Browsing by Author "Ketoja, Jukka A."

Now showing 1 - 10 of 10

Bubble Attachment to Cellulose and Silica Surfaces of Varied Surface Energies : Wetting Transition and Implications in Foam Forming
(2020-07-07) Ketola, Annika E.; Xiang, Wenchao; Hjelt, Tuomo; Pajari, Heikki; Tammelin, Tekla; Rojas, Orlando J.; Ketoja, Jukka A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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.
Chirality and bound water in the hierarchical cellulose structure
(2019-07-15) Paajanen, Antti; Ceccherini, Sara; Maloney, Thaddeus; Ketoja, Jukka A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Abstract: The origins of the unique properties of natural fibres have remained largely unresolved because of the complex interrelations between structural hierarchy, chirality and bound water. In this paper, analysis of the melting endotherms for bleached hardwood pulps indicates that the amount of non-freezing bound water (0.21 g/g) is roughly half of the amount of freezing bound water (0.42 g/g). We link this result to the two smallest constitutive units, microfibrils and their bundles, using molecular dynamics simulations at both hierarchical levels. The molecular water layers found in the simulations correspond quite accurately to the measured amount of non-freezing and freezing bound water. Disorder that results from the microfibril twist and amphiphilicity prevents co-crystallisation, leaving routes for water molecules to diffuse inside the microfibril bundle. Moreover, the simulations predict correctly the magnitude of the right-handed twist at different hierarchical levels. Significant changes in hydroxymethyl group conformations are seen during twisting that compare well with existing experimental data. Our findings go beyond earlier modelling studies in predicting the twist and structure of the microfibril bundle.
Combining scattering analysis and atomistic simulation of wood-water interactions
(2021-01-01) Penttilä, Paavo A.; Paajanen, Antti; Ketoja, Jukka A.
A2 Katsausartikkeli tieteellisessä aikakauslehdessä
Molecular-scale interactions between water and cellulose microfibril bundles in plant cell walls are not fully understood, despite their crucial role for many applications of plant biomass. Recent advances in X-ray and neutron scattering analysis allow more accurate interpretation of experimental data from wood cell walls. At the same time, microfibril bundles including hemicelluloses and water can be modelled at atomistic resolution. Computing scattering patterns from atomistic models enables a new, complementary approach to decipher some of the most fundamental questions at this level of the hierarchical cell wall structure. This article introduces studies related to moisture behavior of wood with small/wide-angle X-ray/neutron scattering and atomistic simulations, recent attempts to combine these two approaches, and perspectives and open questions for future research using this powerful combination. Finally, we discuss the opportunities of the combined method in relation to applications of lignocellulosic materials.
Crossover from mean-field compression to collective phenomena in low-density foam-formed fiber material
(2020-08-07) Mäkinen, Tero; Koivisto, Juha; Pääkkönen, Elina; Ketoja, Jukka A.; Alava, Mikko J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study the compression of low-weight foam-formed materials made out of wood fibers. Initially the stress-strain behavior follows mean-field like response, related to the buckling of fiber segments as dictated by the random three-dimensional geometry. Our Acoustic Emission (AE) measurements correlate with the predicted number of segment bucklings for increasing strain. However, the experiments reveal a transition to collective phenomena as the strain increases sufficiently. This is also seen in the gradual failure of the theory to account for the stress-strain curves. The collective avalanches exhibit scale-free features both as regards the AE energy distribution and the AE waiting time distributions with both exponents having values close to 2. In cyclic compression tests, significant increases in the accumulated acoustic energy are found only when the compression exceeds the displacement of the previous cycle, which further confirms other sources of acoustic events than fiber bending.
Decontamination of a surgical mask with UV-C irradiation : analysis of experimental results with optical simulations
(2024-09-01) Kiiskinen, Titta; Mangs, Oliver; Virkajärvi, Jussi; Elsehrawy, Farid; Salo, Satu; Miettinen, Arttu; Halme, Janne; Harlin, Ali; Ketoja, Jukka A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The suitability of ultraviolet-C (UV-C) irradiation for the decontamination of a surgical face mask was studied by decontamination experiments and carried out using Staphylococcus aureus and MS2 microbes. A moderate dosage level of 0.22 J/cm2 achieved within 2 min led to an over 6-log10 reduction in viable microbe contamination of the inner filtering layer. The underlying reason for this effective decontamination of fibers with small external UV-C dosage was explored with ray-tracing optical simulations, supported by optical measurements on reflection and transmission. The model 3D fiber network was constructed from X-ray tomography images of the layered mask structure consisting of polypropylene fibers. Both simulations and optical measurements indicated that UV light was able to penetrate even the deepest material regions. The simulations show that, despite radiation reflection from the outer mask layer, microbes in the actual filtering layer are affected by the radiation with increased probability due to multiple refraction and scattering of UV light from the inner fibers.
Energy absorption and resilience in quasi-static loading of foam-formed cellulose fibre materials
(2024-07) Pääkkönen, Elina; Ketoja, Jukka A.; Paltakari, Jouni
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
To avoid microplastic pollution, there is an urgent need to replace fossil-based cushioning materials in packaging with easily recyclable alternatives. Here, we investigated the potential of lightweight cellulose fibre materials as a solution for mechanical protection. The quasi-static compression was studied among a vast set of 129 different foam-formed trial points with material density ranging from 21 kg/m3 to 123 kg/m3. The trial points included two different fibre types, bleached softwood kraft pulp (BSKP) and bleached chemithermomechanical pulp (CTMP), with varied refining level, pulp consistency, foaming conditions, surfactant type, strength additives, and final material density and thickness. Besides a correlation analysis of factors affecting compression stress and resilience, the results were reflected against a new theoretical prediction of energy absorption for an ideal low-density random fibre network. The theory predicts the initially-high cushion factor to rapidly drop down to the level of 4‒5 at 40‒80% compression. A similar behaviour was seen among the actual samples, despite their various non-ideal features. At 50% compression, the average cushion factor across the whole data set was 4.84 ± 0.10, being close to the theoretical prediction of 4.61 for the ideal case. The smallest cushion factor of 3.6 was found for a CTMP sample. The recovery from compression varied slightly among the samples and appeared highest for the material density of 60‒100 kg/m3, following the predicted proportion of non-buckled fibre segments. According to the results, cellulose fibre-based cushions have a soft initial response, which is preferable for fragile items. Graphical abstract: (Figure presented.).
High Internal Phase Oil-in-Water Pickering Emulsions Stabilized by Chitin Nanofibrils : 3D Structuring and Solid Foam
(2020-03-04) Zhu, Ya; Huan, Siqi; Bai, Long; Ketola, Annika; Shi, Xuetong; Zhang, Xiao; Ketoja, Jukka A.; Rojas, Orlando J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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.
How Cellulose Nanofibrils Affect Bulk, Surface, and Foam Properties of Anionic Surfactant Solutions
(2019-01-01) Xiang, Wenchao; Preisig, Natalie; Ketola, Annika; Tardy, Blaise L.; Bai, Long; Ketoja, Jukka A.; Stubenrauch, Cosima; Rojas, Orlando J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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.
Nanoscale Mechanism of Moisture-Induced Swelling in Wood Microfibril Bundles
(2022-07-13) Paajanen, Antti; Zitting, Aleksi; Rautkari, Lauri; Ketoja, Jukka A.; Penttilä, Paavo A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Understanding nanoscale moisture interactions is fundamental to most applications of wood, including cellulosic nanomaterials with tailored properties. By combining X-ray scattering experiments with molecular simulations and taking advantage of computed scattering, we studied the moisture-induced changes in cellulose microfibril bundles of softwood secondary cell walls. Our models reproduced the most important experimentally observed changes in diffraction peak locations and widths and gave new insights into their interpretation. We found that changes in the packing of microfibrils dominate at moisture contents above 10–15 whereas deformations in cellulose crystallites take place closer to the dry state. Fibrillar aggregation is a significant source of drying-related changes in the interior of the microfibrils. Our results corroborate the fundamental role of nanoscale phenomena in the swelling behavior and properties of wood-based materials and promote their utilization in nanomaterials development. Simulation-assisted scattering analysis proved an efficient tool for advancing the nanoscale characterization of cellulosic materials.
New tools to study water interactions of microfibril bundles: Molecular modelling based on nanoscale characterization
(2022-09-01) Paajanen, Antti; Penttilä, Paavo; Zitting, Aleksi; Ketoja, Jukka A.
A4 Artikkeli konferenssijulkaisussa
The picture of the smallest structural units of wood fibres, that is, cellulose microfibrils and their bundles, has become more accurate during the last couple of decades, when information gained from several experimental characterisations has been drawn together. This work has been supported by computational methods that allow one to test the behaviour of postulated structures on the nanometre scale, and thus help in interpreting the experimental data. Bound water is an essential component in these models, as it affects both the structural swelling and the mechanical properties of the fibre wall nanostructure. Moreover, mechanisms on this scale can be expected to drive similar properties of macroscopic fibres. We suggest that several large-scale problems in papermaking and converting could be approached with atomistic molecular dynamics simulations for varied chemical compositions and external conditions. We demonstrate this by first showing that simulated moisture diffusion rates agree with measured ones at room temperature, and then determine diffusion rates at elevated temperatures that lack reliable experimental data. These predictions provide key knowledge for further development of high-temperature drying and pressing processes. The results are important also when linking material performance at varied external conditions to the composition of the fibres.