Elementos básicos del flujo proyectado

2.3.1 Preparation o f Polyim ide discs & E xcim er L aser A blations

Single holes w ith target diam eters o f 10pm, 15pm, 20pm and 25pm w ere drilled using a 248 nm excim er laser (Figure 8) (Exitech Ltd, O xford U K) in sample discs (n=6 at each diam eter) punched from 75pm thick polyim ide sheet (G oodfellow Ltd Cam bridge, UK). Sample discs w ere m ounted in a flow rig (Figure 9) designed to m easure the pressure drop across the discs, using filtered and degassed w ater as previously described (Prata 1995). A flow rate o f 1.4 pl/m in was used throughout. R epeated flow m easurem ents (Figure 10) were taken (n=6) for each disc. Specim en discs w ere rem oved from their holder and rem ounted betw een each m easurem ent.

Figure 8. A schematic draw ing o f the 248 nm (K rypton/Fluorine) excim er laser delivery system used (M odel EX -PS-2000, Exitech Ltd, O xford UK) (not to scale). Beam area at the target surface, and hence ablation diam eter, w as controlled by the m ask aperture. The pulse repetition rate w as set at 40H z and a total o f 100 pulses w ere delivered to each disc. Specim ens w ith target diam eters o f 10 and 15 pm were ablated at a fluence o f 200 mJ/cm^ w ith m ask diam eters o f 300 and 450 pm respectively. Those w ith target diam eters o f 20 and 25 pm w ere ablated at 170 mJ/cm^ w ith m ask diam eters o f 600 and 750 pm.

W o rk p ie c e 248 nm Excim er Laser P ro je c tio n L ens C o n d e n s e r L en s M ask H o ld e r

Figure 9. I'he tlo w rig used was similar to that previously deseribed for flow evaluation in G D D s (Prata 1995). The system eonsisted o f a pressure transdueer (Model P23ID. Gould Inc., California. USA), bridge amplifier (Model M L l lO . AD Instruments. N e w South Wales. Australia) and recorder (Model M L200, AD Instruments. N ew South Wales. Australia). Pressure changes were recorded using a Mae Lab software (v3.5 AD Instruments. N ew South Wales, Australia) on a com puter (Pertbrm a 630. Apple Ltd.. California, USA). A 1 ml hypoderm ic syringe (Fortuna. Germany) was used with an adjustable electronic syringe pum p (Model PH D 2000, Harvard Apparatus. Massachusetts. USA) to control the flow rate. The polyimide discs were held in a 13 mm stainless steel filter holder (W hatm an International, Maidstone. UK). A three-w ay lock was positioned between the water reservoir and the rest o f the system. Silicone tubing (Silicone High Strength Tubing. Altee. Alton. UK) was used throughout.

W ater Reservoir

Three-way Lock

ringe on Infusion pump

Pressure Transducer

Filter Holder

Figure 10. A typical flow tracing, dem onstrating stabilisation o f the pressure recording w ithin approxim ately 3 m inutes o f com m encing infusion.

Pressure drop (m m Hg) s D T im e in m in u tes 2.3.2 Flow R ig

The flow rig used w as sim ilar to that previously described for flow evaluation in GDDs (Porter 1997; Prata 1995). The system consisted o f a pressure transducer (M odel P23ID, G ould Inc., California, U SA), bridge am plifier (M odel M L llO , AD Instruments, N ew South W ales, A ustralia) and recorder (M odel M L200, AD Instrum ents, N ew South W ales, A ustralia). Pressure changes w ere recorded using a M acLab software (v3.5 AD Instrum ents, N ew South W ales, A ustralia) on a com puter (Perform a 630, A pple Ltd., California, USA). A 1 ml hypoderm ic syringe (Fortuna, Germany) was used w ith an adjustable electronic syringe pum p (M odel PHD 2000, Harvard A pparatus, M assachusetts, U SA ) to control the flow rate. The polyim ide discs were held in a 13 m m stainless steel filter holder (W hatm an International,

M aidstone, UK). A three-w ay lock w as positioned betw een the w ater reservoir and the rest o f the system. Silicone tubing (Silicone High Strength Tubing, A ltec, Alton, U K) w as used throughout. The transducer and the recording system w ere calibrated using a w ater m anom eter daily prior to experim entation. O nce the system w as filled w ith w ater, all gas bubbles w ere flushed out w ith the disc subm erged in w ater w ithin the filter holder. A t this point, the three-w ay lock w as turned to open the system to atm ospheric pressure and the pressure reading zeroed on the com puter. The three-way lock was then turned to obtain a closed system and infusion pum p started. The pressure reading w as allow ed to stabilise over 20 m inutes and the average reading from the final 10 m inutes was then taken as the pressure drop for the experim ent. This recording was repeated six tim es for each disc.

Pressure Drop = P1-P2

P i = back pressure in m m H g

P2= atm ospheric p ressure in m m H g

2.3.3 Scanning Electron M icroscopy & Im age A nalysis

A fter flow testing, all discs w ere coated in palladium and im aged using a scanning electron m icroscope (JEOL, A kishim a Japan). The dim ensions o f each hole w ere derived using im age analysis softw are (Scion Image, M aryland U SA ) (Figure 11).

Figure 11. 1 he inner edge o f the ablated holes viewed at 90° to the specimen surface by scanning electron microscopy were outlined on each side using image analysis software. The diam eter a perfect circle o f equivalent area was then calculated to estimate entry and exit diameters. Scale bar = 10 pm.