Steady Force Research Articles

Experimental and theoretical investigations on dry gas seal transient performance and its disparity with steady performance

Through high-speed dry gas seal test rig, the real-time film thickness and leakage rate of spiral grooved dry gas seal have been monitored under different closing forces which are changed by adjusting spring compression. Under the conditions of different spring pressures and seal face deflection rates, the change rules of transient opening force and leakage rate are obtained by solving the transient-state Reynolds equation based on the measured real-time film thickness. Meanwhile, the steady opening force and leakage rate are figured out by solving the steady-state Reynolds equation using the average value of the real-time film thickness. Results show that the variation trends of calculated results based on steady-state or transient-state theory coincide well with experimental results. But with the influence of nonlinear effect, the average value of transient opening force and leakage rate are larger than steady opening force and leakage rate calculated with the average value of real-time film thickness, and compared with the value of steady leakage rate, the average value of transient leakage rate considered seal face deflection is closer to the experimental monitoring results.

Null effects of musical groove on cortico-muscular coherence during isometric contraction

Humans have a natural tendency to move to music, which has been linked to the tight coupling between the auditory and motor system and the active role of the motor system in the perception of musical rhythms. High-groove music is particularly successful at inducing spontaneous movement, due to the engagement of (motor) prediction processes. However, how music listening transfers to the muscles even when no movement is intended is less known. Here we used cortico-muscular coherence (CMC) to investigate changes along the cortico-muscular pathway in response to different levels of groove in music listening without intention to move. Electroencephalography (EEG), Electromyography (EMG) from the finger and foot flexors, and continuous force signals were recorded in 18 participants while listening to either high-groove music, low-groove music or silence. Participants were required to hold a steady isometric contraction during all listening conditions. Subjective ratings confirmed that different levels of groove were successfully induced. However, no evidence was found for an effect of music, even high-groove music, on participants' CMC and ability to maintain a steady force for both upper and lower limbs irrespective of musical expertise. These results thus do not support a top-down influence of groove on cortico-muscular coupling. Nevertheless, it remains possible that such influence might occur in the form of dynamic modulations and/or with more active listening. Therefore, these results encourage further research to better understand the effects of groove on the motor system.

Comparison Between Cotton Test and Tap Test for the Assessment of Coronal Syndesmotic Instability: A Cadaveric Study

Category:Ankle; Basic Sciences/Biologics; Sports; TraumaIntroduction/Purpose:Diagnosis of subtle instability of the distal tibiofibular syndesmosis is challenging. In surgically treated rotational malleolar fractures, instability is typically assessed with the intraoperative Cotton test. However, this test can be unreliable due to its dynamic nature and uncontrolled distraction force. The Tap test is an alternative test where a cortical tap is advanced through the fibula with a progressive, stable, and unidirectional distraction force. The objective of this cadaveric study was to compare the DTFS widening when using the Cotton and Tap tests as diagnostic tools for coronal plane syndesmotic instability.Methods:Ten below-knee cadaveric specimens were tested in intact non-stressed, intact stressed, injured non-stressed, and injured stressed conditions, with stressed conditions utilizing both Cotton and Tap tests. In injured conditions, the syndesmotic ligamentous complex was sectioned (anterolateral longitudinal approach). Perfect fluoroscopic Mortise images were acquired for all conditions. For the Tap test, a 2.5 drill bit was used to drill a hole through both distal fibular cortices. A blunt-edged 3.5mm cortical tap was advanced toward the tibia. For the Cotton test, a lateral distraction force was applied to the distal fibula with a towel clamp. Two observers measured Tibiofibular Clear Space (TFCS) 1cm proximal to the ankle joint line. Intra and interobserver reliabilities were assessed by Intraclass Correlation Coefficient (ICC). Syndesmotic TFCS values for all conditions were compared by paired Wilcoxon. Diagnostic performance of the Cotton and Tap tests was assessed (a relative increase of TFCS>2mm). P-values <0.05 were considered significant.Results:The intraclass correlation coefficient (ICC) for intraobserver and interobserver reliability was respectively, 0.96 and 0.78.TFCS measurements were similar in intact non-stressed, intact stressed (both Cotton and Tap tests) and injured non- stressed conditions: intact non-stressed, 3.5mm (CI, 3.0 to 3.9mm); intact stressed, 3.6mm (CI, 3.1 to 4.1mm) (Cotton test) and 4.0mm (CI, 3.5 to 4.5mm) (Tap test); injured non-stressed, 3.8mm (CI, 3.3 to 4.3mm). TFCS was significantly increased (p<0.0001) in injured and stressed ankles for both Cotton and Tap tests, with values of respectively, 6.2mm (CI, 5.8 to 6.7mm) and 6.1mm (CI, 5.7 to 6.6mm). The Cotton test had 73.3% sensitivity, 100% specificity, and 86.7% diagnostic accuracy. The Tap test had 70% sensitivity, 90% specificity, and 80% diagnostic accuracy.Conclusion:Our cadaveric study compared the Cotton and Tap tests for detection of coronal plane syndesmotic instability. Both tests demonstrated similar increases in TFCS measurements in stressed injured conditions when compared to intact and injured non-stressed conditions. Additionally, both tests demonstrated similar diagnostic accuracy for coronal plane syndesmotic instability, with slight favor for the Cotton test. In our experience, the Cotton test can be unreliable due to the difficulty in applying a steady distraction force while maintaining a perfect Mortise view. We recommend the Tap test as a more stable, controlled, and reproducible intraoperative diagnostic test for coronal plane syndesmotic instability.

Investigation of Mach number and Reynolds number effects on flow around a near-critical geometry prism at Reynolds number &lt;i&gt;O&lt;/i&gt;(10&lt;sup&gt;3&lt;/sup&gt;) using a low-density wind tunnel

In this study, steady drag force measurements and time-resolved schlieren visualization were conducted for subsonic compressible flows around a rectangular cylinder with a side length ratio from 0.5 to 1.2. The effect of the compressibility on the critical geometry was evaluated based on the drag coefficient using a low-density wind tunnel in the Mach number range from 0.1 to 0.6 at the Reynolds number of O(103). In addition, the relationship between the change in the drag coefficient and the flow field was discussed. The side length ratio that the drag coefficient takes the maximum was confirmed to change with the Mach numbers. Remarkably, the peak drag coefficient at the Mach number of 0.6 appears at the side length ratio of 1.0, even though the side length ratio of the critical geometry at the incompressible flow is approximately 0.65. However, the sharp peak in the drag coefficient which is observed in the previous incompressible studies could not be reproduced even at the Mach number of 0.2. This is considered to be due to the lack of the spanwise length of the model or the Reynolds number effects. The schlieren images showed the influence of the Mach number on the location of the vortex formation, and the location moves downstream as the Mach number increases. The change in the flow fields behind rectangular cylinders causes a change in the back pressure, and it might result in a sudden change in the drag coefficient.

Effect of Arc Currents on the Mechanical, High Temperature Oxidation and Corrosion Properties of CrSiN Nanocomposite Coatings

In methods for multi-arc ion plating technology, the behavior and characteristics of the arc spot determine the physical characteristics of arc plasma and the properties of the subsequent deposited coatings. In this paper, the effect of arc currents on the hardness, friction coefficient, high temperature oxidation, and corrosion properties of the CrSiN coatings was studied. According to the XRD and SEM results, with the increase of arc currents, the coatings grew preferentially to the CrN (111) crystal direction, and the CrN (220) crystal phase appeared at high currents of 90 A. In addition, the number of large particles increased when the current exceeded 70 A. The HR-TEM results confirmed the formation of nanocomposite structure of nanocrystalline of CrN embedded into the amorphous phase of Si3N4 as explored by XRD. The maximum hardness was achieved at 3120 Hv when the coatings were deposited under currents around 70 A. However, the hardness values decreased with further increase of arc currents. From the contact of ceramic balls with the wear of coatings, the surface of coatings gradually produced friction marks, and the friction force increased from a steady friction force to a dynamic friction force. The high temperature oxidation results showed that fewer oxides were formed on the surface of the coatings when oxidized at 800 °C. It was also found that CrSiN nanocomposite coatings prepared at an arc current of 70 A had a larger corrosion potential and polarization impedance, which could effectively protect the tool matrix.

Viscoacoustic squeeze-film force on a rigid disk undergoing small axial oscillations

This paper investigates the air flow induced by a rigid circular disk or piston vibrating harmonically along its axis of symmetry in the immediate vicinity of a parallel surface. Previous attempts to characterize these so-called ‘squeeze-film’ systems largely relied on simplifications afforded by neglecting either fluid acceleration or viscous forces inside the thin enclosed gas layer. The present viscoacoustic analysis employs the asymptotic limit of small vibration amplitudes to investigate the flow by systematic reduction of the Navier–Stokes equations in two distinct flow regions, namely, the inner gaseous film where streamlines are nearly parallel to the confining walls and the near-edge region of non-slender flow that features gas exchange with the surrounding stagnant atmosphere. The flow in the gaseous film depends on the relevant Stokes number, defined as the ratio of the characteristic viscous time across the film to the characteristic oscillation time, and on a compressibility parameter, defined as the square of the ratio of the acoustic time for radial pressure equilibration to the oscillation time. A Strouhal number based on the local residence time emerges as an additional governing parameter for the near-edge region, which is incompressible at leading order. The method of matched asymptotic expansions is used to describe the solution in both regions, across which the time-averaged pressure exhibits comparable variations that give opposing contributions to the resulting time-averaged force experienced by the disk or piston. A diagram structured with the Stokes number and compressibility parameter as coordinates reveals that this steady squeeze-film force, typically repulsive for small values of the Stokes number, alternates to attraction across a critical separation contour in the parametric domain that exists for all Strouhal numbers. This analysis provides, for the first time, a unifying viscoacoustic theory of axisymmetric squeeze films, which yields a reduced parametric description for the time-averaged repulsion/attraction force that is potentially useful in applications including non-contact fluid bearings and robot locomotion.

RANS Prediction of Wave-Induced Ship Motions, and Steady Wave Forces and Moments in Regular Waves

The wave-induced motions, and steady wave forces and moments for the oil tanker KVLCC2 in regular head and oblique waves are numerically predicted by using the expanded RANS solver based on OpenFOAM. New modules of wave boundary condition are programed into OpenFOAM for this purpose. In the present consideration, the steady wave forces and moments include not only the contribution of hydrodynamic effects but also the contribution of the inertial effects due to wave-induced ship motions. The computed results show that the contribution of the inertial effects due to heave and pitch in head waves is non-negligible when wave-induced motions are of large amplitude, for example, in long waves. The influence of wave amplitude on added resistance in head waves is also analyzed. The dimensionless added resistance becomes smaller with the increasing wave amplitude, indicating that added resistance is not proportional to the square of wave amplitude. However, wave amplitude seems not to affect the heave and pitch RAOs significantly. The steady wave surge force, sway force and yaw moment for the KVLCC2 with zero speed in oblique waves are computed as well. The present RANS results are compared with available experimental data, and very good agreements are found between them.

Influence of swirl and viscosity of annular flow on instability mechanisms of an FGMs micro shell

The influences of the swirl (the fluid rotation) and the steady viscous forces of an annular flow on instability mechanisms of an outer functionally graded materials (FGMs) cylindrical micro shell are investigated. The annular flow passes the annulus between the inner micro shell and outer micro shell. The inviscid fluid-elastic forces related to the shell vibration are modeled in the frame of potential flow theory. The steady viscous forces induced by the fluid viscosity are obtained by using the time-averaged Navier-Stokes equations and then are coupled into the shell model in the form of initial loadings. The pre-loaded shell model is developed by means of the Hamilton's principle in conjunction with the modified couple stress theory. The pre-loaded shell motion equations are described based on the Flügge's thin shell theory. Afterwards, the zero-level contour method and the Galekin's method are employed to solve the problem. A parameter study and numerical comparisons are performed to evaluate the influences of all terms of inviscid fluid-elastic forces on the instability character of micro shell. Especially, the swirl effects are discussed fully. The results display different instability mechanisms of the fluid-micro shell system controlled by the swirl or/and the axial flow velocity. Eventually the fluid viscosity effects are given.

Numerical simulation of a vessel’s manoeuvring performance in regular waves

ABSTRACT This paper deals with the numerical simulation of a ship’s manoeuvring performance in regular waves. This is made possible by employing the time-domain code ELIGMOS, which adopts a hybrid formulation to couple seakeeping and manoeuvring contributions. The first and steady second-order wave-induced forces are incorporated, implementing a multidimensional interpolation scheme to account for their dependency on the instantaneous heading and forward speed values. Two methods have been adopted for the calculation of the added resistance regarding the size of the wavelength (short or long wave seas), namely a far and a near-field one, using the hydrodynamic software NEWDRIFT+ and NEWDRIFT v.7. Low-frequency hydrodynamic manoeuvring forces are incorporated by adopting the expressions suggested by the Japanese MMG (Manoeuvring Modelling Group) method. Validation of the suggested methodology is attempted through the comparison of numerical simulations of turning circle tests in calm water and in regular waves with available experimental evidence for the S-175 container ship.

Equivalence of sessile droplet dynamics under periodic and steady electric fields

The electrohydrodynamics of a sessile droplet under the influence of periodic and steady electric fields in microgravity conditions is theoretically investigated using an inertial lubrication model. Previous studies have revealed that a freely suspended spherical droplet with unequal conductivity and permittivity ratios exhibits distinct dynamics under periodic and equivalent steady forcing in the root mean-square sense. However, it is unclear when (if at all) such distinct dynamics occur for periodic and equivalent steady forcing in the case of sessile droplets. The equivalence between periodic and steady forcing is shown to be governed by the interfacial charge buildup, which further depends on the competition between the charge relaxation and forcing timescales. A circulation-deformation map is introduced for the sessile droplet that acts as a guideline to achieve electric field-induced wetting or dewetting as the case may be. We also demonstrate that a droplet may be rendered either more or less wetting solely by tuning the forcing frequency.

Effect of Fascicle Length Range on Force Generation of Model-Based Biomimetic Controller for Tendon-Driven Prosthetic Hand.

Model-based biomimetic control with neuro-muscular reflex requires accurate representation of muscle fascicle length, which affects both force generation capability of muscle and dynamics of muscle spindle. However, physiological data are insufficient to guide the selection of range of fascicle length for task control. Here a reverse engineering approach was used to investigate the effects of different fascicle length range on controller's force control ability, so as to justify the selection of operating range of muscle length for a grasp force task. We compared 3 different ranges of fascicle length for their effects on force generation, i.e. R1: 0.5 - 1.0 Lo, R2: 0.5 - 1.3 Lo and R3: 0.5 - 1.6 Lo. The rationale to test these range selections was based on both physiological realism and engineering considerations. The steady state force output and transient force responses were evaluated with a range of step inputs as controller input. Results show that the prosthetic finger can produce a linear steady state force response with all 3 ranges of fascicle length. Peak force was the largest with R3. Fascicle length range had no significant effect on the rise time in force generation tasks. Results suggest that a wider range of fascicle length may be more favorable for force capacity, since the contact point of force control may well fall near the optimal length (Lo) region.

Effects of Suppressed Transient Eddies by the Tibetan Plateau on the East Asian Summer Monsoon

AbstractIt is known that the Tibetan Plateau (TP) can weaken the transient eddies (TEs) transported along the westerly jet stream. This study investigates the effects of the persistently suppressed TEs by the TP on the East Asian summer monsoon and the associated mechanisms using the NCAR Community Earth System Model. A nudging method is used to modify the suppression of the TEs without changing the steady dynamic and thermodynamic effects of the TP. The suppressed TEs by the TP weaken the East Asian westerly jet stream through the weakened poleward TE vorticity flux. On the one hand, the weakened jet stream leads to less (more) rainfall in northern (southern) East Asia by inducing anomalous moisture convergence, midtropospheric warm advection, and upper-level divergence, particularly in early summer when the eastward propagation of TE suppression by the TP is strong. On the other hand, the precipitation anomalies can shift the East Asian westerly jet stream southward and promote the moisture convergence in southern East Asia through latent heat release. Therefore, the persistent suppression of the TEs leads to a southward shift of the East Asian rain belt by a convective feedback, as it was previously found that the steady thermodynamic and dynamic forcings of the TP favored a northward shift of the rain belt. This study suggests that the anomalously weak TEs can lead to a rainfall change (more in the south, less in the north) over East Asia.

Verification of a Body Freedom Flutter Numerical Simulation Method Based on Main Influence Parameters

The body freedom flutter characteristics of an airfoil and a fly wing aircraft model were calculated based on a CFD method for the Navier–Stokes equations. Firstly, a rigid elastic coupling dynamic model of a two-dimensional airfoil with a free–free boundary condition was derived in an inertial frame and decoupled by rigid body mode and elastic mode. In the fluid–solid coupling method, the rigid body was trimmed by subtracting the generalized steady aerodynamic force from the structural dynamic equation. The flutter characteristics were predicted by the variable stiffness method at a fixed Mach number and flight altitude. Finally, validation of the predicted body freedom flutter characteristics was performed through a comparison of theoretical solutions based on a Theodorsen unsteady aerodynamic model for airfoil and experimental results for a fly wing aircraft model. The mechanism of the influence of the bending mode stiffness and the position of the center of gravity on the body freedom flutter characteristics were briefly analyzed.

The Effect of Spool Displacement Control to the Flow Rate in the Piezoelectric Stack-Based Valve System Subjected to High Operating Temperature

This work investigates the effect of spool displacement control of the piezoelectric stack actuator (PSA) based valve system on the flow motion of the pressure drop and flow rate. As a first step, the governing equations of the structural parts of the displacement amplifier and spool are derived, followed by the governing equation of the fluid part considering control volume and steady flow force. Then, an appropriate size of the valve is designed and manufactured. An experimental apparatus to control the spool displacement is set up in the heat chamber and tracking control for the spool displacement is evaluated at 20 °C and 100 °C by implementing a proportional-integral-derivative (PID) feedback controller. The tracking controls of the spool displacement associated with the sinusoidal and triangular trajectories are realized at 20 °C and 100 °C. It is demonstrated that the tracking controls for the sinusoidal and triangular trajectories have been well carried out showing the tracking error less than 3 μm at both temperatures. In addition, the flow motions for the pressure drop and the flow rate of the proposed valve system are experimentally investigated. It is identified from this investigation that both pressure drop and flow rate evaluated 20 °C have been decreased up to 18% at 100 °C. This result directly indicates that the temperature effect to control performance of the structural part and fluid part in the proposed PSA based valve system is different and hence careful attention is required to achieve the successful development of advanced valve systems subjected to a wide range of the operating temperature.

Intermittent Retreat Behavior in Experimental Barrier Island Response to Constant Sea Level Rise and Wave Forcing

AbstractBarrier islands are naturally low‐lying features and are susceptible to drowning from sea level rise. However, previous work suggests that barrier islands can retreat upslope to maintain their subaerial extent, though not necessarily at a constant rate even under steady forcing. To explore this idea, we performed physical experiments where barrier islands were subjected to a constant relative sea level rise (RSL) and a constant wave environment. We tracked the islands through time with overhead images and periodic laser elevation scans. Time series of island location show that the islands did not retreat at constant rates through the transgression. Rather, overall long‐term retreat occurred through a punctuated retreat motion comprising steady retreat alternating with accelerated steps. The source of this complex behavior is a cycling between two morphologies: one where an island has a developed ridge and another where no ridge is present. Sea level rise allows waves to overtop the island and erode the ridge that once kept the waves at bay. That sediment is deposited behind the island, moving the barrier landward. Waves continue to push sediment back toward the mainland until the island becomes too wide for waves to carry sediment across the island. This begins a process of backfilling the overwash fan, eventually creating a new ridge. These simple experiments support previous theoretical suggestions that periodic overwash is a key part of a barrier island's behavior during a transgression, and that this can lead to punctuated retreat even when RSL rise is steady.

Investigation of the steady and unsteady forces acting on a pair of circular cylinders in crossflow up to ultra-high Reynolds numbers

Measurements of the steady and unsteady forces acting on a pair of circular cylinders in crossflow are performed from subcritical up to ultra-high Reynolds numbers. The two cylinders with equal diameters d are arranged inline at two centre-to-centre distances: S/d = 2.8 and 4. The trend of the drag curve for the upstream cylinder Cd_{1}(Re) at both distances is similar to that for a single circular cylinder. The development of the drag curves Cd_{2}(Re, S/d = 2.8, 4) of the downstream cylinder is inverse to that of the upstream cylinder. For both cylinder spacing values, the drag on the downstream cylinder is negative for subcritical Reynolds numbers, increases abruptly to positive values at the beginning of the supercritical regime, and shows a significant dip at transcritical Reynolds numbers. This drag inversion indicates that the critical distance Sc decreases sharply in the supercritical Reynolds number range. For S/d = 2.8 at Rerightarrow 10^{7}, the downstream cylinder experiences once more a thrust force. The curve of the Strouhal number St(Re) of the downstream cylinder for S/d = 4 is very close to that of a single cylinder. For Reynolds numbers of Reapprox 1times10^{6} - 7times10^{6}, the Strouhal numbers have nearly equal values of Stapprox 0.22 - 0.24 for both distances. This is followed by a branching. For Rerightarrow 10^{7} and the case S/d = 2.8, the Strouhal numbers dip at St = 0.17. However, for S/d = 4, they increase up to St = 0.27. In the supercritical range, two peaks occur in the power spectra for the large distance S/d = 4. Based on a wavelet analysis, we can conclude that the low-frequency mode, which does not occur for a single cylinder, is an interference effect.Graphic abstract

Enhance the debonding resistance of hydrogel by large-scale bridging

The application of hydrogels has recently expanded markedly owning to the achievement of strong adhesion. In characterizing adhesion, a hydrogel is often subjected to 90° peel, during which the peel force increases, maximizes then drops to a plateau at steady state. The steady state peel force determines adhesion toughness. The maximum peel force determines a debonding resistance that is higher than adhesion toughness, which, however, has been largely unheeded before. This paper studies the mechanics pertaining to the maximum peel force and describes a method to enhance the debonding resistance by invoking the large-scale bridging mechanism. We achieve, by varying the bending stiffness, an increment of debonding resistance from 185 to 856 N/m for a single-network polyacrylamide hydrogel and from 486 to 2054 N/m for a double-network Ca-alginate/PAAm hydrogel on a glass substrate. The increment of debonding resistance depends on the thickness of the hydrogel and the bending stiffness of the backing. As a proof-of-concept deployment of the method, we fabricate a bilayer consisting of a passive hydrogel 2 and a responsive (PAAc/Ca(Ac)2) hydrogel 1. The PAAc/Ca(Ac)2 hydrogel is soft at 25 °C (E ~ 0.5 MPa) but stiffens dramatically at 75 °C (E ~ 100 MPa), serving as the stiff backing to elicit large-scale bridging mechanism to improve the debonding resistance by one order of magnitude. We establish a theoretical model to probe the peel behaviors based on the cohesive-zone model and solve the resultant boundary value problem numerically. Theoretical predications satisfactorily agree with experimental results. We discuss the importance of maximum peel force and the potentials of large-scale bridging mechanism in improving debonding resistance for soft materials.

Effect of Kinesin-5 Tail Domain on Motor Dynamics for Antiparallel Microtubule Sliding.

Kinesin-5 motor consists of two pairs of heads and tail domains, which are situated at the opposite ends of a common stalk. The two pairs of heads can bind to two antiparallel microtubules (MTs) and move on the two MTs independently towards the plus ends, sliding apart the two MTs, which is responsible for chromosome segregation during mitosis. Prior experimental data showed that the tails of kinesin-5 Eg5 can modulate the dynamics of single motors and are critical for multiple motors to generate high steady forces to slide apart two antiparallel MTs. To understand the molecular mechanism of the tails modulating the ability of Eg5 motors, based on our proposed model the dynamics of the single Eg5 with the tails and that without the tails moving on single MTs is studied analytically and compared. Furthermore, the dynamics of antiparallel MT sliding by multiple Eg5 motors with the tails and that without the tails is studied numerically and compared. Both the analytical results for single motors and the numerical results for multiple motors are consistent with the available experimental data.

Nonlinear dynamic response of functionally graded cylindrical microshells conveying steady viscous fluid

Nonlinear free and forced vibrations for functionally graded (FG) cylindrical microshells conveying steady viscous fluid are investigated based on the shear deformation shell and potential flow theory. The size effects are considered by modified couple stress theory. Steady viscous force produced by fluid are added to the inviscid and incompressible microshell-fluid coupled system using time-mean Navier-Stokes equations. Using Hamilton’s principle, the nonlinear partial differential equations are derived for microshell-fluid system. The reduced nonlinear ordinary differential equations are obtained based on static condensation method and Galerkin’s method. The critical flow velocity and natural frequency are here discussed by changing material composition and scale parameters. Moreover, the effects of axial load, scale parameter, fluid flow velocity and viscosity on nonlinear dynamic response are also investigated. Comparisons with corresponding inviscid case and previously literatures are also here discussed

Reducing phenotypic instabilities of a microbial population during continuous cultivation based on cell switching dynamics.

Predicting the fate of individual cells among a microbial population (i.e., growth and gene expression) remains a challenge, especially when this population is exposed to very dynamic environmental conditions, such as those encountered during continuous cultivation. Indeed, the dynamic nature of a continuous cultivation process implies the potential diversification of the microbial population resulting in genotypic and phenotypic heterogeneity. The present work focused on the induction of the arabinose operon in Escherichia coli as a model system to study this diversification process in continuous cultivations. As a preliminary step, the green fluorescent protein (GFP) level triggered by an arabinose-inducible ParaBAD promoter was tracked by flow cytometry in chemostat cultivations with glucose-arabinose co-feeding. For a wide range of glucose-arabinose co-feeding concentrations in the chemostats, the simultaneous occurrence of GFP positive and negative subpopulation was observed. In the second set of experiments, continuous cultivation was performed by adding glucose continuously and arabinose based on the capability of individual cells to switch from low GFP to high GFP expression states, performed with a technology setup called segregostat. In the segregostat cultivation mode, on-line flow cytometry analysis was used for adjusting the arabinose/glucose transitions based on the phenotypic switching profiles of the microbial population. This strategy allowed finding an appropriate arabinose pulsing frequency, leading to prolonged maintenance of the induction level with a limited increase in the phenotypic diversity for more than 60 generations. The results suggest that the steady forcing of individual cells into a given phenotypic trajectory may not be the best strategy for controlling cell populations. Instead, allowing individual cells to switch periodically around a predefined threshold seems to be a more robust strategy leading to oscillations, but within a predictable cell population behavior range.