Filament stretching

Another type of experiment involves a fluid filament being drawn upward against gravity from a reservoir of the fluid (101,213,214), a phenomenon often called the tubeless siphon. The maximum height of the siphon is a measure of the spinnabiUty and extensional viscosity of the fluid. Mote quantitative measures of stress, strain, and strain rate can be determined from the pressure difference and filament diameter. A more recent filament stretching device ia which the specimen is held between two disks that move apart allows measurements ia low viscosity Hquids (215). AH of these methods are limited to spinnable fluids under small total strains and strain rates. High strain rates tend to break the column or filament. 

Polyglycolic acid is a homopolymer of glycolide and was the initial synthetic suture offered for sale under the trade name of Dexon. It is formed into monofilaments from the melt with the filaments stretched as they are formed to assist in aligning the polymer chain, thus increasing the filament strength. Polyglycolic acid (structure 19.33) is also braided into sutures. 

The techniques used in the Ml project were those of fibre spinning, open siphon, filament stretching, contraction flows, converging channel, opposing jets, climbing constants and falling drop methods. The results led to two major findings ... 

One major consequence of the Ml project was the development of a modified filament stretching instrument by Sridhar. In this device, the test sample is held horizontally between two Teflon discs and pulled equally at both ends at a programmable exponential rate such that a constant strain rate is achieved and the stress growth at a constant stretch rate is obtained (40). It appears though that the test sample has to adhere to the plates as the technique does not use aids to clamp samples. Consequently, it is not clear if the technique can be applied to products that are non-sticky or exhibit slip, which could be limiting factors for testing food products. 

The semisolid consistency and the occurrence of slip effects during flow could limit the use of the rotary clamp method, the conveigent flow method or even Sridhar s extensional rheometer for a large number of food products such as mayonnaises and salad dressings. A novel rheometer (15) based on the concept of filament stretching has been developed by Plucinski et al (Fig. 10) to measure the extensional properties... 

The filament stretching device could play a new role in rheometry by providing a new direction for studying substances that exhibit severe slip effects during conventional rotational rheometry. 

DNA synthesis. Mechanochemical transduction in VSMCs is dependent on intact actin filaments. Stretch responses involve Rho activation and Rho/pl60ROCK and mediate stretch-induced ERK activation and cellular vascular hyperplasia in VSMCs. 

Ion lifetimes of the order of nanoseconds have also been measured in experiments which confined the detectable ionization to the region of a filament stretched along the axis of a cylinder [468, 469, 470, 815]. Electron impact ionization was employed and decomposition occurred within the strong field between filament and cylinder. Ethylene, ethane and hexane were among molecules studied. 

Filament stretching and capillary breakup rheometers are two experimental instruments used to impose uniaxial extension to fluids [34-39], In both of these devices a fluid is placed between two surfaces or platens, and the spacing between the platens holding the sample is increased, as shown in Figure 4.16. 

Another simple filament-stretching technique which has the advantage that it does not require measurements of forces acting on the filament, involves... 

Filament stretching has been studied for obtaining extensional properties of polymeric solutions and melts. A comprehensive review of the flow dynamics of filament stretching is provided by McKinley and Sridhar [9]. Subsequently, liquid filament rheometers or capillary breakup extensional rheometers (CaBER) has been developed for the characterization of the dynamics of complex fluids undergoing extensional flows. 

McKineley GH, Sridhar T (2002) Filament-stretching rheometry of complex fluids. Annu Rev Fluid Mech 34 375 15... 

Tirtaatmadja V, Sridhar T (1993) A filament stretching device for measurement of extensional viscosity. J Rheol 37 1081-1102... 

Isotropic model filaments after having been subjected to a permanent extension represent orientated systems. They show anisotropy of swelling, optical anisotropy and their X-ray diagram reveals a preferred orientation of the crystallites. At maximum extension a high degree of orientation may be reached. It is highest in xanthate filaments and lowest in filaments stretched in the air-dry state. 

Recently Hermans et aP have shown by quantitative X-ray analysis of cellulose filaments (stretched in the swollen state to various extents) that very little if any change in orientation occurs on drying, thus proving conclusively that the process... 

Glass fibre and other polymer filters have a uniform felt type of internal stiucture. Modem PTFE filters also resemble a felt, with tihin filaments stretching between modes of PTFE, see Figure 6.18,... 

The force against which the work of filament-stretching is done is r eAf, where e is stretch rate, (l//)d//dr, and Af is cross-section area. This product will be a constant over the length of the fiber. In steady-state plastic deformation, at any instant, eAf exactly balances the applied force on the filament ... 

Non-Newtonian Fluids in Microfluidics, Figure 2 Schematic diagram of two simple flow types, (a) Simple shear flow between parallel rigid surfaces with gap h. (b) Uniaxial elongational flow in a cylindrical fluid filament stretching along its axis z... 

Jones DM, Walters K, WilUams PR (1987) The Extensional Viscosity of Mobile Polymer Solutions. Rheol Acta 26 20-30 McKineley GH, Sridhar T (2002) Filament-Stretching Rheome-try of Complex Huids. Annu Rev Huid Mech 34 375-415 Li L, Larson RG, Sridhar T (2000) Brownian Dynamics Simulation of Dilute Polystyrene Solutions. J Rheol 4 291-322 Gupta RK, Nguyen DA, Sridhar T (1991) Extensional Viscosity of Dilute Polystyrene Solutions Effect of Concentration and Molecular Weight. Phys Huid 12 1296-1318... 

S.H. Spiegelberg, D.C. Abies and G.H. McKinley, The role of end effects on measurements of extensional viscosity in filament stretching rheometers, J. Non-Newtonian Fluid Mech. 64, 229-267 (1996). 

Figure 3.7 Schematic diagram showing the principal features of the filament stretching method for extensional viscosity measurements (a) vertical filament stretching (b) horizontal filament stretching.

There are other variations of the filament stretching technique. For example, filaments are clamped at one end and taken up on a rotating roll [86,87]. This reduces the amoimt of filament stretching to a more uniform level and produces a more constant extensional rate. In fact, when the following filament is taken up on a cold roll [87] a better constancy in the extensional rate is obtained. 

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