Teflon Plug Gradient Coils � R.F. Coil Platinum Electrode
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4mm o.d. NMR TubeFigure :3 .3: Schematic arrangement of the electroosmotic cell in the N M R probe.
field of view and
1282
pixels giving a spatial resolution of78
pm per pixel. Because the velocities to be meas u red were on the order of tens of m icrons per seco n d , i t was necessary to maximise velocity sensitivity i n the face of molecu lar B rownian m otio n . This \vas ach ieved by maximising the separation time, d, be tween the P G S E p u lse pair since B rownian d isplacements i n crease asVii
w hereas clisplacements cl ue to flow increase as d . The u pper l i m i t to d is determined by signal loss d u e to spin relaxation. By using the sti m u lated ech o sequence s hown i n figure 3 . 4 t h e spins were su bjected to T1 rat her than 12 relaxation over m ost of the separation time.In order to obtai n the dependence of the velocity profile o n delay time, t, the time '11 between the start of the EFP p ulse and the start of the soft r J. p ulse was varied between 25 ms and
1000
ms in a set of separate experiments. The short time delay between the rJ. p ulse and the fi rst PGSE pulse was added to '11 i n order to p rovide an estimate of the time delay relevant to the velocity meas u rement. Because of the fi n ite time needed to encode for velocity. it was desirable to keep t h is d u r ation d smaller t h a n '11 in order to o ptimise time resolution . As d was varied it was necessary to adj ust the du ration and maxim u m am plitude of the PGSE pu lses so as to retain optimal velocity sensitivity.Because the electroosmotic n�locities are so smal l . it is i m portant t o establish that measn rements a re free from artifacts ca used . for exam ple, b�' edd y c u rrent induced phase shifts. This \Vas done b�' obtaining a velocit�· image \vi thout the
EFP p ulse for each of the pu lse pa ramet er sets used in t his wor k . The resid u a l apparent velocity i mages, which represented only it \vea k pert u rb ation close to
the noise background . were nOllet heless s u bt racted from the i mages o btained with EFP pulses i n order to correct for any ba.ckgro u nd artifacts.
3. 1. H ES ULTS .5 1 EFP Pul se start acquisition L f. time c, read
c.y
phase s l iceF i g u re 3 . 4 : The stim u lated echo pulse seq uence used to phase-encode the i mage for velocity. The E F P p u lse is switched off s u bsequent to velocity encoding but prior to sign a l acquisitio n . N ote t h a t T) defines t h e ti me d e l a y between t h e start o f t h e E F P pulse a n d the start o f t h e
soft r .f. p u l se . A fu rther delay o f 5.3 ms occurs before the start o f velocity encodi n g .
3 . 4 Results
Figun� 3 . .5 shows a set o f velocity i mages o btained for a t ransverse section o f t h e capil l ary a t s u ccessively i nc reas i n g valu es of delay time, TI . These i mages demonstrate a sym metric , well-behaved velocity d istri b u tion for val u es of Tl < 500
ms, b u t for T1 >
.500
ms there is clear evidence of asym metric flow in which cel l ularseparation of co u n terflmving fl u id is apparen t . possibly d ue to convective effects. T h is w i l l be discussed in t he next sectio n , and in c hapter 8 we will h ave a c loser look at t h e phenomenon o f convection . vVe s h al l therefore con fi ne Oll r q u an t itative an alysis o f velocit:., profi les here to the short time regi me. I n order to com pare o u r res u l ts w i t h t h eoretical pred i ctions, we h ave plotted a seq uence of d i ametral profi les o f wdocity in figure :3 .6. I n each case two theoretical p rofi les are shown i n w hich t h e delay t i m e t i s spt t o the s t art and fi n ish ti mes o f t h e velocity encodi n g PGSE gradient p a i r res pectivel,\', wo uld hope t h at t h e experimen ta.l d ata m ig h t
be rt' presen t ('d b�' a n appropriate mean of t hese c u rves. Note t hat t h(,1'e is a clear t ransitio n frolll p l ug flo\\' to para.bolic flow p rofiles \vith i ncreasing delay time TI '