance between mitochondrial ATP and heat production. Since other treatments affecting mitochondrial ATP did not stimulate wound healing, we propose increased mitochondrial heat production as the most likely primary mechanism of action of AOX in promoting cell migration in these various contexts.