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Rheo-optics Norman J Wagner Rheo-optics blend multidisciplinary talents to address the complex flow, rheology, and microstructure of complex fluids. It is based on the concept that material micros tructure controls rheology as well as opt ical propert ies, and that rheology and microstructure are usually strongly coupled . Recent improvements in optical methods, model system development, statistical mechan ical theories , simulations and the development of new techniques , combined with the explosion in variety and complexity of materials under scrutiny makes for a dynamic and rapidly expanding research field, often with direct technological relevance.

Addresses Associate Profcssor of Chemical Engineering, Center for Molecular and Engineering Thcrmodynamics, University of Delaware, Newark DE 19716, USA; e-mail: [email protected] Current Opinion in Colloid & Interface Science 1998 .3 :391-400 Electronic identifier: 1359-0294·003-00391

© Current Chemistry ISSN 1359·0294 Abbreviations DLS dynamic light scattering DWS diffusing wave spectroscopy LCP liquid crystalline polymer SALS small·angle light scallering SANS small·angle neutron scall ering SOC stress optical coefficient SOR stress optical rule

the past few years gained through a variety of rhea-optic techniques. Note that although many sources of radi at ion can be used to study str uc ture under flow, space limitations restrict thi s review of rhea-optics to methods empl oyin g radiation in the visible wavelengths. Successful rhco-optics can lead to the measurement of rheological properties not otherwise att ainab le, such as deconvolution of rheological properties into partial properties, and rationalization of :J complex rheological behavior in terms of material microstructure and its changes. The first achievement often relies on the clever application of optics (0 resolve the optical tensor when the relevant components of the stress tensor arc d ifficult or intract able to measure by tradit ion al mechanical rheometry, The second achievement exploits the differences in optical signatures in multicornponcnt mixtures or symmetries in the micromcchanical theor ies of rheology to resolve overall rheological behavior into that of ind ividual components or molecular (or colloid al) forces. The latter uchicvcmcnt employs turbidity and scattering methods applied to many materials and flows . This article is organized as follow s: after discussing the fundamentals of rhco-optics, a selection of very recent examples from the literature will be discussed illustrating both achievements . Finally, the use of gcncrulizcd Stokes-Einstein relations along with advances in understanding photon tran sport in translucent materials to infer rheological behavior will be examined.

Introduction Rheology, the science of material deformation and flow, has us onc of its primary goals the measurement and prediction of :J material's rheological properties (i.c . viscosity, modulus, relaxation spectra). In the second half of this century, the use of rheological measurements as a probe of material microstructure and molecular or mesoscale dynamics is manifest. For highly structured complex fluids rheologic al cha racte rization oft en requires a complementary molecular an d/or mesoscale invest igation, such as afforded by optical means. 'Rhco-oprics' or optical rheometry', as the names imply, is the science of rheometry when optical methods arc utilized. A rheologist can exploit the optical changes in a material under deformation (0 quantitatively measure, in the strictest sense of rhco-optics, rhe olo gical propert ies; whereas a looser interpretation, a rheologist can employ optical methods as a diagnostic to qualitatively interpret rhcological behavior in microstructural terms . The choice of experimental technique and experimental realization hinge on a material's spe cific optical propert ies, on wha t rheological property is to be measured and on what level of microstructure is to be quantified. In this necessarily brief and restricted review, I de scribe the recent advances over

Fundamental rheo-optlcs The principle underlying rheo-optics is that at a micrornechunical le ve I rheological properties result from a material deformation, and that material deformation has an optical signature. The former concept stems from Boltzmann's calculation of transport properties in gases via kinetic theory and has since been the staple of kinetic theorist s and those modeling transport coefficients in polymers, surfacmnts, colloids and complex fluids. Stud ies on the application of electromagnetic theory to calculate light's interaction with the deformed matter, which gives the resultant optical signature, and the quantitarive relationship back to the. rheological property of interest were carr ied on "linear polymers over 50 years ago. :\Iorc recent extensions to concentration fluctu ations and particul ate and colloidal systems have been performed. This constitutes the theoretical underpinning of our appl ication of optical dcrcction to qu antitatively resol ving a material's rheological behavior. The dera ils of construct ing a rhco-optical experiment have been described in det ail in the recent monograph by Fuller [II. The simplest example of an 'optical train ' is a

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white light source, a polarizer, a stretching plastic film for the sample, another polarizer oriented at 90· to that of the first to be used as an analyzer, and one's own eye as the detector. In this case, as the film is stretched, the polymer chains will become oriented and thus optically anisotropic such that some of the light will now pass the analyzer and register on the detector. Quantitative measurement of the light transmission can be related to the elongational stress via the stress-optical coefficient (SOC). Optical activity could constitute either the rotation of the polarization state of incident light (birefringence) or the loss of transmitted light intensity, which is dependent upon the incident polarization state (dichroism). The former can be intrinsic (due to anisotropy in monomeric polarizability) or form (due to anisotropy in molecular shape plus refractive index contrast with the solvent). Dichroism can be conservative (due to scattering) or due to optical absorption. A sample's' optical activity, which may be caused by both linear and circular birefringence and dichroism, and the relative orientation angles for each state can be described in terms of a Mueller matrix [1]. This is a useful description enabling Stokes vector calculus of the optical train. Thus, for detection of the desired optical property of the sample, an optical train can be engineered; the recent monograph by Fuller [1] is an appropriate starting place. Therein it is demonstrated how phase modulation combined with lock-in amplification can increase sensitivity so as to detect molecular distortion in the linear regime. For some applications, small angle light scattering (SALS) and direct microscopy [Z] are preferred.. This can be accomplished by light scattering from either a parallel plate rheometer [3] or a stress rheometer with couette cell [4-,5,6-]. Proper optics enable quantitative analysis of the light scattering patterns for both small and wide scattering angles [Z,3,4-].

highly birefringent materials as liquid crystalline polymers (LCPs). This has aided the use of birefringence in non-viscometric flows, such as slit flow or planePoiseuille flow [IZ-]. Deconvolution of the birefringence into true mechanical properties is problematic in non-viscometric flows [13]. The rheo-optical results can, however, be combined with numerical simulations to assess the accuracy of constitutive equations [14-]. For planar flows, an experimental method has been proposed to eliminate end effects for pointwise measurements of the birefringence (HC Ottinger, personal communication).

Stress-optical rules As noted, stress-optical rules, SORs, are constructed by relating microstructure to rheology via micromechanics and to optical properties by electrostatics. Kuhn and Griin (see [1] for a description) constructed the stress optical rule for a model of polymer chains; later Onuki and Kawasaki derived a formalism for the birefringence induced by flowing a classical fluid near the critical point which was later broadened to include both form and intrinsic birefringence in flowing polymer solutions (see [1] for a description). This approach has been robust and fruitful, having been extended to colloids [15,16-]. Theoretical analysis enables deconvoluting the stress tensor into components due to interparticle forces and hydrodynamic interactions in dense, shear-thickening colloidal dispersions via rheo-optics [17],' proving the importance of hydrodynamic interactions at high stresses and concentrations.

Optical methods enable the full deviatoric stress tensor by rheo-optical methods to be measured. The technique of oblique transmission, whereby the laser beam is transmitted obliquely to the flow plane, enables transient measurements of first the viscosity, and second the normal stress differences simultaneously [7-10]; this is not possible mechanically. Combined with transient shear flows, the method provides stringent tests of models for polymer rheology ([11]; D Olsen, E Brown, W Burghardt, personal communication).

Current advances include developing SORs for the anomalous behavior of near-S polymer solutions [18-], among other complex fluids. Quantitative measurement of the dynamics of individual components in miscible polymer blends is possible once stress-optical coefficient (SaCs) are known [19-]. Advances in particle simulation methods now enable Brownian dynamics simulations of the transient response of complex molecules to applied flows, including predicting the rheo-optical response. For example, Andrews et 01. [ZO-] study the effect of flexibility on the rhco-optical behavior of serniflexible polymers in shear flow, as well as the effect of solvent quality (N Andrew, A Mcl-lugh, J Schieber, personal communication). Simulations of elongational flow for both semiflexible polymers and polyelectrolytes are also reported [ZI,ZZ]. This approach promises arnethod to resolve the rheo-optical behavior of more complex materials, for which analytical modeling may be intractable.

Because birefringence is caused by a phase difference, it is detected as a periodic response, thus complicating resolution of the absolute magnitude for thick samples and highly birefringent materials. A full spectrographic method has been demonstrated, however, which resolves the absolute birefringence through the use of a white light source and spectrographic detection, even for such

As with any analytical method In polymer science, polydispersity effects should be included. Theoretical work by Bossart and Ottinger [Z3] resolving the effect of polydispersity on the stress-optical rule for polymer solutions has been turned into a precise method for determining polymer molecular weight and polydispersity [Z4-] through measurements of the 'orientation resistance'.

Optical rheometry: technological advances

Rheo-optics Wagner

Light scattering from shearing colloidal dispersions Light scattering is a powerful method for studying the static and dynamic properties of colloidal dispersions. Sophistication in the development of optically pure, model dispersions has led to substantial investigations into the microstructure of flowing dispersions. A number of recent reviews appearing in this journal discuss aspects of this [25-29], attesting to the intense research interest. Of direct rheo-optic concern is the attempt to quantify the non-equilibrium structure of flowing colloidal dispersions via quantitative light scattering and to compare that with simulations [26] and micromechanical theory [16°] and rheology. Despite many years of intense rigorous theoretical and simulational efforts, theory and experiment often remain far apart [16°,26]. A similar theoretical and experimental approach is employed to study the spinodal decomposition of colloid dispersions, the shift of the spinodal with shear, and critical enhancement of the shear viscosity [30-32]. Near the critical point, strong coupling of the long wavelength fluctuations in colloidal dispersions to the applied shear flow is observed. A fundamental theoretical analysis of the equations of motion are in excellent qualitative agreement with the observed SALS patterns, and the predicted scaling is observed [33°]. In addition to these quantitative and semi-quantitative studies, there is a wealth of qualitative data that is invaluable for rationalizing and understanding the complex rheological behavior of concentrated colloidal dispersions, especially with regard to shear-induced ordering by steady and oscillatory shear flow. A recent review by Ackerson summarizes the main results up until 1996 [25]. In a recent rheo-light scattering study, Paulin et 01. [6°] address the direct connection between the rheological properties (e.g. creep and recoverable compliance, and linear viscoelastic moduli) for crystal structure and particle compressibility for swellable polymethylrncthacrlyate colloidal dispersions. Flow light scattering has been employed to address the issue of spatial homogeneity in ordered dispersions under flow and the possible complications in interpreting rheological data [34-36]. Shearing can be used to induce relatively large, ordered crystals of a variety of symmetries, depending on the particles and the flow type [37]. Recently, f1ow-SALS has demonstrated the existence of a shear-ordered string phase in near hard-sphere dispersions, which had not previously shown shear induced ordering (J Vermant, G Fuller, J Mewis, personal communication). As this string phase is composed of aggregates of strings of particles, this represents a heretofore neither predicted nor observed structure, with important 'consequences for simulations and theory. Future challenges include full 3dimensional reconstruction of the distorted microstructure. Conservative flow dichroism, which is directly calculated as the polarization-dependent integrated scattering, has

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been used with the colloidal stress-optical relationship to quantitatively sort out the relative importance of hydrodynamic and interparticle forces on colloidal dispersion rheology [15,17]. This method has been verified and used by Lee and Yang (personal communication) to establish the flow stability of colloidal silica, whereby changes in the sign of the dichroism are correlated to changes in particle stability, as modified by the steric coating, shear flow, and volume fraction of particles. Rheo-optical measurements were employed to reveal the complex microstructure evolving in shearing laponite clay dispersions. Butterfly patterns were observed in light scattering, arising from stretching of mesoscopic aggregates of primary particles [38], not dissimilar to the butterfly patterns observed in near-S polymer solutions. Novel applications of diffuse optical transmittance have also been employed to infer the shear alignment of dense kaolinite suspensions [39]. Colloidal aggregates are ubiquitous in industrial dispersions used in coatings, .for example. Control of the. extent of aggregation and floc structure is important in order to control the fluid's rheology and product performance. Image analysis of direct microscopy of .shearing colloidal dispersions enables characterizing aggregates under shear, ill situ with rheology [40]. Studies of the fundamental rhco-optics of colloidal dispersions has been complemented by a recent report of a practical application. Extensional alignment of colloidal core-shell rubber particles in a polymer matrix form a dichroic polarizer based on conservative dichroism [41]. The potential application is in display devices.

Thin films and quasi-two dimensional systems Novel applications of flow birefringence to study the dynamics of thin films at interfaces have recently been reviewed by Fuller [42]. In this review he discusses the implications of these advances, such as the success that has been achieved in resolving the flow-induced orientation of rod-like polymers adsorbed at a liquid interface through flow dichroism [43], the relaxation of 20 domains of cholesteric surfactants, and the flow-alignment of fatty acid monolayers [44,45], the latter employing Brewster angle microscopy under flow. A recent report of a micron-gap rheo-optical device [46,47] suggests that similar advances are forthcoming in our understanding of the rheology of complex interfaces and thin films.

Raman spectroscopy Polarized modulation Raman scattering has been improved by polarization modulation, which enabled the extraction of both the second and fourth moments of the orientation distribution function for LCPs [48]. As Raman scattering is spectroscopic, it has the ability to resolve chemical components, as has been demonstrated by Archer and Fuller for a block copolymer melt [49]. This enables probing individual block dynamics in the melt and more rigourous tests of molecular theories. The method has been recently employed to examine flow induced a-helix

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to ~-sheet transinons in flowing poly-l-lysine solutions [50-). A group from BASF has very recently reported the ability to resolve Brownian motion of one particle species in a mulricornponent mixture by Raman correlation spectroscopy, which uses correlation spectroscopy of Raman scattered light to resolve dynamics in complex fluids [51-). Possibilities for probing the dynamics in multicornponent shearing dispersions via Raman scattering arc intriguing by providing insights into the dynamics in mixtures.

Wormy micelles, 'onions', and semiflexible polymers in solution Highly structured surfactant rnesophases arc sensitive to applied flows and are often optically clear making them targets for rheo-optic analysis. The creation of onions or multi lamellar vesicles from lamellar bilayers has been ~nown for some time. Rheo-optical investigations have illuminated the kinetics of onion formation from defect ridden bilayers, connecting the formation and refinement of the onion size with thixotropic shear-thickening. Flowpolarized light scattering shows spherulitic-like scattering which can be analyzed to yield quantitative measures of the onion size, while turbidity and flow birefringence provide additional information about the homogeneity of the phase and distortion of the bilayers, respectively. In this manner a full nonequilibrium phase diagram can be constructed that shows transitions from defect ridden lamellae to onion phases, fracture into a biphasic structure, shear-induced ordering of the onions, and shear-induced alignment of the lamellae [52-54). One study also demonstrated the importance of stress as the parameter controlling the' microstructure of such complex fluids [52). The viscoelastic properties of onion phases have been studied as a function of the onion size, showing the rheological complexity of the phase [55). Recent efforts have examined the effect of the hydrophilicity of one surfactant in a ternary mixture on the lamellar phase structure and rheological response [56). Rheooptical investigations are essential for connecting the mesoscopic texture (SALS and optical microscopy) and net lamellar alignment (birefringence) with the rheology in the nonlinear flow regimes. Worm-like micelles have been studied in part with the view toward their similarity to the ideal, single relaxation time Maxwell model. The ability to break and reform can, in some instances, result in a single, dominant relaxation time. Wheeler etal. [57) demonstrated such behavior for a surfactant solution through flow birefringence and rheology. It was found that the SOR held until a critical shear rate was exceeded, whereupon a violation in the SOR was observed. Flow SALS patterns demonstrated a butterfly pattern as well as a bright streak for shear rates faster than the breaking time of the micelles. This phenomenon was explained in terms of stress-concentration fluctuation coupling, validating theory. Streaks have been observed by others [58), along with similar deviations from ideal behavior at higher shear

rates [59-). For lower shear rates, the butterfly patterns observed in flow-SALS agree qualitatively with recent models of stress-structure coupling. Excess salt has been identified as important for the onset of the complex flow and microstructural behavior, perhaps associated with the development of micelle branching [60). Oda er al. [61) have also reported a strong shear-thickening and increase in optical anistropy in worm-like -rnicelles of gemini surfactants, that they explain as a shear-induced phase transition partially related to branching in the micelles. Note that lyotropic lamellar phases on non ionic surfactants can also exhibit butterfly patterns in flow-SALS resulting from stress-induced concentration fluctuations, but that these are generally absent in ionic surfactants where shear-induced alignment of lamellae is evident at high shear rates [62,63). Berret et 01. [64) studied thermal effects and temperature superposition of the rheology on similar solutions. They interpreted the spatial variations in birefringence in the flow field to have been phase separation into highly aligned and less aligned regions, possibly with different shear gradients, in other words, an inhomogeneous flow. Recent rheo-optical work has been reported on worm-like polymers in solution [65), which provides interesting data for comparison to the worm-like micelles. McHugh and co-workers [66) have studied the effects of temperature and solvent on the flexibility of nematic forming polymers using flow birefringence, including observing coil-helix transitions. The dynamics of rigid rods have been measured by flow birefringence and compared to competing theories for the semi-dilute regime [67). A more complex behavior of a-helix to ~-sheet transition induced by shear flow have been observed by dichroism and flow laser Raman measurements [50-).

Block copolymers and surfactant mesophases Rheo-optics provides direct information about the alignment of block copolymers ([68); ZR Chen, J Kornfield, personal communication) and surfactant lamellar phases under shear [5). Comparing and contrasting the polymeric to the surfactant systems provides insight into both systems. For example, onions form in many surfactant systems and yet are absent in block copolymers, the reasons for which are unclear. The rich behavior of lamellar phase orientation with steady and oscillatory shear flows exhibited by block copolymers is generally not as evident in surfactants, Recent flow birefringence and flow small angle neutron scattering experiments (SANS), however, have demonstrated the existence of a re-entrant transition to parallel alignment in the sodium dodecyl sulfate/decanol/Djf system (j Berghause, J Zipel, P Lindner, W Richering, personal communication). Further, triblock pluronics in the hexagonal phase have also been demonstrated to show shear alignments [69-) similar to hexagonal phase surfactant solutions (G Schmidt, W Richtering, P Lindner, P Alexandridis, personal communication).

Rhea-optics Wagner

Much is known about shear-induced alignment in diblock copolymers from rhco-optic experiments [68]. The dynamics of orientation switching with change in shear frequency has recently been monitored, providing evidence for the molecular mechanisms that stabilize the steady-state structures under flow ([70]; J Zryd, W Berghardt, personal communication). Through comparison of dynamic rheology and flow birefringence, the SOCs of disordered diblocks can be characterized [71]. It was found that although the viscoelasticity was thermorheologically simple (i.e . obeyed time-temperature superposition), which is in contrast to the analogous blends of hornopolyrncrs, the SOC does not exhibit time-temperature superposition. Comparison of experimentalresults with predictions of reptation theory suggest non-uniform friction may be important when the monomeric friction coefficients of the two blocks differ substantially, The knowledge gained from studies · of diblocks can now be applied to the potentially rich behavior of triblock copolymers [68,69-].

Shear-induced structures in polymer solutions and fluids near phase transitions Substantial effort has been devoted to resolving the ability of shear flow to dernix and/or generate structure in simple fluids near phase transitions and semi-dilute polymer solutions. The recent revi ew by Onuki [72] provides a comprehensive review of theoretical efforts to understand the associated shifts in critical behavior, nucleation, and spinodal decomposition with shear. Upon shearing within a certain regime, anomalous d ichroism, birefringence, and SALS patterns C3n be observed. The latter is characterized by the 'butterfly' SALS patterns, shown to arise from el astic coupling between concentration fluctuations and stress. A coherent and comprehensive experimental elucidation of the effect for a model polystyrene solution has been recently reponed by Kume et al. [73-], where various regimes and structures are clearly identified. Transient studies of the rheology and light scattering show congruence between a double-stress overshoot and polymer chain stretching followed by shear-induced structure formation providing a sensitive test of our understanding. Recent theoretical work includes a new SOR for the anomalous stress arising from the anisotropy in the shear-induced structure [IS-]. Flow-induced crystallization in 3 polyethylene melt has also been studied in extensional flow by simultaneous flow-birefringence and dichroism [74], showing how rheooptics can be used to probe this phenomenon.

Liquid crystalline polymers LCPs have been a favorite subject for rheo-optic investigation, with a substantial amount of work on model , main-chain lyotropic ncmatics reponed. Recent reviews by Mewis and Moldenaers (75) of LCPs and Jamieson et al. (76] of nematic monodomains containing LCPs cover much of this work. Conoscopy has been employed to measure the rotation of the director upon flow start up [77] and breakup of nematic monodomains in shear flow [78].

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The molecular mechanisms of stress osciallation damping during transient shear flows has been cludicatcd by conoscopic methods [79-) again providing a sensitive test for our theoretical understanding of LCP dynamics. Recent investigations have also focused on using conservative dichroism [80] to demonstrate the existence of a 'wagging' regime between flow tumbling and flow aligning, as predicted by theory. Flow birefringence in spectrograph mode enables tracking the net molecular alignment as a fun ction of applied shear rate (81), and can be directly compared with SANS and X-ray scattering results giving confidence to the rhco-optic method. The quantitative evolution of the rheology and defect texture at low shear rates has been quantified through ill situ rheo-depolarized light scattering (4-]. Along with complementary SANS me asurements of the net molecular order, this fully enables testing theoretical models for the rheology and microstructure of mesoscopic defect textures in LCPs. Some important aspects of the actual state ,of the LCP and its evolution with appl ied shear suggest the low shear rhcological anomaly, denoted ' re gion I' m3Y be a consequence of other liquid crystalline phases and may not be universal in nature [82-,83). Shear-induced textures and band formation in lyotropic LCPs have been extensively studied by Riti el 01. (84), a phenomenon of practical importance. The situation becomes more complex when liquid crystalline nematic side chain polymers are considered. Control of the rheology can be effected by adjusting either the polymer backbone or the liquid crystalline side groups (85). Rheo-opric studies using rheology, birefringence and SALS provide evidence for the rel ative importance of the polymer backbone to the nematic side groups on the rheology and microstructure [86-), specifically influencing the relative amount of polymer versus LCP behaviour, Finally, the complex rheological behavior of lyotropic LCPs manifests itself in a complex behavior in nonsimple flows. Flow birefringence coupled with laser Doppler velocimctry enables resolving the effects of flow-orientation coupling in flows with mixed shear and extension (87]. The nonlinear rheological behavior results in interesting unsteady behaviors that await theoretical elucidation.

Immiscible polymer blends The mechanics of fluids composed of immiscible polymers is complicated by u mesoscopic morphology that evolves with the applied flow, affecting not only the process rheology but the final product performance as well. To address this problem a series of fundamental studies of model blend systems employed rheology and optical techniques to correlate the evolving blend morphology with the transient rheology [88-92,93°). Model blends of polydimcthylsiloxane dispersed in polyisobutcne are optically suitable for SALS and flow-dichroism (conservative) investigations, as well as direct microscopy [941 .

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Structural changes such as droplet deformation, fibril formation, break-up and coalescence can be monitored in situ (95). Mewis and co-workers [93",96) studied the onset of filament formation upon increases in shear rate and subsequent breakup by Rayleigh instability. Models for the morphology during Rayleigh breakup are developed to quantitatively yield the droplet and filament sizes. From the breakup times, quantitative measurements of the interfacial tension can be obtained (92), which are in good agreement with other methods. Evolution of the transient rheology during shear rate increases can then be interpreted in terms of the droplet stretch and either breakup or filament formation followed by Rayleigh instability [88,89). This data should be a more stringent test of rheological models for immiscible polymer blends (91). A full, quantitative study of rheology and in SiIU blend morphology, however, with direct comparison to the current theory is not yet fully realized. The primary limitation being multiple scattering resultant from the turbidity of blend samples that yield good rheological signatures.

Diffusing wave spectroscopy and probe rheology Optical detection can be used with probe molecules to study the microscopic rheological properties of polymer solutions at multiple length scales, as well as biological systems. Tongelal. [97") report the use of sedimentation to investigate the dependence of the local viscosity measured by a nanometer probe in nonadsorbing polymer solutions. The results show an anomalous behavior not predicted by theory when the correlation length of the polymer solution and the probe size are similar. A substantial amount of recent activity has examined the use .. of tracking a particle's mean-squared displacement and, by assuming a generalized Stokes-Einstein relation between the frequency-dependent diffusivity and the frequency-dependent viscosity, infer the the frequency dependent viscoelasticity of a fluid. Dynamic light scattering (DLS) [98,99], diffusing wave spectroscopy (DWS) (l00·,101), and particle micro tracking [102,103) have been demonstrated as methods to determine the particle's mean squared displacement. In DLS, the probe particle's positional autocorrelation function is determined from the singly scattered laser light, whereas in particle rnicrotracking actual trajectories arc recorded and analyzed. DWS relies on correlations in the phase of multiply scattered light (see the review by Maret for more information'(104)) to yield the probe diffusion in a concentrated system of colloids or emulsion droplets. These methods have been demonstrated to yield accurate results as compared to direct rheological measurements, and have the advantage of always being in the linear viscoelastic regime as the sample's own thermal excitations drive the probe particles motion. DLS and

DWS have yielded measurements of the viscoelasticity over a broad frequency range for emulsions [98,100·), colloidal glasses [100·), gelling gelatin sols (105) and actin filament networks (101). Particle microtracking has been demonstrated for both model polymer solutions as well as concentrated DNA solutions by Mason et 01. (102), where laser deflection was employed to accurately track the tracer. The method can yield rheological information from very small volumes and, because it relies on the natural fluctuations of the viscoelastic medium to drive the particle's Brownian motion, it is not plagued by nonlinear effects associated with driven flows. Particle micro tracking has been proposed for in vivo studies of the rheology inside cells (103). The theoretical underpinnings of these methods are still under refinement. Extension to concentrated dispersions relies on the existence of generalized Stokes-Einstein equations in the form of D = l.'bT/(6TClla) where J:b is Boltzmann's. constant, D is the particle diffusivity, a is the particle radius and T is the temperature. For a single particle, the Stokes-Einstein relation relates the bare particle diffusivity D =Do to the solvent viscosity '11 = Ilo. For concentrated dispersions, D = D~(¢), the long-time sclf-diffusivity, and '11 ='110, the zero shear dispersion viscosity, have been proposed and some experimental evidence has been presented that supports measuring viscosity from measurements of long-time self-diffusivity of the particles. Note that rapid advances in the ability to measure DLS in turbid suspensions permits application to a broadening array of complex fluid systems. More detailed measurements of both rheology and viscosity on model concentrated hard sphere [106) and charge -stabilized dispersions (j Bergenholtz, F Horn, W Richtering, N Willenbacher, N Wagner, unpublished data) clearly show that this and other generalizations of the Stokes-Einstein relation are not strictly valid; this is in agreement with theory. Thus, although the probe method for obtaining rheological information about a complex fluid is attractive, caution must be used in extracting quantitative rheological data from measurements of particle diffusion and motion.

Conclusions Rheo-optics is an expanding field, proving useful for examining complex fluids driven far from equilibrium. Earlier work on colloids and polymer solutions has been broadened to include highly microstructured fluids such as block copolymers, textured liquid crystal polymers, immiscible polymer blends, and a broad range of surfactant mesophases; however, with increased complexity often the quantitative stress-optical relation is not yet on a firm foundation, or, as the case may be, violations of the SOR are evident (which may sometimes be exploited). There is clearly a need for extending the fundamental micrornechanical understanding of the stress-optical relationship to these systems, often with multiple length scales governing rheological response and optical characteristics. Computational advances coupled with more

Rheo-optics Wagner

robust Brownian dynamics simulations are one route to providing stress-optical relations. Substantial work is ongoing and warranted for the extensively studied model colloidal and polymer solutions, with emphasis on shear-induced phase separation and shear-induced structures far from equilibrium. Work in this area is shifting from linear viscoelasticity to detailed examination of fully nonlinear rheology and the related microstructures formed, with additional need for quantitative stress-optical relations valid far from equilibrium. Novel structures formed in sheared colloidal dispersions rriay_~e beneficial to designing nanostructured devices by processing. Shear-induced phase separation and stress-concentration coupling in semi-dilute polymer solutions, colloidal dispersions, and surfactant mesophases are all active topics of research and fruitful debate.

References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: • ••

With the increased degree of complexity in systems of interest to 'rheo-opticians' and the hierarchy of structural lengthscales governing rheology, combination of truly optical methods with other probes sensitive to molecular structure are proving invaluable. Combining X-ray scattering SANS, nuclear magnetic resonance or rheo-optics in the infrared or ultraviolet region with rheology and rheo-optics is becoming essential to deconvoluting the evolving hierarchy of microstructures affected or generated by flow. Such combined methods are expected to become ever more available; for example, devices for combined SALS and rheology are to be offered on SANS lines, such as at National Institute of Standards and Technology.

Fuller G: Optical Rheometry of Complex Fluids. New York: Oxford University Press; 1995.

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Fermigier M, L:HevederC, Jenffer P, Promislow J: A shear device for the microscopic observations of suspensions and emulsions. J Rheo/1998, 42:255-265.

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Wu R, Shaw M, Weiss R: A rheo-Iight-scattering instrument for the study of the phase behavior of polymer blends under simple-shear flow. Rev Sci Instr 1995, 66:2914-2921.

Walker L, Kernick W III, Wagner N: /n situ analysis of the defect texture in liquid crystal polymer solutions under shear. Macromolecules 1997, 30:508-514. Flow-SALS flow-SANS and rheology map out the molecular orientation and defect texture in a shearing, model lyotropic LCP for comparison with theories. 4.

Acknowledgements

Richtering W: Investigation of shear-induced structures in lyotropic meso phases by scattering experiments. Prog Colloid Poly Sci 1997, 104:90-96.

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Paulin S, Ackerson B, Wolfe M: Microstructure-dependent viscosity in concentrated suspensions of soft spheres. Phys Rev E 1997, 55:5812-5819. Flow-SALS and rheology reveal shear-induced changes in ordered colloidal dispersions that depend on initial orientation. The elastic properties are shown to be resultant from both particle microstructure and particle deformation.

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Brown E, Burghardt W, Kahvand H, Venerus D: Comparison of optical and mechanical measurements of second normal stress difference relaxation following step strain. Rhea/ Acta 1995, 34:221·234.

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Takahashi T, Fuller G: Stress measurement using birefringence in oblique transmission. Rhea/ Acta 1996, 35:297-302.

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Kalogrianitis S, van Egmond J: Full tensor optical rheometry of polymer blends. J Rheo/1997, 41 :343-364.

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Janeschitz-Kriegl H: Letter to the editor: Comments on full tensor optical rheometry of polymeric fluids. J Rheo/1998, 42:219-220.

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Brown E, Burghardt W: First and second normal stress difference relaxation in reversing double-step strain flows.

J Rheo/1996, 40:37-54. Bedford B, Cinader 0 Jr, Burghardt W: Unstable slit flow of a liquid crystalline polymer solution. Rheol Acta 1997, 36:384-396. Optical, birefringence and X-ray scattering measurementsof an instability in high shear rates slit flow of a model LCP.

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14.

15.

Thanks to J Bender. L Walker, J Bcrgcnholtz W Kcrnick and A Vaynbcrg for enjoyable. collaborative work in rhco-optics, Financial support from the Fulbright Commission while the author was on sabbaticalat the University of Konstanz and the National Science Foundation (C'I"S-9523968) arc acknowledged.

of special interest of outstanding interest

1.

5.

The broader range of problems has encouraged a development and refinement of methods. Measurerncnts of the full optical tensor, birefringence in thin films as well as complex flows are becoming routine and should yield important dynamical information about such topics as the dynamics of nernatics in kinematically complex flows, the validity of rheological models, and the behavior of surfactants in flowing monolayers, Raman spectroscopy offers a number of possibilities for spectroscopic rheooptics that would enable deconvolution of components in a multicornponent mixture. The optical detection of the Brownian motion of probe molecules to determine the micro-rheology of complex fluids has proven useful, and although more work is required to fully quantify the relationships between viscoelasticity and Brownian motion, the method is already yielding important insight into systems of biological interest.

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