Question 1

Which description best defines a thixotropic fluid in mixing operations, considering both time dependence and recovery after shear?

Accepted Answer

Introduction: Thixotropy is a time-dependent rheological behavior commonly observed in structured fluids such as gels, cell broths, and polymer solutions. In bioprocessing, recognizing thixotropy is important for predicting torque trends during start-up and for interpreting off-line rheology tests that show hysteresis between ascending and descending shear ramps. Given Data / Assumptions: Mixing is performed at approximately constant shear rate over time. Fluid microstructure can break down under shear and rebuild at rest. Process conditions allow observation of recovery when agitation is reduced or stopped. Concept / Approach: Under constant shear, thixotropic fluids exhibit a gradual decline in apparent viscosity as internal structures break down. Once shear is removed, partial or full recovery of the original structure occurs, causing viscosity to rise toward its initial value. Thus, thixotropy encompasses both the time-dependent decrease during mixing and the recovery afterward, which is why the definition emphasizing change and return is most complete. Step-by-Step Solution: 1) Observe viscosity decreases with time at fixed shear, indicating breakdown.2) Cease mixing and allow rest; structure rebuilds and viscosity increases.3) Repeat cycles to demonstrate reversible time dependence distinct from mere shear-thinning.4) Plot ascending and descending curves; hysteresis loop confirms thixotropy.5) Apply in scale-up by considering start-up torque decay and rest periods. Verification / Alternative check: Time sweep tests on a rheometer at fixed shear show viscosity decay, while subsequent low-shear recovery tests demonstrate rebuild toward the initial value, validating the definition. Why Other Options Are Wrong: Constant viscosity: Describes Newtonian, not thixotropic, behavior. Progressive increase with time: That is rheopectic behavior. Decrease with time only: Captures part of thixotropy but ignores recovery, making it incomplete. Common Pitfalls: Confusing thixotropy with pseudoplasticity (which lacks time dependence) or with rheopecty (the opposite time trend). Recovery may be partial and depend on rest duration. Final Answer: Viscosity changes during mixing but returns to its original state after mixing stops

Question 2

In culturing animal cells, which factors most directly influence the sensitivity of cells to hydrodynamic shear during mixing and aeration?

Accepted Answer

Introduction: Animal cells lack rigid walls and are therefore more susceptible to damage from shear stress, bubble interfaces, and turbulent eddies than microbial cells. Identifying the factors that modulate shear sensitivity is essential for setting impeller speeds, gas flow rates, and for choosing protective additives that reduce damage without compromising productivity. Given Data / Assumptions: Suspension culture or microcarrier-based systems are considered. Mixing and aeration introduce shear and bubble-related stress. Medium composition and cell biophysics both play roles. Concept / Approach: Shear sensitivity increases with larger effective cell or aggregate size due to higher stress differentials. Nutrient sufficiency influences membrane composition and cytoskeletal integrity, indirectly affecting robustness. Shear protectants such as non-ionic surfactants reduce bubble coalescence and lower interfacial damage, acting as boundary modifiers that shield cells from harmful interfaces and transient stresses. Step-by-Step Solution: 1) Evaluate cell size and aggregate formation; larger sizes raise shear exposure.2) Ensure adequate nutrients to maintain membrane integrity and energy for repair.3) Add shear protectants (for example, Pluronic F-68) at validated concentrations.4) Optimize aeration strategy to minimize foam and bubble-induced cell damage.5) Monitor viability and product quality to confirm protection is effective. Verification / Alternative check: Comparative runs with and without protectants typically show higher viable cell density and reduced lactate dehydrogenase release when protectants are used and nutrients are sufficient. Why Other Options Are Wrong: Single-factor answers are incomplete; shear sensitivity arises from multiple interacting factors including size, nutrition, and additives. Common Pitfalls: Overdosing surfactants can affect downstream purification or product folding. Neglecting nutrient balance can mask the benefits of shear protection strategies. Final Answer: All of the above

Question 3

A fluid shows an increase in apparent viscosity the longer it is mixed at an approximately constant shear rate. Which time-dependent rheological behavior does this indicate?

Accepted Answer

Introduction: Some complex fluids change viscosity over time even when shear rate is held roughly constant. Distinguishing between thixotropy and rheopecty is important for predicting how torque and power draw evolve during prolonged mixing, and for preventing equipment overloading or insufficient mixing performance. Given Data / Assumptions: Shear rate is maintained approximately constant. Apparent viscosity increases with mixing time. No indication of elastic phenomena dominating the response. Concept / Approach: Rheopecty, sometimes called anti-thixotropy, describes the time-dependent increase in viscosity under sustained shear, implying structure builds up or orders in a way that resists flow. This is the opposite of thixotropy, where viscosity decreases with time due to structural breakdown. Newtonian fluids exhibit no time dependence, and viscoelasticity refers to recoverable elastic response, not necessarily a monotonic time trend in viscosity. Step-by-Step Solution: 1) Observe time trend: viscosity rising over time at fixed shear.2) Exclude thixotropy because that would cause a decrease, not an increase.3) Exclude Newtonian behavior because viscosity would be constant.4) Recognize this pattern as rheopectic behavior.5) Anticipate higher torque as mixing continues; adjust operating envelope accordingly. Verification / Alternative check: Rheometer time sweeps demonstrate a monotonic increase in viscosity at constant shear, and hysteresis loops may invert compared to thixotropic fluids, confirming rheopecty. Why Other Options Are Wrong: Newtonian: No time dependence. Viscoelastic: Describes elastic storage and loss behavior, not necessarily time-dependent thickening. Thixotropic: Viscosity would decrease with time, opposite of the observation. Common Pitfalls: Misidentifying rheopecty as temperature rise effects; always control temperature during tests to isolate time-dependent structure changes. Final Answer: rheopectic

Question 4

In animal cell culture, which laboratory measurement is commonly used to quantify mechanical damage to cells by detecting cytosolic enzyme release into the medium?

Accepted Answer

Introduction: During mixing and aeration, animal cells can be damaged by shear and bubble interfaces, leading to loss of membrane integrity. When membranes are compromised, intracellular enzymes leak into the culture medium. Monitoring a reliable cytosolic marker provides a rapid, quantitative assessment of cell damage in process development and scale-up trials. Given Data / Assumptions: We need a sensitive assay that correlates with membrane disruption. The assay should be compatible with standard spectrophotometric methods. Marker enzyme should be abundant in the cytosol of animal cells. Concept / Approach: Lactate dehydrogenase (LDH) is a cytosolic enzyme that rapidly appears in the extracellular medium when cell membranes are damaged. Measuring LDH activity in the supernatant using NADH-linked reactions offers a convenient indicator of mechanical injury. In contrast, bulk metabolites like lactate reflect metabolism rather than direct lysis, and unrelated enzymes such as lactase or lactate oxidase are not appropriate markers for animal cells. Step-by-Step Solution: 1) Choose a leakage marker located in the cytosol and abundant: LDH.2) After agitation or aeration stress, sample the medium and remove cells by centrifugation or filtration.3) Measure LDH activity via decrease in NADH absorbance at 340 nm.4) Compare against control cultures without shear stress.5) Interpret higher LDH activity as increased cell damage. Verification / Alternative check: Parallel viability assays (for example, trypan blue exclusion) and microscopic observations corroborate LDH leakage results, establishing a consistent picture of shear-induced damage. Why Other Options Are Wrong: Lactate concentration: Reflects metabolic state, not membrane rupture directly. Lactate oxidase activity: Not a standard animal cell leakage marker. Lactase activity: Enzyme not typically relevant in animal cell cytosol. Common Pitfalls: Interpreting LDH without accounting for spontaneous release during normal turnover; include controls. Hemolysis of feeder erythrocytes or serum interference can confound readings; standardize sampling. Final Answer: Activity of lactate dehydrogenase in the medium

Question 5

In bioprocess hydrodynamics, why is the Kolmogorov microscale (eddy size) important when considering shear exposure of cells or particles?

Accepted Answer

Introduction: Turbulent flows contain a spectrum of eddy sizes that transfer energy from large to progressively smaller scales. The Kolmogorov microscale marks the lower end of this cascade where viscous forces dominate and kinetic energy is dissipated. For shear-sensitive cells or particles, comparing their size to this smallest eddy provides a practical criterion for anticipating mechanical stress and potential damage. Given Data / Assumptions: Turbulence exists in the reactor due to agitation and/or aeration. Cells or particles have a characteristic diameter that can be estimated. Energy dissipation rate influences the Kolmogorov scale. Concept / Approach: If cell or particle sizes approach the Kolmogorov microscale, they experience steep velocity gradients across their bodies, raising shear and normal stresses. This can rupture membranes, detach cells from carriers, or fragment fungal hyphae. Understanding this scale helps in setting impeller speed, selecting spargers, and deciding whether low-shear reactors (for example, airlifts) are preferable. Step-by-Step Solution: 1) Estimate energy dissipation rate from power input and volume.2) Compute Kolmogorov scale qualitatively; smaller scale implies harsher microeddies.3) Compare cell or particle diameter with this scale.4) If size ≥ microscale, anticipate higher shear exposure and consider mitigation.5) Adjust operating conditions to increase microscale (lower dissipation) or add shear protectants. Verification / Alternative check: Empirical observation of damage thresholds often correlates with calculated microscale trends across scales, supporting the practical value of this comparison in design and troubleshooting. Why Other Options Are Wrong: Each individual statement is correct but incomplete alone; the comprehensive answer is that all apply. Common Pitfalls: Using only bulk shear rate as a proxy; turbulent microeddies determine local stress. Ignoring aeration effects can underestimate dissipation near gas dispersion zones. Final Answer: All of the above

Question 6

In a stirred-tank bioreactor, compute the impeller Reynolds number for the following operating conditions (use SI units and standard mixing definition Re = (rho * N * D^2) / mu):\r \r Fluid density = 2.5 g/mL (i.e., 2500 kg/m^3)\r Dynamic viscosity …

Accepted Answer

Introduction: The impeller Reynolds number is a key dimensionless group in mixing that indicates the flow regime (laminar, transitional, or fully turbulent). Correctly calculating Re from density, viscosity, impeller size, and rotational speed helps decide correlations for power number, mass transfer, and scale-up in bioprocess engineering. Given Data / Assumptions: rho (fluid density) = 2.5 g/mL = 2500 kg/m^3. mu (dynamic viscosity) = 0.002 kg/(m·s). D (impeller diameter) = 50 cm = 0.50 m. N (rotational speed) = 120 rpm = 2.0 s^-1. Re definition for stirred tanks: Re = (rho * N * D^2) / mu. Concept / Approach: For agitated vessels, the Reynolds number incorporates rotational speed and impeller diameter squared. Unit consistency is essential: convert g/mL to kg/m^3, cm to m, and rpm to s^-1 before substitution. Step-by-Step Solution: Step 1: Convert units: rho = 2500 kg/m^3; D = 0.50 m; N = 120 rpm = 120/60 = 2.0 s^-1; mu = 0.002 kg/(m·s).Step 2: Apply Re = (rho * N * D^2) / mu.Step 3: Compute D^2 = 0.50^2 = 0.25 m^2.Step 4: Numerator = 2500 * 2.0 * 0.25 = 1250.Step 5: Re = 1250 / 0.002 = 625000. Verification / Alternative check: Magnitude check: with high density and moderate viscosity, Re in the 10^5–10^6 range is expected for a 0.5 m impeller at 120 rpm, confirming turbulent flow. Why Other Options Are Wrong: 2500: Off by two orders of magnitude; ignores D^2 and/or unit conversions. 1000000 and 1250000: Overestimates from arithmetic/rounding errors. 100: Severe underestimate; inconsistent with dimensions and speed. Common Pitfalls: Mixing units (rpm vs s^-1), forgetting to square D, or using kinematic viscosity instead of dynamic viscosity without proper conversion (nu = mu/rho). Final Answer: 625000

Question 7

In animal cell culture, which of the following is commonly used as a shear protectorant to reduce interfacial damage during aeration and agitation?

Accepted Answer

Introduction: Shear and interfacial stresses from bubbles and agitation can damage fragile animal cells. Shear protectants mitigate cell adsorption at gas–liquid interfaces and cushion membranes during collisions, thereby improving viability and productivity in bioreactors. Given Data / Assumptions: Cells are shear-sensitive and lack rigid walls. Bioreactors may use sparging and mechanical agitation. Additives can protect cells by modifying interfaces and medium properties. Concept / Approach: Serum contains proteins (e.g., albumin) that preferentially occupy interfaces, reducing direct cell contact with bubbles and impeller-generated eddies. This lowers interfacial tension effects and dampens mechanical damage. Other specialized protectants include poloxamers (e.g., Pluronic F-68), but among the listed options, serum is the well-established choice. Step-by-Step Solution: Step 1: Identify agents known to shield cells at interfaces (proteins, certain surfactants, poloxamers).Step 2: Recognize serum proteins adsorb at interfaces first, protecting cells.Step 3: Compare with other listed compounds and select the established shear protectant. Verification / Alternative check: Culture media formulations historically included fetal bovine serum to enhance robustness under agitation; serum-free processes often replace it with Pluronic F-68 for similar protection. Why Other Options Are Wrong: Silicone oil: Used as antifoam; not a standard cellular shear protectant and can hinder downstream processing. Glucose: Carbon source; no interfacial protection role. Glutamic acid: Amino acid/nutrient, not a shear protectant. Polysorbate 20: A surfactant used in formulations, but not the conventional choice for bioreactor shear protection of cells. Common Pitfalls: Assuming any surfactant or antifoam equals a shear protectant; process additives must be biocompatible and downstream-compatible. Final Answer: Serum

Fluid Flow Questions