Evaporation & Drying

• Rheological Stability of Cake Frosting:
• Bingham / Herschel-Bulkley Plastic Behavior:
• Cake frosting possesses a definite yield stress ($\tau_0$).
• Under gravitational shear stress ($\tau_g \lt \tau_0$), the frosting behaves as a rigid solid, remaining anchored to vertical cake surfaces without slumping.
• Flows smoothly only when applied knife shear exceeds its yield stress.
• Conclusion: Yield stress (Option 2).

Correct option: (2) Yield stress

• Fundamental vs Empirical Rheological Properties:
• Fundamental Rheological Properties:
• Well-defined physical quantities independent of sample geometry, instrument dimensions, or test speed (e.g., shear modulus, true viscosity, fracture stress / true failure stress).
• Empirical Texture Profile Analysis (TPA) Properties:
• Hardness, chewiness, adhesiveness, and gumminess are empirical, instrument-dependent sensory descriptors measured via two-bite force-time curves.
• Conclusion: Fracture stress (Option 4).

Correct option: (4) Fracture stress

• Mechanics of the Maxwell Viscoelastic Model:
• Consists of a Hookean spring (elasticity $E$) and a Newtonian dashpot (viscosity $\eta$) connected in series.
• Series Properties:
• Under tensile load, stress is identical across both components: $\sigma = \sigma_{\text{spring}} = \sigma_{\text{dashpot}}$.
• Strains are additive: Total strain equals the sum of elastic spring deformation and viscous dashpot flow:

$$\gamma_{\text{total}} = \gamma_{\text{elastic}} + \gamma_{\text{viscous}} \implies \frac{d\gamma}{dt} = \frac{1}{E}\frac{d\sigma}{dt} + \frac{\sigma}{\eta}$$

• Conclusion: Strains are additive (Option 1).

Correct option: (1) Strains are additive

• Ideal Hookean Elastic Solids:
• Definition:
• An ideal solid exhibits linear, instantaneous, and fully reversible Hookean elasticity ($\sigma = E \epsilon$).
• Steel as Benchmark:
• Structural steel behaves as a nearly perfect elastic solid within its proportional limit, returning completely to original dimensions upon load release.
• Conclusion: Steel (Option 2).

Correct option: (2) Steel