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DNA quiz 5
| Question | Answer |
|---|---|
| The clothing examiner MUST have some knowledge of | the structure of different kinds of textile fabrics such as weaves, knits and non-woven's, and also an understanding of their mechanical and physical properties, such as elasticity |
| What is a fibre | Unit of matter, either natural or manufactured that forms the basic element of fabrics and textile structures |
| what is a staple fibre | A Staple Fibre is a fibre of defined short length, Essentially all natural fibres are staple fibres |
| the only non staple natural fibre is | silk |
| what is a filament fibre | A filament fibre is a fibre of indefinite length (length is determined when cut). Essentially all man-made fibres are filament fibres |
| natural fibres | Protein (wool, silk, exotics) • Cellulose (cotton, flax, hemp, jute |
| protein fibres | Protein (wool, silk, exotics) • Wool is valuable for BPA examination yet is less than 5% of world consumption |
| cellulose fibres | Cellulose Fibres (cotton, flax, hemp, jute) • Cotton is the most important for BPA examination |
| mineral fibres | Steel • Glass • Asbestos • Carbon |
| regenerated | Viscose Rayon • Bamboo, Spruce, Sugar Cane • Anything that grows fast |
| synthetic | Nylon • Polyester • Polyurethane • Micro-fibres • Fibres • Acrylics • Polyolefin |
| The word "Textile" comes from the Latin word | "texere" and means to weave |
| A Fabric is a | manufactured assembly of fibres and/or yarns that has substantial surface area in relation to its thickness and sufficient inherent cohesion to give the assembly mechanical strength |
| WARP refers to | fibres that run vertically within the fabric |
| WEFT refers to | the fibres that run across the fabric |
| SELVEDGE refers to | the reinforced edge of the fabric to prevent fray |
| BIAS refers to | the 45° line across the fabric relative to the Selvedge and the relative weave provides mechanical stretch to the fabric |
| Hydrophobic fabrics such as polyester, acrylic and nylon have | poor absorbency and are, therefore , stable |
| Hydrophilic fabrics such as wool, cotton and linen | attract moisture and elongate. These fabrics may become unstable, and puddle, sag, or ripple once exposed to water |
| Fabrics properties affect the resultant appearance of bloodstains | Texture Construction Composition Absorption |
| Fabrics are mostly | woven or knitted • Fabrics are made of yarns |
| Yarns are made of | fibres |
| knitted fabric Wales: | Run vertically up the fabric. Each stitch is suspended from the next |
| Courses: | Run horizontally across the fabric. i.e., the path of the yarn through the fabric |
| Gauge: | The number of knitting stitches per inch |
| absorption | The process of gases or liquids being taken up into the pores of a fibre, yarn or fabric |
| Fabrics will absorb blood | but not all fibres are absorbent |
| Many synthetic fibres, particularly 100% polyester, | have little or no absorbency at all. |
| Cotton can absorb | about 25 times it's weight in water |
| Blood flow is influenced by: | Fabric construction Yarn structure & thickness Fibre type |
| Natural fibres | absorb and inhibit blood flow |
| Synthetic fibres | adsorb and promote blood flow |
| How do bloodstains form | When blood comes into contact with a textile a series of physical interactions occur |
| Wetting | blood coats the textile surface |
| Capillary action: | blood penetrates into the textile |
| Wicking: | blood migrates through the textile |
| There are four wetting mechanisms: | Immersion Adhesion Spreading Capillary Penetration |
| Immersion | Occurs when a textile is saturated by a volume of blood large enough to completely submerge part of, or the entire textile |
| One surface of the textile initially contacts a significant volume of blood and as the contact increases, the textile becomes saturated with blood | • E.g. Saturation Stains |
| Adhesion | Defines the attraction between two dissimilar surfaces that are in contact with each other, Dependent on the physical and chemical properties of each contacting surface |
| A blood droplet impacting a porous textile, e.g. a cotton T-shirt, | will penetrate and share strong adhesion to form a spatter bloodstain |
| A blood droplet impacting a non-porous surface | will not penetrate and has poor adhesion |
| spreading | The flow of blood over a textile surface is regarded as a spreading mechanism |
| Spreading is dependent on | blood volume and generally, occurs with larger volumes of blood |
| spreading | Influenced by fabric treatments, surface tension\ • E.g. Flow Patterns |
| capillary penetration | The movement of blood into and along the small capillary spaces between fibres |
| capillary penetration | Wets the fibre surface which allows blood to wick through the textile. • The narrow spaces within a textile act as ductwork. • Blood moves within the spaces of a textile due to the forces of adhesion, cohesion, and surface tension |
| wicking | Describes the overall flow of the blood through the textile substrate facilitated by capillary action |
| Longitudinal wicking | - the flow or spread of blood through the length and width of a textile |
| Transverse wicking | - the flow or spread of blood through the thickness of the textile. |
| wicking | Occurs immediately after wetting • Occurs in both the longitudinal and transverse planes of a textile |
| wicking | As blood wicks between yarns and fibres within the yarns, it will fill the pore spaces between yarns as it migrates through the textile |
| Preferential Diffusion | Wicking facilitates the preferential flow of blood along the yarns away from the centre of the forming stain Blood preferentially wicks along and in between fibres within yarns rather than across the yarns, i.e., across the weave or knit |
| Preferential Diffusion | Most noticeable in fabrics made of long filament fibres, such as synthetics, synthetic blends and silks Can result in stain distortion |
| Classifying Spatter (Macroscopic Features) | • Size • Shape • Uniform perimeter • Satellite spatter • Spines • Even penetration into the fabric |
| The presence of rim features is useful, | but not diagnostic |
| Tiny satellite spatter is often generated when spatter impacts a surface | when spatter impacts a surface On fabrics, these can adhere to adjacent fibres and threads May appear as a globule encasing the thread or fibre |
| Satellite spatter can help indicate which | side the stain was deposited on • Can help differentiate between transfer and spatter • They are not always present on fabrics |
| Other Features to aid Classifying Spatter | Coated fibres around the stain perimeter - Caused by droplet retraction |
| Other Features to aid Classifying Spatter | Hollowed spatter - indicates wet blood was deposited and has dried |
| coated fibres | May be observed in some transfer stains - More common in lateral transfers - More common in fabrics comprised of staple fibres - Less common in fabrics comprised of filament fibres |
| problematic issues with Micro Satellites | - Not always present around an individual spatter stain - Often present in a spatter pattern - No guarantee they will be present |
| problem issues with Coated Fibres | Not always present around a individual transfer stain - They are present in or adjacent to a group of transfer stains - No guarantee they will be present |
| Where micro satellites or coated fibres are not present around a stain, | the stain can be difficult to classify |
| Fibre type and fabric construction | will influence the final appearance of the stain and its diagnostic features |
| spatter | mirco satellites created stains hollow spheres |
| transfers | coated firbres upper surface deposition |
| liquids and gases are | both fluids |
| Liquids: | molecules close together, experience attractive force |
| Gases: | molecules far apart, experience mild attractive forces |
| Fluid dynamics = | The study of forces on and in fluids |
| Body Forces: | Act throughout the element. Tend to accelerate fluid elements (e.g., Weight) |
| Normal Forces: | Act at right angles to surfaces. Tend to accelerate or compress fluid elements (e.g., Pressure) |
| Shear Forces: | Act parallel to surfaces. Tend to accelerate or deform fluid elements (e.g., Viscosity) |
| First Law (Inertia) | An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force |
| Second law (Force) | The acceleration of an object depends on the mass of the object and the amount of force applied. |
| Third law (Action & Reaction) | Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. |
| Surface tension occurs only at the interface where a liquid meets another substance, such as: | • Liquid-liquid interfaces • Gas-liquid interfaces • Solid-liquid interfaces |
| When a droplet forms, surface tension causes it to oscillate, stretching and compressing slightly. | Viscosity resists this movement, gradually damping out the oscillations and stabilizing the droplet into its final shape |
| cast off | Rather than gravity accelerating the blood away from the object, an external force accelerates the object away from the blood |
| cast off | Surface tension and viscosity play a similar role in cast-off as in passive dripping, causing the liquid to break into ligaments and droplets |
| cast off Sequence: | Sheet → Ligaments → Isolated Ligament → Drop |
| jet breakup | The process that occurs in an arterial spurt, where surface tension waves create bulges along the jet stream. These bulges then break apart into droplets. |
| sheet breakup | As waves travel along the sheet, they form ligaments. These ligaments then break into bulges, which evolve into droplets of varying sizes. |
| The amount of energy used to break up a liquid affects how big or small the droplets are. | More energy means a larger surface area, which creates more droplets, but each one will be smaller in size. When energy is added, it increases the energy of the molecules on the surface, making it easier to break the liquid apart |
| impact to a blood source | 1. Upon impact, the blood sheet expands. 2. Surface tension and viscosity resist expansion, leading to: 3. Sheet → Ligament → Droplet formation |
| gunshot spatter | 1. Energy is transferred to the blood via inertial force when the projectile strikes. 2. Viscous force results from the bullet moving through the blood. 3. Higher kinetic energy density results in smaller droplet size. |
| Viscous forces always act | against motion |
| Drag force is | the resistance force exerted by a fluid (such as air or water) against an object moving through it. It slows the object down as it moves |
| Three types of drag: | • Skin friction drag, due to viscous force • Pressure (form) drag, due to inertial force • Induced drag (acts on bodies which produce lift: not relevant in BPA) |
| Skin Friction Drag | When a blood droplet is in flight, skin friction drag refers to the resistance it encounters as it moves through the air. As the droplet travels, the air closest to its surface sticks slightly due to friction, which slows down its movement |
| droplet trajectory | Once in flight droplet trajectory is determined only by weight and drag forces, can be calculated if the initial droplet velocity, size, wind speed and wind direction are known In BPA this prediction is difficult because size and velocity aren’t known |
| The Weber number and Reynolds number are both used to | describe the behavior of fluids, but they focus on different factors. |
| the Weber number | tells us about the stability of a liquid's surface in a moving fluid |
| the Reynolds number | tells us about the type of flow the fluid will have |
Created by:
skyep25