Transient Rheology Research Articles

Contemporary modeling and analysis of steady state and transient human blood rheology

The rheological characterization of a human blood, through modeling and analysis of transient flows and large-amplitude oscillatory shear (LAOS) flow, has made tremendous progress recently. We show how various components, and modifications of two recent scalar, structural kinetic, thixotropic models, can offer several modeling and prediction improvements, and compare our results to the Maxwell-like Bautista-Manero-Puig (BMP) model, and a recent transient model based on the Herschel-Bulkley. We explore the weakness of the legacy blood models, and then, we apply this newly improved model to recently published data from the literature in order to demonstrate its efficacy in modeling steady state, transient, and oscillatory shear flow. Following this effort, we demonstrate a novel approach using the sequence of physical phenomena (SPP) to facilitate interpretation, characterization, mapping, and “fingerprinting” of transient blood data from the literature. We compare the SPP approach to other LAOS analysis techniques in the literature and show how our approach can function as a mechanical-property diagnostic blood analysis tool. The goal of this work is a deeper understanding of the microstructural basis and validity of structural thixotropic blood models, and transient flow analysis techniques and procedures.

Modeling Seismic Cycles of Great Megathrust Earthquakes Across the Scales With Focus at Postseismic Phase

AbstractSubduction is substantially multiscale process where the stresses are built by long‐term tectonic motions, modified by sudden jerky deformations during earthquakes, and then restored by following multiple relaxation processes. Here we develop a cross‐scale thermomechanical model aimed to simulate the subduction process from 1 min to million years' time scale. The model employs elasticity, nonlinear transient viscous rheology, and rate‐and‐state friction. It generates spontaneous earthquake sequences and by using an adaptive time step algorithm, recreates the deformation process as observed naturally during the seismic cycle and multiple seismic cycles. The model predicts that viscosity in the mantle wedge drops by more than three orders of magnitude during the great earthquake with a magnitude above 9. As a result, the surface velocities just an hour or day after the earthquake are controlled by viscoelastic relaxation in the several hundred km of mantle landward of the trench and not by the afterslip localized at the fault as is currently believed. Our model replicates centuries‐long seismic cycles exhibited by the greatest earthquakes and is consistent with the postseismic surface displacements recorded after the Great Tohoku Earthquake. We demonstrate that there is no contradiction between extremely low mechanical coupling at the subduction megathrust in South Chile inferred from long‐term geodynamic models and appearance of the largest earthquakes, like the Great Chile 1960 Earthquake.

Effect of the degree of substitution on the rheology of sodium carboxymethylcellulose solutions in propylene glycol/water mixtures

The linear dynamic viscoelastic properties and non-linear transient rheology of sodium carboxymethylcellulose solutions (Na-CMC) in propylene glycol/water mixtures were investigated. Measurements were carried out for the solutions of Na-CMC with three different degrees of substitution (DS), namely 0.62, 0.79, 1.04, and the similar average molecular weight (Mw ≈ 250,000 g/mol). The strong synergism between the molecules of Na-CMC with DS of 0.62 and 0.79, and propylene glycol has been observed. The occurrence of the overshoot shear stress and the low loss tangent values indicate the physical cross-linking of the polymer chains. The increase of propylene glycol concentration over 80 wt% and sodium carboxymethylcellulose (DS = 0.7) over 1.6 wt% leads to the formation of a physical cross-link network. The absence of overshoot shear stress and terminal behaviour in SAOS flow of the Na-CMC1.04 solutions in the PG/water mixture shows that no intermolecular cross-linking of polymer chains occurred in them.

Transient rheology of the oceanic asthenosphere following the 2012 Indian Ocean Earthquake inferred from geodetic data

Postseismic motion in the middle-field (100–500km from the epicenter) geodetic data resulting from the 2012 Indian Ocean earthquake exhibited rapid change during the two months following the rupture. This pattern probably indicates multiple postseismic deformation mechanisms and might have been controlled by transient rheology. Therefore, the relative contribution of transient rheology in the oceanic asthenosphere and afterslip in the oceanic lithosphere should be incorporated to explain short- and long-term transitional features of postseismic signals. In this study, using two years of post-earthquake geodetic data from northern Sumatra, a three-dimensional spherical-earth finite-element model was constructed based on a heterogeneous structure and incorporating transient rheology. A rheology model combined with stress-driven afterslip was estimated. Our best-fit model suggests an oceanic lithosphere thickness of 75km with oceanic asthenosphere viscosity values of 1×1017Pas and 2×1018Pas for the Kelvin and Maxwell viscosity models, respectively. The model results indicate that horizontal landward motion and vertical uplift in northern Sumatra require viscoelastic relaxation of the oceanic asthenosphere coupled with afterslip in the lithosphere. The present study demonstrates that transient rheology is essential for reproducing the rapidly changing motion of postseismic deformation in the middle-field area.

Rheological properties of nanocrystalline cellulose suspensions

Rheological behavior, including linear and nonlinear, as well as transient rheology of nanocrystalline cellulose (NCC) suspensions was studied in this work. Two kinds of polymer solutions, aqueous poly(vinyl alcohol) (PVA) with flexible chain structure and aqueous carboxymethyl cellulose (CMC) with semi-rigid chain structure, were used as the suspension media to further explore the role that the interactions among NCC and polymers played during shear flow. The results reveal that NCC has lower values of percolation threshold in the PVA solution than in the CMC one during small amplitude oscillatory shear (SAOS) flow because the flexible PVA chain has higher adsorbed level onto NCC particles than the negatively charged semi-rigid CMC chain, which is further confirmed by the Fourier transformed infrared (FT-IR) spectroscopy tests. As a result, the NCC suspension shows a weak strain overshoot in PVA solution during large amplitude oscillatory shear (LAOS) flow, which cannot be seen on the one in CMC solution. During startup shear flow, both of these two suspensions show evident stress overshoot behavior with the strain-scaling characteristics, indicating the formation of ordered long-term structure of rod-like NCC particles with self-similarity during flow. However, NCC suspension have far stronger stress overshoot response in CMC solution relative to the one in PVA solution. A possible synergy mechanism between NCC and CMC chain is hence proposed.

Rheologic constraints on the upper mantle from 5 years of postseismic deformation following the El Mayor‐Cucapah earthquake

AbstractWe analyze five years of Southern California GPS data following the Mw=7.2 El Mayor‐Cucapah earthquake. We observe transient postseismic deformation which persists for 3 years at epicentral distances greater than ∼200 km. In the near field, rapid postseismic transience decays to a sustained rate which exceeds its preseismic trend. We attempt to determine the mechanisms driving this deformation, where we consider afterslip at seismogenic depths and viscoelastic relaxation in the lower crust and upper mantle as candidate mechanisms. We find that early, rapid, near‐field deformation can be explained with afterslip on the fault that ruptured coseismically. The later, sustained, near‐field deformation can be explained with viscoelastic relaxation in the lower crust with a steady‐state viscosity of ∼1019 Pa s and possibly continued afterslip. The later postseismic deformation in the far field is best explained with a transient viscosity of ∼1018 Pa s in the upper mantle. We argue that a transient rheology in the mantle is preferable over a Maxwell rheology because it better predicts the decay in postseismic deformation and also because it does not conflict with the generally higher, steady‐state viscosities inferred from studies of geophysical processes occurring over longer timescales.

The transient shear rheology of a thermotropic liquid crystalline polymer in the super-cooled state

Abstract To determine the processing temperature of a representative thermotropic liquid crystalline polymer (TLCP) in the super-cooled state, small amplitude oscillatory shear (SAOS), the startup of shear flow and differential scanning calorimetry (DSC) measurements were performed. Good agreement on the onset crystallization temperature was obtained from the SAOS and the startup of shear flow. Compared with DSC, the rheological measurements were more sensitive for detecting crystallization in the super-cooled state. The rheological study suggested that crystallization did not occur unless the TLCP was cooled 30 °C below its melting point, and its viscosity was significantly increased as the material was super cooled. Additionally, super cooling the TLCP did not significantly affect the relaxation of shear stress after preshearing. However, the recovery of transient shear stress in the interrupted shear measurements was suppressed to a great extent in the super-cooled state.

Rheological flow laws for multiphase magmas: An empirical approach

The physical properties of magmas play a fundamental role in controlling the eruptive dynamics of volcanoes. Magmas are multiphase mixtures of crystals and gas bubbles suspended in a silicate melt and, to date, no flow laws describe their rheological behaviour. In this study we present a set of equations quantifying the flow of high-viscosity (>105Pa·s) silica-rich multiphase magmas, containing both crystals (24–65vol.%) and gas bubbles (9–12vol.%). Flow laws were obtained using deformation experiments performed at high temperature (673–1023K) and pressure (200–250MPa) over a range of strain-rates (5·10−6s−1 to 4·10−3s−1), conditions that are relevant for volcanic conduit processes of silica-rich systems ranging from crystal-rich lava domes to crystal-poor obsidian flows. We propose flow laws in which stress exponent, activation energy, and pre-exponential factor depend on a parameter that includes the volume fraction of weak phases (i.e. melt and gas bubbles) present in the magma. The bubble volume fraction has opposing effects depending on the relative crystal volume fraction: at low crystallinity bubble deformation generates gas connectivity and permeability pathways, whereas at high crystallinity bubbles do not connect and act as “lubricant” objects during strain localisation within shear bands. We show that such difference in the evolution of texture is mainly controlled by the strain-rate (i.e. the local stress within shear bands) at which the experiments are performed, and affect the empirical parameters used for the flow laws. At low crystallinity (<44vol.%) we observe an increase of viscosity with increasing strain-rate, while at high crystallinity (>44vol.%) the viscosity decreases with increasing strain-rate. Because these behaviours are also associated with modifications of sample textures during the experiment and, thus, are not purely the result of different deformation rates, we refer to “apparent shear-thickening” and “apparent shear-thinning” for the behaviours observed at low and high crystallinity, respectively. At low crystallinity, increasing deformation rate favours the transfer of gas bubbles in regions of high strain localisation, which, in turn, leads to outgassing and the observed increase of viscosity with increasing strain-rate. At high crystallinity gas bubbles remain trapped within crystals and no outgassing occurs, leading to strain localisation in melt-rich shear bands and to a decrease of viscosity with increasing strain-rate, behaviour observed also in crystal-bearing suspensions.Increasing the volume fraction of weak phases induces limited variation of the stress exponent and pre-exponential factor in both apparent shear-thickening and apparent shear-thinning regimes; conversely, the activation energy is strongly dependent on gas bubble and melt volume fractions. A transient rheology from apparent shear-thickening to apparent shear-thinning behaviour is observed for a crystallinity of 44vol.%. The proposed equations can be implemented in numerical models dealing with the flow of crystal- and bubble-bearing magmas. We present results of analytical simulations showing the effect of the rheology of three-phase magmas on conduit flow dynamics, and show that limited bubble volumes (<10vol.%) lead to strain localisation at the conduit margins during the ascent of crystal-rich lava domes and crystal-poor obsidian flows.

Stress‐driven relaxation of heterogeneous upper mantle and time‐dependent afterslip following the 2011 Tohoku earthquake

AbstractUnderstanding of postseismic deformation following great subduction zone earthquakes is complicated by the combined effects of viscoelastic relaxation of earthquake‐induced stresses in the upper mantle and time‐dependent afterslip on the megathrust. We integrate geodetic observations and constraints on afterslip from small repeating earthquakes on the megathrust to better distinguish contributions from these two postseismic processes. We have developed a three‐dimensional, spherical viscoelastic finite element model to study the postseismic deformation of the 2011 Mw9.0 Tohoku earthquake that has been recorded at unprecedented high resolution in space and time. We model stress‐driven afterslip in a 2 km thick weak shear zone away from historic rupture zones on the megathrust. We model both the viscoelastic relaxation of the upper mantle and shear zone deformation with a transient Burgers body rheology. The transient Kelvin viscosity is assumed to be one order of magnitude lower than that of the Maxwell viscosity. Viscoelastic relaxation of the mantle wedge alone causes postseismic uplift and seaward motion in the upper plate, opposite to the pattern from relaxation of just the oceanic upper mantle. Afterslip on the fault produces uplift updip of the afterslip zone and subsidence over its downdip edge and mostly seaward motion above the afterslip zone. The best fit Maxwell viscosity of the shear zone at depths ≤50 km is 1017 Pa s, constrained by afterslip estimates from repeating earthquakes. The optimal viscosities of the deep weak shear zone, continental mantle wedge, and oceanic upper mantle are determined to be 5 × 1017 Pa s, 3 × 1019 Pa s, and 5 × 1019 Pa s, respectively. The stress‐driven afterslip in the shear zone is up to ~3.5 m in the first 2 years after the earthquake, equivalent to an Mw8.4. Our model reproduces the first‐order pattern of the GPS observations both in horizontal and in vertical directions. Seafloor geodetic observations of subsidence and landward motions near the high‐slip zone of the earthquake provide evidence for a low‐viscosity asthenosphere below the oceanic lithosphere.

A nonlinear shear and elongation rheological study of interfacial failure in compatible bilayer systems

This work aims to examine whether or not interfacial failure can occur in a compatible polymer bilayer system under large shear and elongation deformations, as well as to probe the sensitivity of nonlinear transient rheology to the presence of interface/interphase at neighboring layers. For this, stress relaxation after a step strain and fast startup in simple shear and uniaxial extension experiments have been performed on healed and coextruded poly(methyl methacrylate)/poly(vinylidene fluoride) bilayers with the presence of a robust diffuse interphase as evaluated by energy dispersive X ray. For unhealed bilayers, interfacial failure occurred in shear flows at intermediate deformations, while for healed bilayers the interphase greatly delayed the onset of interfacial failure to larger deformation steps and to a higher deformation rate in the startup shear. Extensional rheology demonstrated that the presence of an interphase in the bilayers greatly enhanced the transient extensional viscosity ηE+(t) as well as the tensile relaxation modulus E(t) of the structure, even though the entanglement density was relatively low. Moreover, models are presented to predict the nonlinear relaxation behavior of bilayers and to estimate the relaxation behavior of the interphase. Fitting of the tube model to the shear relaxation indicates a dilated tube diameter in the interphase, confirming its weak entanglement intensity and its readiness to flow-induced disentanglement under large external deformations. Finally, the physics of the interfacial failure was assessed based on some recent molecular dynamic theories.

Obtaining repeatable initial fiber orientation for the transient rheology of fiber suspensions in simple shear flow

Many researchers reporting the transient rheology of fiber suspensions have not experimentally verified the initial fiber orientation. An assumption for the initial orientation is then required to simulate the fiber orientation evolution and stress response. Measurements of fiber orientation obtained prior to testing in a rheometer can confirm the homogeneity of fiber orientation throughout a sample and repeatability between multiple samples. In this work, the transient rheology of glass fiber suspensions above 0.5 mm in length was measured in a sliding plate rheometer with the initial fiber orientation generated through compression molding, flow reversal, and injection molding. Measurements of shear stress and fiber orientation were obtained to evaluate each sample preparation method and to gain insight into the stress-microstructure relationship. Preshearing and applying flow reversal was used in an effort to control the initial fiber orientation for transient shear stress measurements, but fiber orient...

High molecular weight effects on steady and transient rheology of lyotropic poly(p-phenylene terephthamide) solution

This study investigated some special effects of high molecular weight poly(p-phenylene terephthamide) (PPTA) on steady-state and transient rheology of lyotropic PPTA/H2SO4 solutions. The solutions with the same concentration were prepared to possess different ratios of the high molecular weight PPTA (Hi-PPTA, ηinh = 7.15 dL/g) to ordinary PPTA (Ord-PPTA, ηinh = 5.43 dL/g). With increased Hi-PPTA amount in the solutions, viscosity of the solutions was reduced in the “Newtonian” plateau of the flow curve. The first normal stress difference N1 in the high shear rate range was strongly affected by Hi-PPTA. Increasing the Hi-PPTA amount, the PPTA/H2SO4 solutions showed a significant decrease in the negative N1 minimum and increase in the corresponding shear rate γ˙min. After cessation of steady-state shearing, the recoverable strain obviously increased for the solutions with the increasing Hi-PPTA amount. Accordingly, the dynamic modulus (G″) maximum and time corresponding to the G″ maximum became smaller for the relaxation process.

Transient dynamics of a colloidal particle driven through a viscoelastic fluid

We study the transient motion of a colloidal particle actively dragged by an optical trap through different viscoelastic fluids (wormlike micelles, polymer solutions, and entangled λ-phage DNA). We observe that, after sudden removal of the moving trap, the particle recoils due to the recovery of the deformed fluid microstructure. We find that the transient dynamics of the particle proceeds via a double-exponential relaxation, whose relaxation times remain independent of the initial particle velocity whereas their amplitudes strongly depend on it. While the fastest relaxation mirrors the viscous damping of the particle by the solvent, the slow relaxation results from the recovery of the strained viscoelastic matrix. We show that this transient information, which has no counterpart in Newtonian fluids, can be exploited to investigate linear and nonlinear rheological properties of the embedding fluid, thus providing a novel method to perform transient rheology at the micron-scale.

Percolation networks and transient rheology of polylactide composites containing graphite nanosheets with various thicknesses

The polylactide (PLA) composites with various layered graphite particles were prepared by the approach of solution mixing for the viscoelasticity study. Four kinds of particles with various layered structures, including natural graphite flakes, and graphite nanosheets with the thickness of ∼25 nm and ∼5 nm, as well as graphene were used as the filler, aiming at establishing the relationship between viscoelasticity of composites and filler structure and networks. The results reveal that transient rheological response of PLA composites shows evident sheet thickness dependence, and the stress overshoot behavior is closely related to the percolation network density during startup and reverse flow. But the strain-scaling characteristic of overshoots is independent of thickness of nanosheets and their networks. The linear dynamic rheology (molten state) and creep measurements (solid state) as well as thermal analyses were then performed to further explore the difference in dispersion and distribution among those layered graphite fillers.

Reconciling viscoelastic models of postseismic and interseismic deformation: Effects of viscous shear zones and finite length ruptures

AbstractWe have developed a suite of earthquake cycle models for strike‐slip faults to investigate how finite ruptures and lithosphere‐scale viscous shear zones affect interseismic deformation. In particular, we assess whether localized and stationary interseismic deformation and large‐scale, rapidly decaying postseismic transients may be explained with models incorporating either or both of these features. Models incorporating viscous shear zones give more stationary interseismic deformation than layered half‐space, Maxwell viscoelastic models producing similar early postseismic deformation in the near field. This tendency is accentuated when the effective viscosity per unit width of the shear zone increases either with depth or with interseismic time. Models with finite (200 km long) ruptures produce time‐dependent deformation similar to that from models with infinitely long ruptures, though with smaller‐magnitude and somewhat more localized surface velocity fluctuations. Models incorporating a stiff lithosphere, a low‐viscosity mantle asthenosphere, and a crust‐ to lithosphere‐scale viscous shear zone can replicate postseismic and interseismic deformation typical of large, strike‐slip earthquakes. Such models require depth‐dependent, power law, or transient rheologies for the viscous shear zone material in the crust, as long as the effective viscosity per unit shear zone width is approximately 1015 Pa s/m at crustal depths during the postseismic interval. Below the Moho, the shear zone effective viscosity per unit width must be higher throughout the seismic cycle, at least over a short depth interval. For example, if shear zone viscosity per unit width is 5×1016 Pa s/m in the mantle lithosphere, a model with a 50 km thick lithosphere and asthenosphere effective viscosities of 1 to 5×1018 Pa s can reproduce reference velocity profiles for both postseismic and later interseismic deformation.

An analytic solution for capillary thinning and breakup of FENE-P fluids

The FENE-P model of a fluid is particularly suitable for describing the rheology of dilute polymer solutions (Newtonian solvents containing small amounts of dissolved polymer) as a result of its ability to capture nonlinear effects arising from the finite extensibility of the polymer chains. In extensional flows, these polymer solutions exhibit dramatically different behavior from the corresponding Newtonian solvents alone, notably through the creation of persistent filaments when stretched. By using the technique of capillary thinning to study the dynamics of the thinning process of these filaments, the transient extensional rheology of the fluid can be characterized. We show that under conditions of uniaxial elongational flow, a composite analytic solution can be developed to predict the time evolution of the radius of the filament. Furthermore we derive an analytic expression for the finite time to breakup of the fluid filaments. This breakup time agrees very well with results obtained from full numerical simulations, and both numerics and theory predict an increase in the time to breakup as the finite extensibility parameter b, related to the molecular weight of the polymer, is increased. As b→∞, the results converge to an asymptotic result for the breakup time which shows that the breakup time grows as tbreak∼ln(MW), where MW is the molecular weight of the dilute polymer solution.

Along-strike variations in temperature and tectonic tremor activity along the Hikurangi subduction zone, New Zealand

In the Hikurangi subduction zone, situated along the east coast of the North Island, New Zealand, where the old oceanic Pacific Plate is subducting beneath the Australian Plate, several slow slip events and tectonic tremors have recently been documented. These observations are somewhat surprising because such slow seismic phenomena tend to be common in subduction zones where relatively young oceanic plate is subducting. The locations of tectonic tremors, down-dip limit of slow slip events and seismic coupling transition change along strike from greater depths in the south to shallower depths in the north, suggesting significant along-strike variations in the characteristics of the plate interface. Similar along-strike variations have been observed for other characteristic features of the Hikurangi subduction zone. Here, we demonstrate that along-strike variations observed for tectonic tremors, slow slip events, and seismic coupling can be explained by lateral differences in the thermal structure of the subduction zone, which are controlled mainly by variations in convergence rate and friction along the plate interface. To demonstrate this, we first confirm that tectonic tremors occur around the plate interface. Then, we calculate the thermal structure of the Hikurangi subduction zone using a two-dimensional finite difference code. To explain the along-strike variation in the heat flow observed in the forearc region, temperatures along the plate interface should be systematically higher in the northern region than in the southern region, which we interpret as a consequence of higher convergence rates and greater frictional heating in the northern region. We compare the along-strike variation of seismic characteristics with calculated thermal structure and highlight that this along-strike variation in temperature controls the depth of the brittle-ductile transition, which is consistent with the observed spatial variations in tectonic tremors, down-dip limit of slow slip events and seismic coupling. Our results suggest that tectonic tremors recorded within subduction zones reflect the transient rheology of the materials being subducted, which is controlled by variations in temperature along the plate interface.

Prevalence of viscoelastic relaxation after the 2011 Tohoku-oki earthquake.

After a large subduction earthquake, crustal deformation continues to occur, with a complex pattern of evolution. This postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture and continuing slip (afterslip) or relocking of different parts of the fault. When postseismic geodetic observations are used to study Earth's rheology and fault behaviour, it is commonly assumed that short-term (a few years) deformation near the rupture zone is caused mainly by afterslip, and that viscoelasticity is important only for longer-term deformation. However, it is difficult to test the validity of this assumption against conventional geodetic data. Here we show that new seafloor GPS (Global Positioning System) observations immediately after the great Tohoku-oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short-term postseismic deformation. These data reveal fast landward motion of the trench area, opposing the seaward motion of GPS sites on land. Using numerical models of transient viscoelastic mantle rheology, we demonstrate that the landward motion is a consequence of relaxation of stresses induced by the asymmetric rupture of the thrust earthquake, a process previously unknown because of the lack of near-field observations. Our findings indicate that previous models assuming an elastic Earth will have substantially overestimated afterslip downdip of the rupture zone, and underestimated afterslip updip of the rupture zone; our knowledge of fault friction based on these estimates therefore needs to be revised.

Flow-induced morphological instability in nanosilica-filled polyamide 6/polystyrene blends

Morphological stability of nanosilica-filled polyamide 6/polystyrene (PA6/PS) blends under flow was evaluated via online optical-shear technique and rheology methods. In the presence of nanosilica, the droplet size of as-prepared blends was refined at low hydrophilic nanosilica concentrations (1–3 vol.%). When a slow shear flow was imposed, the morphology of hydrophilic nanosilica-filled blends became unstable and displayed a promoted coalescence dynamics. These droplets broke up at high shear rates but the breakup dynamics was suppressed at high nanosilica concentrations. The flow-induced morphological discrepancies at different shear rates for the hydrophilic nanosilica-filled blends were discussed in terms of the significantly enhanced viscoelasticity of the droplets and the decreased interfacial tension of the blends induced by nanosilica. Correlation between the transient rheology and morphological evolution of these filled blends was also examined.

Blood rheology effect of submaximal exercise on young subjects

Nowadays cardiac and metabolic diseases are a matter of concern. Exercise is a valid treatment and method of prevention for not only adults, but also young subjects. Physical activity causes transient blood rheology impairment in adults. However little is known about the effects of exercise on blood flow characteristics in young subjects. The aim of the study was to assess the effects of a light aerobic exercise session on blood rheology in young subjects. Ten young subjects (aged 12-16 years) performed 1 hour of submaximal aerobic exercise (70% HRmax). Blood samples were drawn just before and after exercise. We determined blood and plasma viscosity, fibrinogen, erythrocyte deformability and aggregability. No changes in blood viscosity (p > 0.05), erythrocyte aggregation (p > 0.05) and fibrinogen (p > 0.05) were observed. Hematocrit (p = 0.025) and plasma viscosity (p = 0.018) rose with exercise, while erythrocyte elongation index lowered (p < 0.001). Plasma volume slightly reduced which may explain the lack of changes in blood viscosity. The results of the present study indicate a similar hemorheological response to submaximal exercise in both young people and adults.