Structure and rheology of liquid crystal hydroglass formed in aqueous nanocrystalline cellulose suspensions

Journal of Colloid and Interface Science 555 (2019) 702–713

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Structure and rheology of liquid crystal hydroglass formed in aqueous nanocrystalline cellulose suspensions Yuan Xu a, Aleks Atrens b, Jason R. Stokes a,⇑ a b

School of Chemical Engineering, The University of Queensland, Brisbane, Australia School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Australia

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 4 June 2019 Revised 6 August 2019 Accepted 7 August 2019 Available online 7 August 2019 Keywords: Nanocrystalline cellulose Cellulose nanocrystal Rheology Liquid crystal Phase separation Anisotropic soft material

a b s t r a c t Hypothesis: Liquid crystal hydroglass (LCH) is a biphasic soft material with flow programmable anisotropy that forms via phase separation in suspensions of charged colloidal rods upon increases in ionic strength. The unique structure and rheology of the LCH gel formed using nanocrystalline cellulose (NCC) is hypothesised to be dependent on colloidal stability that is modulated using specific ion effects arising from Hofmeister phenomena. Experiments: LCHs are prepared in NCC suspensions in aqueous media containing varying levels of sodium chloride (NaCl) or sodium thiocyanate (NaSCN). The NCC suspensions are characterised using rheology and structural analysis techniques that includes polarised optical microscopy, zeta potential, dynamic light scattering and small-angle X-ray scattering. Findings: The two salts have a profound effect on the formation process and structure of the LCH. Differences in network density and size of the liquid crystal domains are observed within the LCH for each of the salts, which is associated with the strength of interaction between NCC particles during LCH formation. In comparison to Cl
at the same salinity, the chaotropic nature of the weakly hydrated SCN
enhances colloidal stability by rendering NCC particles more hydrated and repulsive, but this also leads to weaker gel strength of the LCH. The results suggest that salts are a means in which to control the formation, structure and rheology of these anisotropic soft materials. Ó 2019 Published by Elsevier Inc.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (J.R. Stokes). https://doi.org/10.1016/j.jcis.2019.08.022 0021-9797/Ó 2019 Published by Elsevier Inc.

Structural anisotropy is crucial for many natural and artificial bio-structures such as cornea [1] and muscle [2] to perform their function including light transmission, mass transfer and mechani-

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

cal deformation [3,4]. Fabricating hydrogels with anisotropic morphology is desirable because of the limiting ability of isotropic hydrogels to replicate biological functions for applications in biotechnology and biomimicry. Anisotropic hydrogels are a class of soft materials comprised of a three-dimensional cross-linked network and an anisotropic component. The network provides solid-like rheological behaviour, while the anisotropic component produces directionally-dependent response to external stimuli, such as mechanical forces, electric fields, and temperature [5–7]. Due to this unique combination of mechanical durability and structural order, anisotropic hydrogels have potential uses in several biophysical and biomedical applications, including mimicry of biological systems having a hierarchy of orientational anisotropy over a range of length scales [8]. Anisotropic hydrogels are fabricated in two steps: firstly, aligning the gelling components with an external field, such as shear flow [9,10] or electric/magnetic fields [11,12]; and secondly, fixing the alignment through gelation via polymerisation or phase segregation. Successful preparation of anisotropic hydrogels has proven difficult because materials are infrequently conducive to being easily aligned and then gelled [13–15]. For example, flow-induced alignment of anisotropic colloids such as cellulose nanofibers [16], peptide [17], liquid crystals [18] or surfactant bilayers [19] is temporary, and particles tend to relax to random orientation rapidly upon cessation of shear. Producing anisotropic hydrogels with these materials requires sophisticated design to ensure gelation is achieved so the structure is kinetically trapped within a narrow time frame. Electric fields have been used to orient some dipolar particles such as surface-modified carbon nanotubes [20], which allows creation of homogeneous anisotropy that is otherwise rarely achievable. However, it is difficult to produce complex structural order, particularly spatial heterogeneity, using electric fields [13,21,22]. Phase separation is a common method to fabricate colloidal hydrogels [23–25]. Aqueous colloidal suspensions can form a hydrogel following phase separation through spinodal decomposition. This consists of quenching (e.g., increase ionic strength) a homogeneous suspension into a thermodynamically unstable state so that it separates into colloid-rich and colloid-poor phases [26]. With an appropriate combination of colloidal size, shape, concentration and interparticle forces, the kinetics of spinodal decomposition are such that the phases do not sediment (from fast phase separation) nor form a close-to-homogeneous structure (from slow phase separation). With these favourable kinetics of growth and separation for both phases, a biphasic metastable hydrogel may form [27]. Spinodal decomposition of colloidal suspensions has been extensively investigated, including the development of new classes of isotropic and anisotropic soft materials [28–31]. The use of spinodal decomposition in suspensions of anisotropic colloids such as cellulose nanofibers to form an anisotropic hydrogel has not been previously realised. This is probably because the factors necessary to enable phase separation, such as increasing ionic strength, typically drive anisotropic colloids to isotropic behaviour due to reduced colloidal stability, which may occur before the conditions necessary for spinodal decomposition being achieved. Unless these specific conditions are realised, the likely outcome is consequently an isotropic hydrogel [32,33]. We recently demonstrated a new class of biphasic anisotropic soft material, which we termed a Liquid Crystal Hydroglass (LCH) [34]. The LCH is formed via spinodal decomposition of an aqueous suspension of charged colloidal rods, in this case nanocrystalline cellulose (NCC), within a specific range of NCC and NaCl concentration in the phase diagram [34]. LCH is a gel because its low volume fraction of NCC rods, but it is also a biphasic material with its solidlike rheological behaviour provided by the attractive glass matrix phase and anisotropy arisen from an embedded liquid crystalline phase. Aqueous NCC suspensions show a lyotropic liquid crystal

703

behaviour, forming a liquid crystal phase above a critical solids content. The liquid crystal phase observed in NCC suspensions is chiral nematic, which arises from the surface charge and twisting of NCC rods [35–37]. Further increases in NCC concentration (higher solid content) leads to formation of an anisotropic repulsive soft glass phase due to the excluded volume effect [38]. Salinity plays an important role in the gelation of NCC aqueous suspensions and introduces additional complexity to the phase diagram. The negatively charged NCC surface enables mediation of inter-particle repulsion by adjusting solution ionic strength [39–42]. Liquid crystallinity of NCC suspensions is non-linearly dependent on salinity, with transitions from liquid crystal, to isotropic, and to re-entrant liquid crystal phases with increasing salinity [34,43,44]. The presence of liquid crystal phase at high salinities allows spinodal decomposition to coincide with an anisotropic phase. This consequently leads to formation of a biphasic anisotropic material, LCH, containing an attractive glass phase and a liquid crystal phase. The attractive glass matrix in LCH is reversibly breakable, allowing the liquid crystal phase to align under external forces such as shear flow, but quickly jamming the liquid crystal phase in its orientation after external force ceases. Anisotropic orientations created in this manner are metastable, but are temporally persistent over observable time frames. This feature provides an efficient solution to the technical challenge in fabricating anisotropic soft materials, and enables the introduction of arbitrarily complex structures by applying different flow fields. An example is shown in graphical abstract which features a cockatoo drawn in an LCH (observed via crossed polarisers). This method of creating useful anisotropy in a soft material is thus considered promising in many applications such as biomedical and biomimicry [45]. This NCC hydroglass differs substantially from previously-reported NCC isotropic hydrogels [41,46–48], anisotropic hydrogels [49,50], and anisotropic composite hydrogels formed via freeze-casting [5,51], as none of these hydrogels demonstrates reversibly-tuneable structural anisotropy and mechanical properties after the network is formed. Our previous article [34] reported the existence of LCHs, some of their fundamental properties and behaviours, and revealed their potential for tuneable anisotropy in soft materials. However, a more detailed understanding is needed of the formation processes, including tuning of phase separation/gelation processes and their effect on LCH properties. Here we aim to provide a detailed characterisation of this anisotropic soft material, LCH, formed from NCC aqueous suspensions. Firstly, we seek to demonstrate the formation of LCH with tuneable anisotropy and mechanical properties varying over three orders of magnitude. Secondly, we investigate the controllable phase separation process in terms of colloidal stability and liquid crystallinity in response to ion specificity. We also aim to provide a link between the structural behaviour of the LCHs and their rheological response. 2. Materials and methods 2.1. Material The nanocrystalline cellulose (NCC) was sourced from Maine University Process Development Centre (Orono, ME) as 11.9 wt% NCC aqueous suspension with 0.9 wt% sulphur on dry NCC sodium form which is equivalent to 4.62 SO
3 per 100 anhydroglucose units. The NCC suspension was made by re-dispersing freezedried powder from hydrolysis of cellulose. The NCC has an average (sd = standard deviation) length and diameter of 210 (sd = 10) and 15 (sd = 4) nm respectively, characterised by AFM imaging in our previosu work [40]. The zeta potential of this particle is –45 mV dispersed in deionised water with no salt added. Sodium chloride (Merck KgaA, >99.5%) and sodium thiocyanate (Sigma Aldrich, >98%) were used to adjust the ionic strength and to determine

704

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

the ion specificities of NCC gelation. Diluted samples were prepared using deionized water from a reverse osmosis water system, which has a resistivity of 18.2 MX cm (Sartorius Stedim). 2.2. Sample preparation The suspension was prepared by diluting 11.9 wt% NCC with deionized water to achieve desired solids content. The ionic strength of each suspension was adjusted by adding preprepared salt solution at 2 M into the NCC suspension using a pipette. The salt-induced volume change for suspensions at the highest ionic strength (0.1 M) used was 5%, which is assumed to have negligible consequences on behaviours of interest. All samples prepared were rested for 24 h before testing. The pH value for all samples was between 6 and 6.2. Unless specified otherwise, all experiments were at room temperature. 2.3. Optical observation Crossed polarised photographs were taken using a Cannon 70D camera, with the sample placed between two polarisers. Polarisers were from Edmund Optics, with light transmission 0.01% across two polarisers with their axes perpendicularly aligned. The light source was a panel LED backlight from Edmund Optics with a color temperature 6500 K and <1% difference in light intensity over the panel. Crossed polarised micrographs were taken by microscope (Olympus BX40) equipped with a polariser above the light source and a polarised filter before the image sensor. 2.4. Zeta potential Zetasizer Nano ZS (Malvern Instrument, UK) was used to measure the electrophoretic mobility of NCC particles in suspension at different salt concentrations via dynamic light scattering. Zeta (f) potentials were obtained by converting mobility values using the Smoluchowski equation. Each sample was tested 3 times with 14 runs per measurement. 2.5. Dynamic light scattering (DLS) DLS measurements were made using a BI-200SM light scattering system (Brookhaven Instrument, US) equipped with a 532 nm laser at 25 °C. The detection was made at a scattering angle of 90°. The intensity size distributions were obtained using CONTIN algorithm to analyse correlation function in the instrument software. 2.6. Diffusive-wave spectroscopy (DWS) A commercial DWS instrument (Rheolaser, Formulaction, France) equipped with 650 nm laser was used to characterise the motion of NCC particles in suspensions and hydrogels at 25 . With known particle size from DLS measurement, the photon transport mean free path l* is estimated from the colloid refractive index (cellulose, 1.47) and solvent (water, 1.33), wavelength, and concentration based on Mie-theory [52,53]. For 5 wt% NCC suspension, l* is c. a. 600 lm, which satisfied the requirement of L/l* > 10 (L is the vial thickness, approximately 2 cm) [54]. The colloidal motion of NCC particles can be represented by the mean-square displacement (MSD) as a function of the correlation time. Samples were rested for 30 min before measurement to eliminate initial effects. 5 measurements were made for each curve to confirm reproducibility. 2.7. Small angle X-ray scattering (SAXS) NCC suspensions and hydrogels were characterised at 25 °C using small angle x-ray scattering (SAXSess mc2, Anton-Paar, Aus-

tria). The X-ray (k = 0.1542 nm) was incident from a 40 kV generator. The obtained 2D SAXS data was converted to 1D scattering curve using Saxsquant software by radial integration. The background scattering (for DI water with according salinities) was subtracted from SAXS data before analysis in Saxsquant software. 2.8. Rheology Samples are measured at MCR-502 compact modulus rheometer (Anton-Paar, Austria). Steel parallel plate geometry (diameter 50 mm) was used with 1 mm gap for measurement to determine the phase diagram. To prevent evaporation, silicon oil was placed on the edge of the plate during tests. Small amplitude oscillatory shear tests were performed for all samples for frequency ranges from 100 to 0.1 rad/s. The strain used in tests were in the linear viscoelastic region, which was determined by performing strain swap test; 0.5% strain was found to be suitable for this range of composition of NCC samples 3. Background - formation of NCC liquid crystal hydroglass (LCH) The LCH comprised of NCC colloidal rods is results from the interplay between liquid crystallinity and gelation behaviour of NCC aqueous suspension. NCC solid content and inter-particle attraction ultimately determine the formation of LCHs. The negative charged surface of NCC allows inter-particle attraction to be manipulated by adding electrolytes, compressing the surface double layer of the NCC and consequently reducing repulsion from Debye screening. Therefore, the presence of electrolyte in NCC aqueous suspensions can trigger transition between liquid/solid phases with their characteristic rheological properties. A liquid/solid phase diagram is demonstrated in our previous works [40,42] where a predominant role of DLVO type interaction is shown to determine colloidal stability of each distinct phase and kinetics of transitions between phases. Besides the liquid/solid gelation transition, LCH formation depends on liquid crystallinity (anisotropy) of the NCC suspension as a function of solid content and particle attraction. This is shown in Fig. 1, with liquid/solid phases marked by their isotropic/anisotropic behaviours. The presence of a re-entrant liquid crystal phase at high ionic strength fulfils the prerequisites for the formation of LCH: spinodal decomposition (and subsequent gelation) coinciding with an anisotropic liquid crystal phase. Scheme A-D in Fig. 1 illustrates the interplay between liquid crystallinity and gelation of NCC suspensions with the phase transition of 7 wt% NCC suspensions with NaCl used as an example (pictures in Fig. 1). The progressive addition of salt to a liquid crystal suspension (A) results in an initial reduction (B) and subsequent increase (C) of the proportion of liquid crystal phase. The suspension is then ‘quenched’ into the thermodynamically unstable region (D) causing spinodal decomposition and separation into DL and DS phases, resulting in a biphasic LCH. DS is a colloid-rich phase with an attractive glass morphology, which forms the matrix of LCHs and provides solidlike rheological behaviour. DL is a liquid crystal phase that provides LCHs with anisotropic characteristics. 4. Results and discussion 4.1. Optical observations Fig. 2 shows photographs and microscopy images taken through cross-polarisers, illustrating the formation of LCHs at two NCC concentrations and with two types of salt (NaCl and NaSCN). Liquid crystal (anisotropic) phase is bright under cross-polarisers while the attractive glass matrix (isotropic) is dark. Two general trends

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

705

Fig. 1. Schematic phase diagram of NCC aqueous suspension as a function of solid content and inter-particle attraction (represented by ionic strength). Relative positions of each phase and shape of boundaries are according to our previous work [34,42], and theoretical works in gelation [26] and liquid crystallinity [86] of colloidal suspensions. Pictures A-D on the right are taken through cross-polarisers and with corresponding compositions located on A-D respectively on the phase diagram.

Fig. 2. Cross-polarised pictures and micrographs for LCHs with salinity from 15 to 100 mM (across), NCC solid content (right), and type of salt (left). Scale bars represent 100 lm.

are observed from these pictures. Firstly, the proportion of liquid crystal phase in LCH decreases when ‘quenched’ to a higher salinity, eventually forming an isotropic gel at 100 mM salt concentration. This demonstrates a continuum of phase transition, under which the anisotropy becomes negligible as the colloid-poor phase

DL approaches isotropic behaviour. Secondly, compared with NaCl, NaSCN produces finer ‘grains’ and a more uniform distribution of the liquid crystal phase. This is attributed to different phase separation schemes induced by these two types of salt, driven by differences in colloidal stability and ion specificities.

706

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

4.2. Colloidal stability Understanding NCC colloidal stability, and change in stability resulting from addition of electrolyte, is important for proper interpretation of the formation of LCHs, as the colloidal stability plays a key role in phase separation and gelation. Though NCC colloidal stability [33,55], phase separation [40,43,56] and their correlation with rheology [42,57] have been explored in the literature, previous studies have not realised the formation of LCHs or characterised the underlying phase separation process leading to this anisotropic state of soft solid. With few exceptions [58,59], the effect of ion specificity on the stability of suspension of charged colloids is rarely discussed, though relevant relationships are well documented for solutions of protein and other natural or artificial macromolecules [60–62]. Additionally, the effect of ion specificities on the phase separation and gelation equilibrium of charged colloids including NCC has not been studied. The particle size distributions (PSDs) of NCC dispersed in salt solutions with concentrations of 0–100 mM are shown in Fig. 3. The addition of NaCl (Fig. 3a) versus NaSCN (Fig. 3b) causes different particle aggregation schemes. The addition of 10 mM NaSCN shows a sharper peak in the PSD at c.a. 100 nm, indicating a narrower dispersity of NCC compared with the zero-salt-added sample, while 10 mM NaCl does not substantially alter the PSD. NaCl concentrations of 25 mM or higher lead to agglomerations of NCC, which is signalled by the broadened peaks at larger hydrodynamic radius (c.a. 200 nm). Smaller peaks in the PSDs of NCC dispersions with this range of NaCl concentrations is attributed to un-aggregated particles [63,64]. The 25 and 50 mM NaSCN conditions have broad but still well-defined peaks in the PSDs, which

is characteristic of the formation of stable clusters, indicating a balance between long-range repulsion and short-range attraction. As such, aggregation takes place at a small scale, without macroscopic phase separation. NCC is electrostatically stabilised in aqueous solution due to its negatively charged surface. Consequently, variation of n potential in response to solution salinity provides a direct measurement of the colloidal stability and the critical aggregation threshold. Colloidal suspensions are considered stable if the absolute n potential of dispersed colloids is above 30 mV [65]. The values of n potential of NCC in 0–50 mM NaCl and NaSCN suspensions are illustrated in Fig. 4. For salinity above 1 mM, a large deviation in n potential can be seen for NCC dispersed in NaCl and NaSCN solutions: the NCC loses its stability at 10 mM NaCl, however it can remain stable at up to 25 mM in NaSCN solution (the magnitude of n potential is just slightly below 30 mV even at 50 mM NaSCN). The n potential measurements suggest that the NCC particles in NaSCN solution can retain electrostatic stability to a higher salinity that those in NaCl, consequently inhibiting phase separation. The colloidal behaviour of NCC particles in NaSCN solutions is in accordance with theoretical works [26,66] regarding phase separation processes during physical gelation of colloidal dispersions. The rate of phase separation is closely dependent on the relationship between colloidal interparticle repulsion and environmental parameters (e.g., temperature, ionic strength). Sharp changes in repulsion in response to external perturbation lead to rapid phase separation, whilst gradual changes lead to slow phase separation. The former leads to the macroscopic phase separation as even a very small step change in the environmental parameter above a threshold provides sufficient interparticle attraction to form large

Fig. 3. Particle size distribution curves of NCC dispersed in aqueous NaCl (a) and NaSCN (b) solutions of salinity from 0 to 100 mM. The y-axis is the distribution function determined from scattered light intensity (in arbitrary units). Curves are shifted along the y-axis to make them distinguishable, with the relative peak size unchanged.

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

Fig. 4. n potential measurement of NCC dispersed in aqueous NaCl and NaSCN solutions with salinities from 0 to 100 mM. Data points and error bars correspond to the average and standard deviation, respectively, obtained from 42 measured values (i.e., each sample was tested 3 times with 14 runs per measurement).

clusters or flocs, as a precursor to gelation. However, in the latter circumstances, the macroscopic phase separation is supressed because a small step change in the environmental parameter above the threshold of destabilisation does not provide enough ‘degree of sub-cooling’ (deviation from equilibrium state) for formation of observable aggregates. Therefore, under conditions in which phase separation is supressed, colloidal suspensions can undergo a phase separation process closer to the equilibrium route (micro-phase separation) in which it is possible to obtain stable clusters, and then lead to a more homogeneous gel structure if quenched further. The close-to-equilibrium phase separation route indicates that the NaSCN can promote colloidal stability and supress phase separation in NCC dispersion, which is not achievable by NaCl. Different phase separation routes resulting from NaCl and NaSCN can originate from specific ion effects such that the hydration of ions can influence the stability of charged colloids or macromolecules dispersed in the solution, which is known as Hofmeister phenomena [60–62]. According to Hofmeister phenomena, SCN
is a chaotropic (weakly hydrated) ion which is deemed to promote the surface hydration of dispersed colloids and enhance the colloidal stability, especially for negatively charged particles such as NCC; while Cl
is a kosmotropic (strongly hydrated) ion that reduces the colloidal stability and promotes the particle aggregation and phase separation. The suppression of macroscopic phase separation by NaSCN allows the segregation of liquid crystal phase and attractive glass phase in a smaller scale than those formed in NaCl solution, which causes a more spatially uniform distribution of LC phase. This scheme of phase separation is supported by the clusters observed in NaSCN solutions that had a definite size over a range of salinity (as seen in PSDs), and by the more negative values of f potentials for the NCC in NaSCN solutions.

4.3. Microstructure of NCC liquid crystal hydroglasses The overall phase behaviour of LCHs is also a function of the microstructural signatures within each phase, including colloidal rod separation and arrangement. Diffusive wave spectroscopy (DWS) has been used to examine the microstructure of biphasic LCHs by tracking the motion of colloids within the matrix. DWS has previously been used to reveal slow dynamics of various colloidal suspensions [67,68] including NCC [69], with a key advantage that its application is not limited to dilute systems, as in

707

DLS, and thus DWS enables probing concentration dependent structural formations such as liquid crystal and attractive glass phases. Mean square displacement (MSD, hr(s)2i) is a measurement of the distance travelled by the tracked particles within the time s. The spectrum of MSD are expected to include three regions [26]. First, an initial pure diffusive region (hr(s)2i / s1) over a short time scale where particle moves freely; this region is not measureable here due to instrument limitation. Secondly, a sub-diffusive region in which particle movement is mediated by inter-particle forces, resistance from which makes hr(s)2i / s
708

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

Fig. 5. (a) Dimensionless mean square displacement as a function of correlation time of 5 wt% NCC particles in aqueous suspensions containing 0–100 mM of NaCl or NaSCN. Dimensionless mean squared displacement is represented by hr2i/L2 where L is the hydrodynamic size of NCC rods (200 nm). (b) Cross-polarised pictures taken for NCC suspension. Samples A-G correspond to curves in (a) with respective compositions.

suspensions of 50 mM salinity show a liquid crystal region in cross-polarised pictures but demonstrate predominant attractive glass behaviour on the MSD curve, and why the 25 mM NaSCN makes the NCC suspensions macroscopically solid-like but shows predominantly liquid-like behaviour in MSD curve. Fig. 6 shows the Lorentz-corrected small-angle X-ray scattering curves of NCC dispersions at a range of compositions. The inverse of the q values of peaks of the scattering curves indicate the ‘preferred’ separation distance between rods, and can characterise rod packing within NCC suspensions [74]. From Fig. 5a, the transition from liquid crystal suspension (zero added salt) to attractive glass phase (100 mM NaCl) shifts the peak to a higher q value (i.e., shorter distance between particles). This indicates closer packing of rods occurs in the attractive glass phase due to interparticle attraction. In the suspensions with 25 and 50 mM NaCl (i.e., within LCH region), two peaks are present on the scattering curves, with q values characteristic of liquid crystal and attractive glass phases. This bimodal curve indicates formation of a biphasic structure in LCHs. Fig. 6a shows the gradual change in relative proportions of liquid crystal and attractive glass phases, whereby the LC peak decays while the attractive glass peak appears during the transition from liquid crystal suspension to LCH and then to isotropic gel. This continuum of phase transition agrees with crosspolarised photographs (Fig. 2) and mean square displacement curves (Fig. 5), whereby the liquid crystal proportion gradually decreases as the ionic strength increases, and eventually resulting in isotropic behaviour. Minimal differences are seen in scattering curves between NCC suspensions with 100 mM NaCl versus

100 mM NaSCN (Fig. 6b). This suggests that rod packing in highsalinity isotropic gels are independent of salt type, which may arise because at high salinity both NaCl and NaSCN can destabilise the NCC suspension and trigger phase separation. At 10 mM salinity, 7 wt% NCC suspensions are in a liquid crystal state for both NaCl and NaSCN (Fig. 6b photographic insert). However, the peak for 10 mM NaCl is located at a higher q (shorter inter-particle distance), indicating closer rod packing in a nematic phase (inserted schematics in Fig. 6b). The difference in separation distance of rods in the nematic phase caused by choice of salt type (NaCl vs. NaSCN) in NCC suspensions can arise from ion specificities, such that NaSCN results in a more negative surface potential at this salinity (Fig. 4). This leads to a stronger particle repulsion and consequently separates particles. 4.4. Rheology characterisation Rheological measurements reveal the phase transition continuum that leads to the formation of LCHs, and characterises the response of resultant LCHs to mechanical perturbation. Fig. 7 shows the storage (G’) and loss (G’’) moduli for 5 wt% NCC suspensions with 0–50 mM NaSCN (Fig. 7a) and NaCl (Fig. 7b) respectively. Macroscopically solid-like rheological response (G’ > G’’ and G’ / x0) appears at 10 mM NaCl. However, NCC suspensions with 10 mM NaSCN show liquid-like rheological responses (G’’ > G’ and G’ / xn, with n > 0). This indicates that the segregation of an attractive glass phase from the liquid crystal phase starts at a lower salinity in NaCl solutions compared to NaSCN solutions. This

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

709

Fig. 6. SAXS scattering curves for (a) 5 wt% NCC suspensions with NaCl; (b) 7 wt% NCC suspensions with NaCl and NaSCN, inserted cross-polarised photographs are for 10 mM NaCl and NaSCN respectively. Scattering curves are plotted in Lorentz-corrected form (I(q)  q2 v.s.q).

Fig. 7. Storage (G’) and loss (G’’) moduli for 5 wt% NCC with (a) NaSCN and (b) NaCl. Dashed lines are guide for eyes.

is attributed to NaCl more profoundly reducing the f potential of NCC, thus destabilising NCC suspensions. Further addition of either salt shifts the system to solid-like behaviour. However, NaCl is observed to have a more profound effect on rheology than NaSCN. For example, a concentration of 25 mM of NaCl leads to a G’ an order higher in magnitude that that resulting from a concentration of 25 mM NaSCN, despite both salts resulting in an LCH state. At 50 mM salinity, NCC suspensions have similar rheological responses with both salt types, which indicates that phase separation occurs in a comparable extent for suspensions with either salt, and the distribution of phases has less effect on the rheological behaviour. It is an ongoing challenge to use rheological measurements to analyse the phase transition continuum in soft matter systems. For example, a plot of G’ and G’’ against salinity can illustrate the build-up of structural strength (like that is shown in Fig. 8c), but it is inherently limited to reveal the phase transition, as both G’ and G’’ are functions of frequency. The values of G’ and G’’ at a fixed frequency are consequently unable to reveal the formation continuum of LCHs. Here, instead of using G’ and G’’ or tan (d) = G”/G’ values at specific frequencies, we use tan(d) values across a range of different frequencies to evaluate the phase transition continuum. This approach does not present specific values of G’ or G’’ but information about the shape and relative position

of spectra of G’ and G’’ as a function of frequency and solution environment. Tan(d) is the ratio between G’’ and G’, which shows the relative contribution of elastic and viscous components of the material to the overall mechanical response. Therefore, using tan (d) at various frequencies can reveal the relative elastic/viscous behaviour of the material over a range of investigated scales, providing an overview of the phase evolution process. Tan(d) is nearly independent of frequency across a range of frequencies for states near liquid-solid transition, indicating a close-to-self-similar relaxation where elastic and viscous contributions to relaxation are the same over an observable length scale. Besides the liquidsolid transition points [75], self-similar relaxation also occurs in homogeneous liquid crystal phase at certain concentrations [76–78] as indicated by a parallel spectra of G’ and G’’ across measurable frequencies. The tan(d) values for 7 wt% NCC as a function of solution salinity are plotted in Fig. 8 for NaSCN (Fig. 8a) and NaCl (Fig. 8b). Four general trends are observed in these figures, as follows: (1) 5 wt% NCC is originally in a liquid crystal state, signalled by tan(d) values almost identical across frequencies; (2) the initial addition of salt (1mM) destabilises the liquid crystal phase, causing the divergence of tan(d) as a function of frequency;

710

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

Fig. 8. Tan(d) values as a function of frequency and salinity for 7 wt% NCC suspension with (a) NaSCN and (b) NaCl. (c) Variation in storage (G’) modulus of 7 wt% NCC against salinity. (d) Stress–strain curve from static yielding tests for 7 wt% NCC with 25 mM NaCl or NaSCN. Shaded region indicates the non-linear viscoelastic behaviour.

(3) further increase in salinity (10 mM) leads to a convergence of tan(d), which indicates the re-growth of the liquid crystal phase; (4) tan(d) values of 1 indicate the formation LCH, and the onset of solid-like behaviour. Tan(d) does not converge in the region of LCH, which arises from the formation of a biphasic structure, in which relaxation schemes differ within each phase. Fig. 8b shows that liquid crystal phase is more easily destabilised by NaCl, resulting in a larger divergence in tan(d) as a function of frequency. In contrast, NaSCN causes less destabilisation of the liquid crystal phase (Fig. 8a), signalled by much smaller divergence of tan(d) within the biphasic region. The destabilisation of the liquid crystal phase caused by NaCl reduces formation of a re-entrant liquid crystal phase. Consequently, the NaCl system has a higher salinity boundary of re-entrant liquid crystal phase and a narrower compositional range of this phase. Note that the compositional range for LCH is only trivially affected by type of salt from the plot of tan(d), though the specific mechanical responses, e.g., G’ and G’’, differ between each type of salt within this region (Fig. 8c). This is because the chaotropic NaSCN can allow the suspensions to undergo phase separation in a route to being closer to equilibrium, which leads to a more homogeneous phase distribution compared with NaCl. The resultant meso-structural difference influences the macroscopic mechanical response, but cannot alter the critical salinity under which the attractive glass phase starts to segregate from liquid crystal phase forming LCHs. Fig. 8c illustrates the build-up of mechanical strength (represented by G’) of NCC suspensions as a function of increasing concentration of NaSCN and NaCl. The addition of NaSCN instead of

NaCl effectively avoids an initial drop in G’ due to the destabilisation of NCC suspension [40]. In the LCH region, NCC suspensions containing NaSCN show a substantially lower G’ than those containing NaCl. This is because different meso-structures form with each salt type. NaSCN produces a comparative more uniform and smaller mesh size in a network of attractive glass matrix (Fig. 2). This results in comparatively weaker network connections compared to the NaCl system. Under linear deformation, the stronger and more rigid ‘network’ connections in LCHs with NaCl lead to a higher mechanical strength. Fig. 8d shows the ‘static’ yielding behaviour of NCC LCHs for different shear rates (static yield stress defined here as the peak stress in the stress-strain curve), which further characterises the effect of salt type on the mechanical properties of LCHs. LCH prepared with NaCl has a higher modulus (slope before yielding point) than those prepared with NaSCN, although these yield at a larger strain. The use of NaSCN instead of NaCl can extend the linear viscoelastic region (shaded region in Fig. 8d), especially for the lower-stress and larger-strain region. 5.1. Scheme of ion specificities on phase separation and properties of LCHs Fig. 9 shows an overall scheme of how the chaotropic/kosmotropic salts influence the phase separation continuum, by altering the colloidal stability and the distribution of LC and attractive glass phases, and hence the mesostructure of resultant LCHs. Materials with strong and rigid network connections tend to have higher strength, whilst a more uniform distribution load-bearing structure leads to a larger number of ‘network’ connection and consequently a better ductility and toughness [79,80]. We believe this principle applies here for LCHs. Salts containing kosmotropic

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

711

Fig. 9. Schematics about phase separation scheme and formation of LCHs with NaSCN (chaotropic) or NaCl (kosmotropic) salt. Red rods represent NCC in liquid crystal phase, while green rods represent NCC in attractive glass phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ions such as NaCl steeply reduce the colloidal stability of NCC when solution ionic strength crosses a certain threshold (as shown by surface potential vs. salinity in Fig. 4). This causes phase separation to occur in a strongly non-equilibrium manner and the formation of an attractive glass phase in a larger fibril-like morphology. The strength and rigidity of this attractive glass phase leads to a higher elastic modulus of resultant LCHs. On the other hand, salts comprised of chaotropic ions such as NaSCN provide a more gradual destabilisation process at an elevated ionic strength (Fig. 4), which allows phase separation to occur at a microscopic scale (as seen by the formation of clusters of finite size in Fig. 3). This type of phase separation scheme results in a network composed of attractive glass matrix with a more homogenous mesh size and larger number of network connections. These can distribute load more uniformly throughout the material, extending the yielding point to a larger strain (Fig. 8d). 6. Conclusion This work has used various microscopic, scattering and rheological measurements to study the formation and properties of LCHs, and builds a correlation between the results from each method. Whilst our previous work revealed the LCH phase using NaCl [34], this article also shows that they can be prepared using NaSCN as the modulator of salinity. The variation in colloidal stability as a function of solution ionic strength plays an essential role in the phase separation continuum of NCC suspensions, and hence the formation of LCHs, which is revealed in detail by the evolution of particle size distribution and colloidal surface potential against salt concentration. A key hypothesis is that the specific ion effect arising from Hofmeister phenomena can be used to control the dependency of colloidal stability as a function of ionic strength. It is thus expected that the structure and property of LCHs can be tuned by using different salts. We show that the addition of NaSCN instead of NaCl can retain the stability of NCC colloids in aqueous solution up to a salinity as high as 30 mM whereas NaCl destabilises dispersed NCC at 10 mM salinity. As the result of a more gradual dependency of surface potential on solution ionic strength, the addition of NaSCN provide a smaller ‘degree of sub-cooling’ (deviation from equilibrium). This enables NCC suspensions to undergo a phase transition process closer to an equilibrium route, which allows the segregation of attractive glass phase to occur at a smaller scale, and postponing macroscopic phase separation to higher salinity.

The distinct effects of NaCl and NaSCN on the colloidal stability of NCC is attributed to the chaotropic/ kosmotropic nature of Cl
and SCN
ions according to Hofmeister phenomena. LCHs prepared with NaCl or NaSCN show characteristically different mesostructures, whereby NaSCN produces a comparatively more uniform distribution of LC and attractive colloidal glass phases. The microstructural signatures revealed by DWS and SAXS show that there are minimal differences for rod packings between NaCl and NaSCN salts within phases present. This indicates that the type of salt determines the distribution of LC and attractive glass phases, and hence the mesostructures of LCHs and corresponding rheological behaviours, but the salt type cannot alter the microstructure of each distinct phase nor the critical salt concentration that triggers particle aggregation. The plot of tan(d) of NCC suspensions as a function of salinity and frequency reveal the phase transition and the formation of LCHs, and shows a good agreement with the microscopic and scattering measurements. From the tan(d) curves, small additions of NaCl (1 mM) destabilises the original liquid crystal phase (in contrast to NaSCN), and consequently narrows the compositional range of the re-entrant liquid crystal region. The existence of the re-entrant liquid crystal phase at elevated ionic strength is an essential prerequisite to the formation of LCHs. Consequently, it may be possible to use various salt types according to the Hofmeister series to tailor colloidal stability to control the compositional window of the re-entrant liquid crystal phase. This may enable the formation of LCHs with a wider array of materials and compositions. We anticipate that this approach may be of interest in fabrication of anisotropic soft materials with distinct micro- and meso- structures. The characterisation of specific ion effect based on Hofmeister phenomena advances from existing studies on colloidal stability of NCC particles in aqueous suspensions [55,81]; and that the rheological behaviours of LCHs reported in this article also extended upon existing literatures of NCC suspension rheology [16,38,42], none of which have characterised the formation and properties of LCH phase. LCH is a novel anisotropic soft material with its structural ordering programmable by shear flow. This flexible control of structural anisotropy enables LCH to have potential in creating complex structural order, which is difficult to fabricate otherwise in materials that is of interest in many biological and engineering applications [21,22]. Our future work aims to investigate the existence of LCH in NCC suspensions derived from different sources, as well as for other colloidal rods. The size, aspect ratio

712

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713

and polydispersity of NCC rods are dependent on their raw materials [82], which are anticipated to affect the formation scheme of LCH. In addition, future work could investigate whether how the mean size of NCC rods varies between those in the LC phase and those in the attractive glass phase within the LCH, as we note it is reported that the NCC rod in isotropic phase is smaller than that in coexisting LC phase [43]. Future work should seek to develop novel applications based on newly found NCC liquid crystalline hydroglass structures. This could be achieved by utilising the flow-programmable ordering of liquid crystalline domains to template assembly of other nanoparticles and thus enable various functionality [83–85]. Acknowledgements Yuan Xu thanks the University of Queensland (UQ) for an International (UQI) Scholarship. The research originated from support via the Australian Research Council Centre of Excellence in Plant Cell Walls (CE110001007). Yuan Xu salary is currently supported by the Australian Research Council Discovery Project DP180101919. References [1] S.G. Rudisill, M.D. DiVito, A. Hubel, A. Stein, In vitro collagen fibril alignment via incorporation of nanocrystalline cellulose, Acta Biomater. 12 (2015) 122– 128. [2] J.L. Gennisson, T. Deffieux, E. Mace, G. Montaldo, M. Fink, M. Tanter, Viscoelastic and anisotropic mechanical properties of in vivo muscle tissue assessed by supersonic shear imaging, Ultrasoun. Med. Biol. 36 (2010) 789– 801. [3] A.H. Torbati, P.T. Mather, A hydrogel-forming liquid crystalline elastomer exhibiting soft shape memory, J. Polym. Sci. B: Polym. Phys. 54 (2016) 38–52. [4] A.H. Clark, S.B. Ross-Murphy, Struct. Mech. Proper. Biopolym. Gel. 83 (1987) 57–192. [5] M. Chau, K.J. De France, B. Kopera, V.R. Machado, S. Rosenfeldt, L. Reyes, et al., Composite hydrogels with tunable anisotropic morphologies and mechanical properties, Chem. Mater. 28 (2016) 3406–3415. [6] Y.S. Kim, M. Liu, Y. Ishida, Y. Ebina, M. Osada, T. Sasaki, et al., Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel, Nat. Mater. 14 (2015) 1002–1007. [7] R.S. Kularatne, H. Kim, M. Ammanamanchi, H.N. Hayenga, T.H. Ware, Shapemorphing chromonic liquid crystal hydrogels, Chem. Mater. 28 (2016) 8489– 8492. [8] K.J. Walker, S.V. Madihally, Anisotropic temperature sensitive chitosan-based injectable hydrogels mimicking cartilage matrix, J. Biomed. Mater. Res. B Appl. Biomater. 103 (2015) 1149–1160. [9] X.Y. Lin, Z.J. Wang, P. Pan, Z.L. Wu, Q. Zheng, Monodomain hydrogels prepared by shear-induced orientation and subsequent gelation, RSC Adv. 6 (2016) 95239–95245. [10] Y. Tanaka, A. Kubota, M. Matsusaki, T. Duncan, Y. Hatakeyama, K. Fukuyama, et al., Anisotropic mechanical properties of collagen hydrogels induced by uniaxial-flow for ocular applications, J. Biomater. Sci. Polym. Ed. 22 (2011) 1427–1442. [11] Q. Lu, S. Bai, Z. Ding, H. Guo, Z. Shao, H. Zhu, et al., Hydrogel assembly with hierarchical alignment by balancing electrostatic forces, Adv. Mater. Interf. 3 (2016) 1500687. [12] M. Liu, Y. Ishida, Y. Ebina, T. Sasaki, T. Hikima, M. Takata, et al., An anisotropic hydrogel with electrostatic repulsion between cofacially aligned nanosheets, Nature 517 (2015) 68–72. [13] K. Sano, Y. Ishida, T. Aida, Synthesis of anisotropic hydrogels and their applications, Angew. Chem. (2017). [14] M.A. Haque, T. Kurokawa, J.P. Gong, Anisotropic hydrogel based on bilayers: color, strength, toughness, and fatigue resistance, Soft Mat. 8 (2012) 8008. [15] M. Chen, J. Zhu, G. Qi, C. He, H. Wang, Anisotropic hydrogels fabricated with directional freezing and radiation-induced polymerization and crosslinking method, Mater. Lett. 89 (2012) 104–107. [16] S. Shafiei-Sabet, W.Y. Hamad, S.G. Hatzikiriakos, Rheology of nanocrystalline cellulose aqueous suspensions, Langmuir: ACS J. Surf. Colloid. 28 (2012) 17124–17133. [17] M.T. McClendon, S.I. Stupp, Tubular hydrogels of circumferentially aligned nanofibers to encapsulate and orient vascular cells, Biomaterials 33 (2012) 5713–5722. [18] J.A. Muller, R.S. Stein, H.H. Winter, Director dynamics of uniformly aligned nematic liquid crystals in transient shear flow, Rheol. Acta 33 (1994) 473–484. [19] B. Medronho, S. Shafaei, R. Szopko, M.G. Miguel, U. Olsson, C. Schmidt, Shearinduced transitions between a planar lamellar phase and multilamellar vesicles: continuous versus discontinuous transformation, Langmuir: ACS J. Surf. Colloid. 24 (2008) 6480–6486.

[20] S. Ahadian, J. Ramon-Azcon, M. Estili, X. Liang, S. Ostrovidov, H. Shiku, et al., Hybrid hydrogels containing vertically aligned carbon nanotubes with anisotropic electrical conductivity for muscle myofiber fabrication, Sci. Rep. 4 (2014) 4271. [21] T.H. Ware, M.E. McConney, J.J. Wie, V.P. Tondiglia, T.J. White, Actuating materials. Voxelated liquid crystal elastomers, Science 347 (2015) 982–984. [22] T.J. White, D.J. Broer, Programmable and adaptive mechanics with liquid crystal polymer networks and elastomers, Nat. Mater. 14 (2015) 1087–1098. [23] H. Tanaka, Y. Nishikawa, T. Koyama, Network-forming phase separation of colloidal suspensions, J. Phys.: Condens. Mat. 17 (2005) L143–L153. [24] E.M. Ahmed, Hydrogel: Preparation, characterization, and applications: a review, J. Adv. Res. 6 (2015) 105–121. [25] R. Piazza, G.D. Pietro, Phase separation and gel-like structures in mixtures of colloids and surfactant, Europhys. Lett. (EPL) 28 (1994) 445–450. [26] E. Zaccarelli, Colloidal gels: equilibrium and non-equilibrium routes, J. Phys.: Condens. Matt. 19 (2007) 323101. [27] Y. Gao, J. Kim, M.E. Helgeson, Microdynamics and arrest of coarsening during spinodal decomposition in thermoreversible colloidal gels, Soft Mat. 11 (2015) 6360–6370. [28] F. Sciortino, P. Tartaglia, Glassy colloidal systems, Adv. Phys. 54 (2007) 471– 524. [29] E. Bukusoglu, S.K. Pal, J.J. de Pablo, N.L. Abbott, Colloid-in-liquid crystal gels formed via spinodal decomposition, Soft Mat. 10 (2014) 1602–1610. [30] A.M. Lapeña, S.C. Glotzer, S.A. Langer, A.J. Liu, Effect of ordering on spinodal decomposition of liquid-crystal/polymer mixtures, Phys. Rev. E 60 (1999) R29–R32. [31] A. Matsuyama, T. Kato, Early stages of spinodal decomposition in binary liquid crystal mixtures, J. Chem. Phys. 113 (2000) 9300–9309. [32] Z. Dogic, S. Fraden, Ordered phases of filamentous viruses, Curr. Opin. Coll. Interf. 11 (2006) 47–55. [33] X.M. Dong, T. Kimura, J.-F. Revol, D.G. Gray, Effects of ionic strength on the isotropic
chiral nematic phase transition of suspensions of cellulose crystallites, Langmuir: ACS J. Surf. Colloid. 12 (1996) 2076–2082. [34] Y. Xu, A.D. Atrens, J.R. Stokes, Liquid crystal hydroglass formed via phase separation of nanocellulose colloidal rods, Soft Mat. 15 (2019) 1716–1720. [35] W.J. Orts, L. Godbout, R.H. Marchessault, J.F. Revol, Enhanced ordering of liquid crystalline suspensions of cellulose microfibrils: a small angle neutron scattering study, Macromolecules 31 (1998) 5717–5725. [36] J. Araki, S. Kuga, Effect of trace electrolyte on liquid crystal type of cellulose microcrystals, Langmuir:ACS J. Surf. Colloid. 17 (2001) 4493–4496. [37] I. Usov, G. Nystrom, J. Adamcik, S. Handschin, C. Schutz, A. Fall, et al., Understanding nanocellulose chirality and structure-properties relationship at the single fibril level, Nat. Commun. 6 (2015) 7564. [38] E.E. Ureña-Benavides, G. Ao, V.A. Davis, C.L. Kitchens, Rheology and phase behavior of lyotropic cellulose nanocrystal suspensions, Macromolecules 44 (2011) 8990–8998. [39] Y. Habibi, Key advances in the chemical modification of nanocelluloses, Chem. Soc. Rev. 43 (2014) 1519–1542. [40] Y. Xu, A.D. Atrens, J.R. Stokes, Rheology and microstructure of aqueous suspensions of nanocrystalline cellulose rods, J. Colloid Interf. Sci. 496 (2017) 130–140. [41] M. Chau, S.E. Sriskandha, D. Pichugin, H. Therien-Aubin, D. Nykypanchuk, G. Chauve, et al., Ion-mediated gelation of aqueous suspensions of cellulose nanocrystals, Biomacromolecules 16 (2015) 2455–2462. [42] Y. Xu, A.D. Atrens, J.R. Stokes, ‘‘Liquid, gel and soft glass” phase transitions and rheology of nanocrystalline cellulose suspensions as a function of concentration and salinity, Soft Mat. 14 (2018) 1953–1963. [43] A. Hirai, O. Inui, F. Horii, M. Tsuji, Phase separation behavior in aqueous suspensions of bacterial cellulose nanocrystals prepared by sulfuric acid treatment, Langmuir: ACS J. Surf. Colloid. 25 (2009) 497–502. [44] C. Honorato-Rios, A. Kuhnhold, J.R. Bruckner, R. Dannert, T. Schilling, J.P.F. Lagerwall, Equilibrium liquid crystal phase diagrams and detection of kinetic arrest in cellulose nanocrystal suspensions, Front. Mater. (2016) 3. [45] S.J. Woltman, G.D. Jay, G.P. Crawford, Liquid-crystal materials find a new order in biomedical applications, Nat. Mater. 6 (2007) 929–938. [46] X. Yang, E. Bakaic, T. Hoare, E.D. Cranston, Injectable polysaccharide hydrogels reinforced with cellulose nanocrystals: morphology, rheology, degradation, and cytotoxicity, Biomacromolecules 14 (2013) 4447–4455. [47] C. Zhou, Q. Wu, Y. Yue, Q. Zhang, Application of rod-shaped cellulose nanocrystals in polyacrylamide hydrogels, J. Colloid Interf. Sci. 353 (2011) 116–123. [48] J. Yang, C.R. Han, F. Xu, R.C. Sun, Simple approach to reinforce hydrogels with cellulose nanocrystals, Nanoscale 6 (2014) 5934–5943. [49] C. Salas, T. Nypelö, C. Rodriguez-Abreu, C. Carrillo, O.J. Rojas, Nanocellulose properties and applications in colloids and interfaces, Curr. Opin. Colloid Interf. 19 (2014) 383–396. [50] T. Saito, T. Uematsu, S. Kimura, T. Enomae, A. Isogai, Self-aligned integration of native cellulose nanofibrils towards producing diverse bulk materials, Soft Mat. 7 (2011) 8804. [51] T. Kohnke, T. Elder, H. Theliander, A.J. Ragauskas, Ice templated and crosslinked xylan/nanocrystalline cellulose hydrogels, Carbohydr. Polym. 100 (2014) 24–30. [52] C. Oelschlaeger, M. Schopferer, F. Scheffold, N. Willenbacher, Linear-tobranched micelles transition: a rheometry and diffusing wave spectroscopy (DWS) study, Langmuir: ACS J. Surf Colloid. 25 (2009) 716–723. [53] P.D. Kaplan, M.H. Kao, A.G. Yodh, D.J. Pine, Geometric constraints for the design of diffusing-wave spectroscopy experiments, Appl. Opt. 32 (1993) 3828–3836.

Y. Xu et al. / Journal of Colloid and Interface Science 555 (2019) 702–713 [54] I. Martiel, L. Sagalowicz, R. Mezzenga, Viscoelasticity and interface bending properties of lecithin reverse wormlike micelles studied by diffusive wave spectroscopy in hydrophobic environment, Langmuir: ACS J. Surf. Colloid. 30 (2014) 10751–10759. [55] H. Oguzlu, C. Danumah, Y. Boluk, Colloidal behavior of aqueous cellulose nanocrystal suspensions, Curr. Opin. Colloid Interf. Sci. 29 (2017) 46–56. [56] T. Abitbol, D. Kam, Y. Levi-Kalisman, D.G. Gray, O. Shoseyov, Surface charge influence on the phase separation and viscosity of cellulose nanocrystals, Langmuir (2018). [57] S. Shafiei-Sabet, W.Y. Hamad, S.G. Hatzikiriakos, Ionic strength effects on the microstructure and shear rheology of cellulose nanocrystal suspensions, Cellulose 21 (2014) 3347–3359. [58] T. Phan-Xuan, A. Thuresson, M. Skepö, A. Labrador, R. Bordes, A. Matic, Aggregation behavior of aqueous cellulose nanocrystals: the effect of inorganic salts, Cellulose 23 (2016) 3653–3663. [59] R. Prathapan, R. Thapa, G. Garnier, R.F. Tabor, Modulating the zeta potential of cellulose nanocrystals using salts and surfactants, Colloids Surf. A 509 (2016) 11–18. [60] P. Lo Nostro, B.W. Ninham, Hofmeister phenomena: an update on ion specificity in biology, Chem. Rev. 112 (2012) 2286–2322. [61] L. Liu, R. Kou, G. Liu, Ion specificities of artificial macromolecules, Soft Mat. 13 (2016) 68–80. [62] J.C. Lutter, T.Y. Wu, Y. Zhang, Hydration of cations: a key to understanding of specific cation effects on aggregation behaviors of PEO-PPO-PEO triblock copolymers, J. Phys. Chem. B. 117 (2013) 10132–10141. [63] J. Cai, L. Zhang, S. Liu, Y. Liu, X. Xu, X. Chen, et al., Dynamic self-assembly induced rapid dissolution of cellulose at low temperatures, Macromolecules 41 (2008) 9345–9351. [64] A. Lu, Y. Liu, L. Zhang, A. Potthast, Investigation on metastable solution of cellulose dissolved in NaOH/urea aqueous system at low temperature, J. Phys. Chem. B 115 (2011) 12801–12808. [65] J. Mewis, N.J. Wagner, Colloidal Suspension Rheology, Cambridge University Press, New York, 2012. [66] E. Zaccarelli, S.V. Buldyrev, E. La Nave, A.J. Moreno, I. Saika-Voivod, F. Sciortino, et al., Model for reversible colloidal gelation, Phys. Rev. Lett. 94 (2005) 218301. [67] S. Romer, C. Urban, H. Bissig, A. Stradner, F. Scheffold, P. Schurtenberger, Dynamics of concentrated colloidal suspensions: diffusion, aggregation and gelation, Philos. Trans. R. Soc. A: Math., Phys. Eng. Sci. 359 (2001) 977–984. [68] H.M. Wyss, S. Romer, F. Scheffold, P. Schurtenberger, L.J. Gauckler, Diffusingwave spectroscopy of concentrated alumina suspensions during gelation, J. Colloid Interf. Sci. 241 (2001) 89–97. [69] B. Derakhshandeh, G. Petekidis, S. Shafiei Sabet, W.Y. Hamad, S.G. Hatzikiriakos, Ageing, yielding, and rheology of nanocrystalline cellulose suspensions, J. Rheol. 57 (2013) 131–148.

713

[70] J. He, M. Mak, Y. Liu, J.X. Tang, Counterion-dependent microrheological properties of F-actin solutions across the isotropic-nematic phase transition, Phys. Rev. E: Stat. Nonlin. Soft Mat. Phys. 78 (2008) 011908. [71] M.M. Alam, R. Mezzenga, Particle tracking microrheology of lyotropic liquid crystals, Langmuir: ACS J. Surf. Colloid. 27 (2011) 6171–6178. [72] R. Mezzenga, C. Meyer, C. Servais, A.I. Romoscanu, L. Sagalowicz, R.C. Hayward, Shear rheology of lyotropic liquid crystals: a case study, Langmuir: ACS J. Surf. Colloid. 21 (2005) 3322–3333. [73] K.N. Pham, A.M. Puertas, J. Bergenholtz, S.U. Egelhaaf, A. Moussaid, P.N. Pusey, et al., Multiple glassy states in a simple model system, Science 296 (2002) 104–106. [74] C. Schutz, M. Agthe, A.B. Fall, K. Gordeyeva, V. Guccini, M. Salajkova, et al., Rod packing in chiral nematic cellulose nanocrystal dispersions studied by smallangle x-ray scattering and laser diffraction, Langmuir: ACS J. Surf. Colloid. 31 (2015) 6507–6513. [75] C. Schwittay, M. Mours, H.H. Winter, Rheological expression of physical gelation in polymers, Faraday Discuss. 101 (1995) 93. [76] S.V. Muniandy, C.S. Kan, S.C. Lim, S. Radiman, Fractal analysis of lyotropic lamellar liquid crystal textures, Physica A 323 (2003) 107–123. [77] M. Nakagawa, K. Kobayashi, M. Hori, M. Okabe, Y. Hori, On the fractal distribution of embryos at nematic-isotropic critical point, Mol. Cryst. Liq. Cryst. 195 (2006) 15–25. [78] S.M. Hashemi, U. Jagodic, M.R. Mozaffari, M.R. Ejtehadi, I. Musevic, M. Ravnik, Fractal nematic colloids, Nat. Commun. 8 (2017) 14026. [79] N. Koumakis, G. Petekidis, Two step yielding in attractive colloids: transition from gels to attractive glasses, Soft Mat. 7 (2011) 2456. [80] K.N. Pham, G. Petekidis, D. Vlassopoulos, S.U. Egelhaaf, P.N. Pusey, W.C.K. Poon, Yielding of colloidal glasses, Europhys. Lett. (EPL). 75 (2006) 624–630. [81] L. Zhong, S. Fu, X. Peng, H. Zhan, R. Sun, Colloidal stability of negatively charged cellulose nanocrystalline in aqueous systems, Carbohydr. Polym. 90 (2012) 644–649. [82] Y. Zhang, Q. Cheng, C. Chang, L. Zhang, Phase transition identification of cellulose nanocrystal suspensions derived from various raw materials, J. Appl. Polym. Sci. 135 (2018) 45702. [83] K.E. Shopsowitz, H. Qi, W.Y. Hamad, M.J. Maclachlan, Free-standing mesoporous silica films with tunable chiral nematic structures, Nature 468 (2010) 422–425. [84] D.G. Gray, X. Mu, Chiral nematic structure of cellulose nanocrystal suspensions and films; polarized light and atomic force microscopy, Materials 8 (2015) 7873–7888. [85] G.R. Meseck, A.S. Terpstra, M.J. MacLachlan, Liquid crystal templating of nanomaterials with nature’s toolbox, Curr. Opin. Colloid Interf. Sci. 29 (2017) 9–20. [86] T. Drwenski, S. Dussi, M. Hermes, M. Dijkstra, R. van Roij, Phase diagrams of charged colloidal rods: Can a uniaxial charge distribution break chiral symmetry?, J Chem. Phys. 144 (2016) 094901.