Absence Of Flow Research Articles

Duplex Sonography of Vertebral Arteries for Evaluation of Patients with Acute Vertigo

We evaluated the role of vertebral artery extracranial color-coded duplex sonography (VAECCS) in predicting vertebrobasilar stroke in consecutive patients presenting to the emergency department with vertigo of suspected ischemic origin. The final diagnosis was established by a panel of experts consisting of an emergency physician, a neurologist, and an otoneurologist. Vertebrobasilar stroke was diagnosed when an acute brain ischemic lesion congruent with symptoms was detected by neuroimaging during the index visit or a stroke was diagnosed within a 3-mo period after emergency department presentation. Among 126 patients, 28 (22%) were diagnosed with vertebrobasilar stroke. Fifteen (75%) of 20 patients with abnormal VAECCS results and 13 (12%) of 106 with normal VAECCS results had a final diagnosis of vertebrobasilar stroke. The sensitivity and specificity of VAECCS were 53.6% and 94.9%, respectively. Detecting an abnormal flow pattern at VAECCS significantly increased the risk of vertebrobasilar stroke (odds ratio = 21.5). The flow patterns most frequently related to vertebrobasilar stroke were absence of flow and high resistance pattern velocity (odds ratio = 9.3 and 22.7, respectively). VAECCS predicts vertebrobasilar stroke and could be a useful bedside screening tool in patients with vertigo.

Impact of catholyte recirculation on different 3-dimensional stainless steel cathodes in microbial electrolysis cells

While stainless steel (SS) cathodes have shown great promise due to their low cost and high specific surface areas in microbial electrolysis cells (MECs), they have mainly been examined under static (no flow) conditions. Several different SS materials with different 3-dimensional structures (mesh, fiber felt, wool, and brushes) were compared in the absence and presence of fluid flow (0.05, 0.1 and 0.2 cm/s) past the cathode by catholyte recirculation. MECs with wool produced the highest hydrogen production rate with 1.3 ± 0.3 L-H2/L-reactor/d, which was the same as the Pt control at a catholyte recirculation of 40 mL/min (applied voltage of 0.9 V). In the absence of flow, hydrogen production rates of SS materials decreased by 52% (wool) to 28% (brush). The high hydrogen production rate using wool was likely a result of its high specific surface area (480 m2/m3-reactor volume), and reduced cathode overpotential due to gas removal by catholyte recirculation.

Effects of Flow and Bulk Vesicle Concentration on Supported Lipid Bilayer Formation.

Supported lipid bilayers (SLBs) have been used extensively in a variety of biotechnology applications and fundamental studies exploring lipid behavior. Despite their widespread use, various physicochemical parameters have yet to be thoroughly investigated for their impact on SLB formation. In this work, we have studied the importance of flow in inducing the rupture of surface adsorbed chicken egg-derived l-α-phosphatidylcholine (egg PC) vesicles on silica and gold surfaces via quartz crystal microbalance with dissipation monitoring (QCM-D). On silica at 25 °C, egg PC vesicles were found to adsorb in a flattened configuration (∼13 nm thick, compared to bulk vesicle diameters of ∼165 nm) but only undergo a transition to a stable SLB under flow conditions. In the absence of flow, an increase in system temperature to 37 °C was able to promote vesicle rupture and SLB formation on silica with a 10 times lower rupture time, compared to rupture under continuous flow (175 μL/min flow rate). Gold surfaces, with their increased hydrophobicity, led to less vesicle flattening once adsorbed (structures ∼60 nm thick), and did not support vesicle rupture or SLB formation, even at flow rates of up to 650 μL/min. We also showed that, under continuous flow conditions, vesicle adsorption rates on silica surfaces follow Langmuir kinetics, with an inverse dependence on bulk vesicle concentration, while an empirical power law dependence of vesicle rupture time on bulk vesicle concentration was observed. Ultimately, this work elicits fundamental insight into the importance of flow and bulk vesicle concentration in the adsorbed vesicle rupture process during SLB formation using QCM-D.

Splenic Vein Thrombosis Following Pancreas Transplantation: Identification of Factors That Support Conservative Management.

Prophylaxis for graft portal/splenic venous thrombosis following pancreas transplant varies between institutions. Similarly, treatment of venous thrombosis ranges from early re-exploration to conservative management with anticoagulation. We wished to determine the prevalence of graft splenic vein (SV) thrombosis, as well as the clinical significance of non-occlusive thrombus observed on routine imaging. Records of 112 pancreas transplant recipients over a 5-year period at a single center were reviewed. Venous thrombosis was defined as absence of flow or presence of thrombus identified in any part of the graft SV on ultrasound. Thirty patients (27%) had some degree of thrombus or absence of flow in the SV on postoperative ultrasound. There were 5 graft losses in this group. Four were due to venous thrombosis, and occurred within 20 days of transplant. All patients with non-occlusive partial SV thrombus but normal arterial signal on Doppler ultrasound were successfully treated with IV heparin followed by warfarin for 3-6 months, and remained insulin independent. Findings of arterial signal abnormalities, such as absence or reversal of diastolic flow within the graft, require urgent operative intervention since this finding can be associated with more extensive thrombus that may lead to graft loss.

Experimental evaluation of the use of vision and barbels as references for rheotaxis in green sturgeon

Rheotropism (the ability to detect and respond to a current) and rheotaxis (deliberate orientation relative to a current) are widespread in fishes and aquatic organisms, but the relative importance of different sensory modalities as references for the rheotaxis response in fishes is largely unknown. While mechanical stimuli (including water flows) have been used to evaluate rheotaxis behavior in fishes, comparison between sensory modalities is rare, and there has been little or no investigation into the mechanosensory role of barbels in rheotaxis for bottom-oriented fish. We conducted two experiments to evaluate the role of visual stimuli (in the form of an optomotor belt) and barbels in juvenile green sturgeon rheotaxis behavior. The green sturgeon did not exhibit a clear optomotor response, and spent a higher proportion of time positively oriented toward a flowing current than they did toward a moving background in the absence of flow. Removal of barbels increased the average individual tendency to orient positively in the presence of flow. While visual cues almost certainly play a role in rheotaxis behavior at large, individuals vary greatly in their degree of responsiveness to stimuli, and the optomotor stimuli used in our experiments were not as effective as the mechanosensory stimuli in provoking positive rheotaxis. Further, the barbels of green sturgeon do not appear to influence their ability to display positive rheotaxis in the presence of water current.

Outcomes of Ultrasound-Guided Thrombin Injection of Nongroin Arterial Pseudoaneurysms

PurposeTo evaluate success and complication rates of percutaneous ultrasound-guided thrombin injection of nongroin pseudoaneurysms (PSAs). Materials and MethodsRetrospective review of a prospectively maintained institutional database yielded 39 cases of arterial PSAs occurring at nongroin sites that were treated with percutaneous ultrasound-guided thrombin injection between 2000 and 2016 (average patient age 69.2 y ± 14.0). Of PSAs, 74.4% (29/39) arose in the upper extremities, and 92.3% (36/39) were iatrogenic. The brachial artery was the most commonly affected vessel (51.3% [20/39]), and arterial access was the most common cause (56.4% [22/39]). Average overall PSA size was 2.4 cm (range, 0.5–7.2 cm); average amount of thrombin injected was 320 IU (range, 50–2,000 IU). Technical success was defined as absence of flow within the PSA immediately after thrombin injection. Treatment success was defined as sustained thrombosis on follow-up imaging obtained at 1–3 days after treatment. ResultsTechnical and treatment success rates of thrombin injections were 100% (39/39) and 84.8% (28/33), respectively. Longer term follow-up imaging (average 71 d; range, 12–201 d) was available for 7 of the treatment successes with 100% (7/7) showing sustained thrombosis. Comparing treatment successes and failures, there was no significant difference in average PSA size (2.3 cm vs 2.0 cm, P = .51) or average amount of thrombin injected (360 IU vs 180 IU, P = .14). There were no complications. ConclusionsUltrasound-guided thrombin injection is a safe, efficacious treatment option for PSAs arising in nongroin locations.

A randomised comparison of Conventional versus Intentional straTegy in patients with high Risk prEdiction of Side branch OccLusion in coronary bifurcation interVEntion: rationale and design of the CIT-RESOLVE trial

IntroductionThe intentional strategy (aggressive side branch (SB) protection strategy: elective two-stent strategy or jailed balloon technique) is thought to be associated with lower SB occlusion rate than conventional strategy (provisional...

Dynamics of a treadmilling microswimmer near a no-slip wall in simple shear

Induction of flow is commonly used to control the migration of a microswimmer in a confined system such as a microchannel. The motion of a swimmer, in general, is governed by nonlinear equations due to non-trivial hydrodynamic interactions between the flow and the swimmer near a wall. This paper derives analytical expressions for the equations of motion governing a circular treadmilling swimmer in simple shear near a no-slip wall by combining the reciprocal theorem for Stokes flow with an exact solution for the dragging problem of a cylinder near a wall. We demonstrate that the reduced dynamical system possesses a Hamiltonian structure, which we use to show that the swimmer cannot migrate stably at a constant distance from a wall but only exhibit periodic oscillatory motion along the wall, or to escape from it. A treadmilling swimmer with the lowest two treadmilling modes is investigated in detail by means of a bifurcation analysis of the reduced dynamical system. It is found that the swimming direction of oscillatory motion is clarified by the sign of the Hamiltonian in the absence of flow, and that the induction of the flow suppresses upstream migration but aligns swimmer orientations in downstream migration. These results could inform strategies for the transport and control of micro-organisms and micromachines.

A numerical investigation of Helmholtz resonators in the presence of grazing flow by means of the lattice Boltzmann method

Helmholtz resonators remain widely used in noise control. In applications such as aircraft engines and exhaust systems, the presence of a grazing flow significantly changes their behavior, and a correct prediction of their acoustic properties is essential to improve noise reduction. With the purpose of understanding the physical phenomena associated with the acoustic-flow interaction, the simulation of a single 2D Helmholtz resonator was considered by means of an in-house numerical code based on the lattice Boltzmann method (LBM). The results have been validated against published data based on both experimental results and direct numerical simulation (DNS) for normally incident acoustic waves in the absence of flow. The investigations will proceed by taking into account grazing acoustic waves in the presence of a grazing flow, similarly to the conditions found in a liner test rig. Efforts will be focused on typical aircraft engine inlet conditions, i.e., high Mach numbers and SPL. Both experimental and numerical results will be compared in terms of absorption coefficient and impedance.

The FAST D protocol: a simple method to rule out traumatic vascular injuries of the lower extremities

BackgroundThe aim of this study is to assess the accuracy of a Fast Doppler protocol for the examination of an injured lower limb, namely 2-Point Fast Doppler (2PFD), in order to rapidly triage arterial lesions after penetrating trauma.MethodsThe presence of flow and the aspects of the Doppler waveform of the dorsalis pedis artery (DPA) and posterior tibial artery (PTA) of the injured lower limb (2PFD) were evaluated immediately before the execution of a standardized Color Duplex Doppler (SD) evaluation in 149 limbs of 140 patients with gunshot penetrating injuries. We considered 2PFD normal exams as the ones with triphasic patterns in both the DPA and PTA, and 2PFD pathologic exams as the ones with absent, biphasic, or monophasic flow patterns in the DPA and/or PTA. 2PFD data were then analyzed to assess accuracy variables, using SD results as matching test reference. According to the trauma center standard protocols, SD positive cases underwent also angiography and surgical exploration, whose findings were used to further match the 2PFD specificity.ResultsThe 2PFD protocol showed a sensitivity of 100%, and a specificity of 100% compared with the SD, in the diagnostic workup of arterial injuries of the lower limbs after penetrating trauma. Furthermore, all the pathologic cases that resulted in all true positives (TP), compared with SD, were confirmed as TP also when matched with the angiography evaluation results.ConclusionsThe 2PFD protocol can rapidly identify arterial flow and differentiate between normal and pathologic spectral Doppler analyses in distal arteries. The presence of the normal triphasic flows in DPA and PTA is as sensitive as the standardized Color Doppler Duplex assessment of the entire limb in ruling out arterial lesions in lower-limb penetrating trauma. The absence of flow or the presence of a biphasic or monophasic pathologic flow in DPA and PTA is pathologic and should be always followed by further investigation. 2PFD is faster and easier to perform compared with the SD approach. It could become a new first-line screening technique, both in pre-hospital and hospital critical scenarios, particularly in contexts where advanced diagnostic performance is limited by time concerns or scarce resources.

Impact of straight, unconnected, radially-oriented, and tapered mesopores on column efficiency: A theoretical investigation

Superficially porous particles (SPPs) can be prepared from a pseudomorphic transformation (PMT) which produces straight, unconnected, and radially-oriented mesopores (ROMs). ROMs can be either both ends open in fully porous particles (FPPs) or one-end-closed in SPPs. The impact of ROMs on the longitudinal diffusion (B/u), solid–liquid mass transfer resistance (Csu), and on the eddy dispersion (A(u)) height equivalent to a theoretical plate (HETP) of 3D randomly packed columns was investigated based on theoretical viewpoints. Torquato's theory of effective diffusion in packed beds (B term), Giddings’ coupling theory of eddy dispersion (A term), and Giddings’ generalized nonequilibrium theory (Cs term) are applied to make predictions. First, it is found that the A term is nearly independent on the internal structure of the particle. Secondly, in the absence of flow, infinitely narrow and both ends open (no constriction effect) ROMs induce an internal hindrance factor of 23 regarding diffusion along the axial direction. Experimental data reveal that one-end-closed and 80Å wide ROMs in SPPs lead to a measurable internal hindrance factor of 27 regarding diffusion in the porous shell. Thirdly, above the optimum speed, the Cs coefficient is dependent on the geometry (cylinders, cones, etc.) of the ROMs: when ROMs are conical in SPPs, Cs is expected to decrease by 80% with respect to cylindrical ROMs. From an application perspective, PMT-SPPs prepared with narrow ROMs are well suited for the analysis of small molecules at or below optimum speed (lowest B term) while PMT-SPPs made of wide and conical ROMs are ideal for the analysis of large molecules above optimum speed (smallest Cs term).

Load Response of the Flagellar Beat.

Cilia and flagella exhibit regular bending waves that perform mechanical work on the surrounding fluid, to propel cellular swimmers and pump fluids inside organisms. Here, we quantify a force-velocity relationship of the beating flagellum, by exposing flagellated Chlamydomonas cells to controlled microfluidic flows. A simple theory of flagellar limit-cycle oscillations, calibrated by measurements in the absence of flow, reproduces this relationship quantitatively. We derive a link between the energy efficiency of the flagellar beat and its ability to synchronize to oscillatory flows.

Abstract 12507: Spatial Gradients in Wall Shear Stress Promote Collective Human Lymphatic Microvascular Endothelial Cell Migration and Alignment Through Sphingosine 1-phosphate Receptor 1

Introduction: Spatial variations in fluid flow are thought to help trigger the formation of valves within the blood and lymphatic circulatory systems during embryonic development. However, how the physical stimulus drives the collective cell movements and changes in gene expression that underlie valve formation is not known. This knowledge gap is of considerable importance since valve failure within the lymphatic and venous vessels is significant in a wide variety of cardiovascular disease states. Methods: In this study, we developed in vitro fluid flow devices that allow us to replicate key aspects of the fluid flow environment found in the vicinity of developing valves within the lymphatic and venous vasculatures. We developed an impinging flow chamber as a model for vessel bifurcations, applying a spatial gradient in wall shear stress (WSS). We have also developed a device which applies flow through a constriction such as is commonly found at sites of valve initiation. Results: Using these devices, we examined the migration of human lymphatic microvascular ECs (MVECs) subjected to spatial gradients in WSS. We found that lymphatic MVECs subjected to a variety of WSS ranges from 0 to 9, 33, or 72 dynes/cm 2 align and collectively migrate against the flow direction over the course of 20 hours. When subjected to flow through a constriction, lymphatic MVECs turned perpendicular to the flow direction at regions near the site of maximum constriction. We determined that the G-Protein Coupled Receptor, Sphingosine 1-Phosphate Receptor 1 (S1PR1) was required for both lymphatic MVEC alignment and collective migration: a 70% S1PR1 mRNA knockdown relative to scrambled siRNA control was sufficient to prevent the collective migration and perpendicular alignment phenotypes we observed in our in vitro flow devices. In addition, these shear stress gradients resulted in higher nuclear levels of Prospero Homeobox Protein 1, a transcription factor known to be required for valvulogenesis, relative to cells in the absence of flow (Impinging Flow; 9.8 +/- 2.2 fold, Constriction; 5.1 +/- 0.8 fold). Conclusions: Our observations implicate S1PR1 as a key component in both the phenotypic and transcriptional responses of LECs to gradients in WSS, such as those at sites of valve formation.

Endovascular Recanalization of Chronic Non-malignant Obstruction of the Inferior Vena Cava

70 J/cm) for SSV. The complete occlusion of the treated saphenous vein was defined as absence of flow on color Doppler imaging and was achieved in 174/177 (98.3%) subjects at 4 weeks and 132/135 (97.7) patients at 12 months. Partial occlusion was observed in three patients and was defined as reflux >3 cm distal to the junction. The mean length of non-occluded stump at the junction between saphenous vein and deep vein was 0.5 cm (range 3.6 to 5.0 cm) 4 weeks postoperative. Mean postoperative reduction of the diameter of GSV and SSV at 1 year was 40% and 56% respectively (p< 0.05%). In the early postoperative phase paraesthesia was observed in 15/177 (8.5%), ecchymosis in 4/177 (2.3%), lymphocoele in 1/ 177 (0.6%), hyperpigmentation in 1/177 (0.6%). Endovenous heat induced thrombus (EHIT) propagation was observed in 4/177 (2.3%) subjects. There were no skin burns or phlebitis. No patient reported severe pain. The mean postoperative pain intensity (Visual Analog Scale) was 1.3. The mean convalescence period was 2 days (1e30 d). No long-term complications and recurrent reflux was observed at 1 year follow-up. Conclusion: ELT using Thulium laser (1940 nm) with radial fiber efficiently eliminates the reflux in the saphenous veins by occlusion and significant diameter reduction with no reflux recurrence at 1 year follow up. The complication rates are less or comparable to the other endothermal and surgical procedures. These benefits are offered with low postoperative pain, analgesia requirement and rapid convalescence.

A non-traditional view on the modeling of nematic disclination dynamics

Non-singular disclination dynamics in a uniaxial nematic liquid crystal is modeled within a mathematical framework where the kinematics is a direct extension of the classical way of identifying these line defects with singularities of a unit vector field representing the nematic director. It is well known that the universally accepted Oseen-Frank energy is infinite for configurations that contain disclination line defects. We devise a natural augmentation of the Oseen-Frank energy to account for physical situations where, under certain conditions, infinite director gradients have zero associated energy cost, as would be necessary for modeling half-integer strength disclinations within the framework of the director theory. Equilibria and dynamics (in the absence of flow) of line defects are studied within the proposed model. Using appropriate initial/boundary data, the gradient-flow dynamics of this energy leads to non-singular, line defect equilibrium solutions, including those of half-integer strength. However, we demonstrate that the gradient flow dynamics for this energy is not able to adequately describe defect evolution. Motivated by similarity with dislocation dynamics in solids, a novel 2D-model of disclination dynamics in nematics is proposed. The model is based on the extended Oseen-Frank energy and takes into account thermodynamics and the kinematics of conservation of defect topological charge. We validate this model through computations of disclination equilibria, annihilation, repulsion, and splitting. We show that the energy function we devise, suitably interpreted, can serve as well for the modeling of equilibria and dynamics of dislocation line defects in solids, making the conclusions of this paper relevant to mechanics of both solids and liquid crystals.

Examining the Colonization and Survival of E. coli from Varying Host Sources in Drainage Basin Sediments and Stormwater.

It is widely understood that stormwater drainage has a significant impact on the health of tidal creek systems via regular inputs of runoff from the surrounding watershed. Due to this hydrologic connection, contamination of the upstream drainage basin will have a direct effect on estuaries and tidal creeks that often act as receiving waters. This study focuses on the importance of drainage basin sediments as they enhance the persistence and transport of the fecal indicator bacteria E. coli within a watershed. Experiments presented use microcosm environments with drainage basin sediments and stormwater to investigate E. coli colonization of stagnant waters and to examine the importance of host sources to bacterial survival. A novel method for establishing microcosms using environmental sediments with in situ bacterial populations and sterile overlying waters is used to examine E. coli colonization of the water column in the absence of flow. Colonization of sterile sediment environments also is examined using two common host sources (human and avian). Each experiment uses sediments of varying grain size and organic content to examine the influence of physical characteristics on bacterial prevalence. Results suggest host source of bacteria may be more important to initial bacterial colonization while physical characteristics of drainage basin sediments better explains extended E. coli persistence. Findings also suggest an indirect control of water column bacterial concentration by sediment type and erodibility.

Gravity Effects on Fiber Dynamics in Wall Turbulence

The present study investigated gravity effects on the dynamical behavior of inertial fibers suspended in a vertical channel flow. Direct numerical simulations were performed to obtain the turbulent flow field and the fibers were modelled as prolate spheroidal point particles. For each of the four fiber classes, three different gravity configurations were considered: upward flow with gravity opposing, downward flow with aiding gravity, and channel flow in absence of gravity. Results for the fiber distribution and the translational and rotational fiber motion were reported. In the near-wall region, the presence of gravity resulted in an increased fiber density in the downward flow but a nearly uniform distribution of fibers in upward flow. However, the preferential clustering of fibers in near-wall low-speed streaks was unaffected by gravity. The mean wall-normal or drift velocity of the fibers was higher in the downward flow and lower in the upward flow as compared to the case with no gravity. The suppressed drift velocity in the upward flow resulted in a more uniform fiber distribution throughout the channel in contrast to the near-wall accumulation of fibers in the two other cases. Overall gravity turned out to have negligible effects on some of the statistics of the least inertial fibers whereas the inclusion of gravity had a strong impact for heavier fibers.

A Technique for Comparing Wall Pressure Distributions in Steady Flow Through Rigid Versus Flexible Patient-based Abdominal Aortic Aneurysm Phantoms

Abdominal aortic aneurysms (AAAs) represent permanent, localized dilations of the abdominal aorta. Here, we describe a procedure for noninvasively measuring the flow-induced wall pressure distribution in both effectively rigid, thick-wall and flexible, thin-wall phantoms under perfusion conditions dynamically simulating the in vivo abdominal aorta. Both phantoms accurately replicated the shape of patient AAAs including the renal and iliac arteries, and the flexible phantoms reflected patient tissue mechanical properties as well. As an example of their use, wall pressure distributions measured in rigid and flexible phantoms derived from one representative patient under flow conditions emulating the aorta at rest are presented. In both phantoms, there was a net pressure decrease from the upstream end of the bulge to the downstream end. However, there was a five times larger variation of wall pressure magnitude along the bulge region of the flexible phantom than along the rigid phantom, 6–7 mmHg versus more than 30 mmHg. In addition, in the rigid phantom, pressure signal fluctuations were of the same order of magnitude as the pressure transducer inherent noise level. In the flexible phantom, they were approximately 10 times the noise level in the absence of flow, suggesting that flow in the flexible phantom was unstable even at Reynolds number 500.

Flow of colloidal solids and fluids through constrictions: dynamical density functional theory versus simulation

Using both dynamical density functional theory and particle-resolved Brownian dynamics simulations, we explore the flow of two-dimensional colloidal solids and fluids driven through a linear channel with a constriction. The flow is generated by a constant external force acting on all colloids. The initial configuration is equilibrated in the absence of flow and then the external force is switched on instantaneously. Upon starting the flow, we observe four different scenarios: a complete blockade, a monotonic decay to a constant particle flux (typical for a fluid), a damped oscillatory behaviour in the particle flux, and a long-lived stop-and-go behaviour in the flow (typical for a solid). The dynamical density functional theory describes all four situations but predicts infinitely long undamped oscillations in the flow which are always damped in the simulations. We attribute the mechanisms of the underlying stop-and-go flow to symmetry conditions on the flowing solid. Our predictions are verifiable in real-space experiments on magnetic colloidal monolayers which are driven through structured microchannels and can be exploited to steer the flow throughput in microfluidics.

Effects of shear flow on phase nucleation and crystallization.

Classical nucleation theory offers a good framework for understanding the common features of new phase formation processes in metastable homogeneous media at rest. However, nucleation processes in liquids are ubiquitously affected by hydrodynamic flow, and there is no satisfactory understanding of whether shear promotes or slows down the nucleation process. We developed a classical nucleation theory for sheared systems starting from the molecular level of the Becker-Doering master kinetic equation and we analytically derived a closed-form expression for the nucleation rate. The theory accounts for the effect of flow-mediated transport of molecules to the nucleus of the new phase, as well as for the mechanical deformation imparted to the nucleus by the flow field. The competition between flow-induced molecular transport, which accelerates nucleation, and flow-induced nucleus straining, which lowers the nucleation rate by increasing the nucleation energy barrier, gives rise to a marked nonmonotonic dependence of the nucleation rate on the shear rate. The theory predicts an optimal shear rate at which the nucleation rate is one order of magnitude larger than in the absence of flow.