Science of Medicines Flashcards

Question

Describe the flow curve of dilatant (shear thickening) fluids?

Answer 1

(Stress (σ) vs. Shear Rate (γ̇): The curve starts at the origin and rises non-linearly, curving upwards more steeply at higher shear rates Viscosity (η) vs. Shear rate (γ̇) Upwards curve which increases with increasing γ̇ (half of a U shape)

Answer 2

At rest (or low shear rates) particles are loosely packed/lubricated by fluid When shear stress is applied, particles are forced closer together and cannot rearrange easily + decreased lubrication This leads to crowding, the formation of temporary particle clusters and collisions which resist flow. As a result, the viscosity increases with increasing shear rate

Answer 3

Behave like a solid, until a certain minimum force (the yield stress) is applied Once this yield stress is exceeded, the fluid flows like a pseudoplastic or Newtonian fluid, depending on the material E.g. toothpaste, mayonaise

Answer 4

How it behaves at rest High η₀ = better resistance to particle settling → greater physical stability in suspensions High η₀ = thicker, more structured feel at rest (e.g. thick creams that don’t drip easily) → affects ease of scooping/spreading and consumer perception High η₀ (thick at rest): * Stable at rest – resists sedimentation and phase separation * Structured feel – good for creams, pastes and suspensions * May need more force to pour or spread – useful in products that should stay in place * Applications = rich moisturising creams, suspensions, syrups Low η₀ (thin at rest): * Less stable – higher risk of sedimentation or creaming * More runny or pourable – ideal for products requiring easy flow * Feels lighter – suitable for lotions * Applications = light lotions, oral syrups, thin emulsions

Answer 5

Describes the fluid's behaviour at high shear rates High η∞ = high resistance to flow at high shear: * Strong internal structure – resistant to flow under high shear, indicates strong interactions between particles/molecules * Stable under mechanical stress - useful for products that need to maintain form under stress (e.g. during mixing, spraying or handling) * Applications = thick pastes, gels Low η∞ = low resistance to flow at high shear * Easily deforms or spreads under mechanical stress * Useful for easy spreading and handling in applications requiring fluidity * Applications = lotions, thin oils

Answer 6

The threshold stress that must be exceeded for the fluid to start flowing High σᵧ: * Requires significant force to initiate flow – fluid remains solid-like until yield stress is exceeded * Resistant to deformation – doesn’t flow easily under low shear. * Useful for products that need to stay in place without spreading or dripping (e.g. thick creams or pastes). * Applications = toothpastes, gels Low σᵧ: * Easier to initiate flow – fluid flows under lower force or shear * Can spread or pour easily with little force * Applications = lotions

Answer 7

Oil/Water ratio * Affects the internal structure + molecular packing * More oil droplets = higher viscosity Presence of additional excipients (e.g. polymers, preservatives, solvents) * Solvents may reduce viscosity * Polymers may increase viscosity Nature of the oil phase Nature + quantity of emulsifier * Excessive emulsifier can lead to over-stabilisation, where the emulsion becomes too viscous and gel-like Temperature * Viscosity decreases with increasing temperature

Answer 8

High volume fractions Particle size Particle size distribution Electrostatic interactions Particle shape Steric Hindrance

Answer 9

As the volume fraction of particles increases, interparticle interactions become more significant This leads to increased viscosity due to particle crowding and resistance to flow

Answer 10

Smaller particles have a larger surface area relative to their volume, which can lead to stronger interactions and higher viscosity Larger particles might lead to lower viscosity but could cause shear thinning due to their ability to slide past each other more easily at high shear rates

Answer 11

Narrow particle size distribution (more uniform size) leads to more ordered arrangements and can result in higher viscosity and better stability Broad particle size distribution can lead to packing inefficiencies, resulting in lower viscosity and potentially less stability due to larger particles destabilising the system

Answer 12

Electrostatic repulsion between particles (e.g. due to surface charge) can reduce aggregation and increase stability This leads to a lower viscosity in some cases, as particles are prevented from forming aggregates that could increase resistance to flow However, if repulsive forces are weak, particles may flocculate, increasing viscosity

Answer 13

Non-spherical particles tend to increase viscosity more than spherical particles due to increased friction and anisotropic interactions Irregular shapes can cause more complex packing and increase resistance to flow, leading to higher viscosity

Answer 14

Polymeric stabilisers or adsorbed surfactants can create a protective layer around particles, preventing aggregation Steric hindrance from these layers increases viscosity by making it harder for particles to move relative to each other

Answer 15

Rotational Rheometers Measure how a fluid responds to applied shear by rotating one surface against another A sample is placed between two surfaces – typically: * Cone and plate * Parallel plates * Coaxial cylinders The instrument applies either: * A controlled shear rate (measures resulting stress) * A controlled shear stress (measures resulting shear rate) From this, it calculates: * Viscosity (η = σ / γ̇) * Yield stress * Flow curves * Viscoelasticity

Answer 16

Thixotropy - viscosity decreases with time under constant shear stress Rheopecty - viscosity increases with time under constant shear stress

Answer 17

1. Apply a constant shear rate for a set time and measure viscosity over time 2. Increase and then decrease shear rate gradually while recording shear stress or viscosity 3. If the curves do not coincide/overlap and there is a loop present (area between up and down curves) it indicates time-dependent behaviour

Answer 18

The property of a material that exhibits both viscous behaviour (like a liquid – flows and dissipates energy) and elastic behaviour (like a solid – deforms and stores energy) A viscoelastic material deforms like a liquid but also resists flow and recovers like a solid

Answer 19

If stress is high for an infinitesimally short time, no flow occurs - the material behaves like a solid If stress is very small over a long time, it flows - the material behaves like a liquid Fast deformation → more elastic behaviour Slow deformation → more viscous behaviour

Answer 20

Creep test: Studying the resultant displacement (strain) of a material after application of a small constant stress for a determined amount of time Oscillatory measurements: * Apply sinusoidal stress and measure response in strain * The response is mathematically treated to give elastic and viscous components

Answer 21

Contamination Chemical instability Impurities Toxic adducts Physical instability (poor dosing reproducibility)

Answer 22

Aggregation + Coagulation + Flocculation * Clumping of colloidal particles due to attractive forces * Solutions - increasing repulsive forces through surface charge (e.g. surfactants) or steric hindrance (e.g. polymers) Sedimentation * Particles settle due to gravity * Solutions - reducing particle size, increasing viscosity of the medium, suspending agents

Answer 23

D = kT ÷ 6π ηa Where: * k = Boltzmann constant * T = Temperature * η = Viscosity * a = Particle radius

Answer 24

Decreasing particle size: * Greater Brownian motion → improved dispersion → resists sedimentation * Increased surface area → higher surface energy → increased risk of aggregation

Answer 25

Increasing temperature: * Increased Brownian motion * In short term: improved dispersion → greater stability * In long term: breakdown of stabilisers (e.g. surfactants/polymers) → aggregation/coalescence → instability * Reduced viscosity of medium → can increase sedimentation

Answer 26

Decreasing viscosity: * Less resistance to gravity and motion → increased sedimentation rate (Stokes’ Law) * Faster particle movement → higher collision frequency → risk of aggregation

Answer 27

Increasing electrical charge: * Stronger electrostatic repulsion between particles → resists aggregation → improved stability * Larger energy barrier (DLVO theory) → less aggregation * High zeta potential → improved stability * Excessive charge → charge screening + sudden flocculation

Answer 28

DLVO theory considers two opposing forces between colloidal particles: * Attractive van der Waals forces → promote aggregation * Repulsive electrostatic forces (from surface charge) → prevent aggregation The balance between these determines whether particles remain dispersed or aggregate: * A high energy barrier (due to strong repulsion) → stable colloid * A low or no energy barrier → particles fall into the primary minimum → irreversible aggregation

Answer 29

A two-phase system where solid particles (dispersed phase) are dispersed in a liquid (continuous phase) The particles are not soluble and are >1 µm in size

Answer 30

Colloids have smaller particles (<1 µm) that remain dispersed without settling, while suspensions have larger particles (>1 µm) that may settle over time due to gravity

Answer 31

Uniform particle size distribution Homogeneous during dosing Proper viscosity Slow and uniform sedimentation Easily redispersible/to re-suspend upon shaking Physically and chemically stable Palatable for patients (pleasant appearance + acceptable taste)

Answer 32

Particle size and shape Density difference between solid and liquid Viscosity of the dispersion medium Zeta potential (surface charge) Presence of flocculating or deflocculating agents

Answer 33

Sedimentation - settling of particles over time Creaming - upward movement of dispersed particles (less dense than medium) Caking - hard, compact sediment that is difficult to redisperse Flocculation - loose aggregation of particles Ostwald ripening - growth of large particles at the expense of smaller ones due to differences in solubility

Answer 34

Flocculation is the reversible aggregation of particles into loose clusters. Solutions: * Adjusting particle size * Addition of electrolytes * Addition of flocculating agents (e.g. surfactants, polymers) * Optimising viscosity * Optimising pH

Answer 35

The attractive van der Waals forces between particles overcome the repulsive electrostatic forces (secondary minimum). This leads to the formation of loose, reversible aggregates (flocs) that can settle faster but are easily redispersed.

Answer 36

Low concentrations: * Large diffuse layer * High primary maximum (strong repulsion) * No secondary minimum → stable, deflocculated suspension Intermediate concentrations: * Partial compression of diffuse layer * Lower primary maximum * Appearance of secondary minimum * Promotes flocculation (reversible aggregation) High concentrations: * Complete compression of diffuse layer → reducing zeta potential * No primary maximum * Particles approach closely → leads to irreversible aggregation or caking → unstable suspension

Answer 37

Stokes' Law describes sedimentation rate: v = 2a^2g (σ - ρ) ÷ 9η Where: * v = sedimentation velocity * a = particle radius * σ = density of particles/dispersed phase * ρ = density of medium/continuous phase * η = viscosity * g = gravity

Answer 38

Creaming occurs when particles are less dense than the medium and move upward Caking occurs when particles settle down and form a dense, compact sediment that’s hard to redisperse

Answer 39

Smaller particles move more due to Brownian motion → Less time in contact → Less chance to form flocs They settle more slowly → Fewer collisions at the bottom → Reduces floc formation Higher surface charge (zeta potential) → Stronger repulsion between particles → Prevents particles from clumping into flocs

Answer 40

Electrolytes - reduce zeta potential, encouraging floc formation Polymers - increase viscosity + create bridges between particles (steric mechanism) Surfactants - alter particle surface charges and promote flocculation

Answer 41

Surfactants that lower the surface tension between the solid particles and the liquid medium, helping the liquid spread over and wet the solid This improves dispersion and prevents clumping of hydrophobic powders

Science of Medicines Flashcards

(67 cards)