Inlet Electrode A hydrodynamic AFM flow cell for the quantitative measurement of interfacial kinetics Barry A. Coles,* Richard G. Compton, Jonathan Booth, Qi Hong and Giles H. W. Sanders Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, UK OX1 3QZ A novel liquid flow cell is developed which allows atomic force microscopy images to be obtained under defined and modelled hydrodynamic flow conditions, enabling measured reaction fluxes to be compared with those calculated from proposed heterogeneous reaction mechanisms. AFM of reacting surfaces has been carried out without defined flow conditions, for example by Hansma and coworkers 1 who observed that steps on a calcite surface under aqueous solution moved laterally as dissolution proceeded. This technique is extremely powerful since it can identify key structural features, for example steps or etch pits, which contribute to the dissolution process. This approach remains qualitative since no attempts have been reported in which quantitative measures of the reaction flux are related to a chemical model of the interfacial reaction. The latter dictates defined hydrodynamics and mass transport so that the variations of concentrations in space and time above the surface can be modelled. The value of defined hydrodynamic conditions has been demonstrated by measurements in channel flow cells 2 in which a laminar flow of solution passed through a rectangular channel with a calcite crystal forming part of one wall. The reaction was monitored electrochemically by an electrode flush with the wall and immediately downstream of the calcite surface. Precise knowledge of the flow pattern enabled it to be shown that the reaction between H + and calcite was a first-order heterogeneous reaction of H + . However, the channel geometry does not facilitate simultaneous observation of the changes in surface topography or a direct physical measurement of the rate of surface dissolution, since the insertion of a scanning probe would invalidate the analytic expression for flow velocities. We have developed a novel flow cell, based on the standard Topometrix 3 Discoverer liquid immersion cell, in which AFM images may be obtained in the presence of a flowing liquid and in which the flow pattern may be calculated. The standard cell has tangential inlet and outlet ports at the periphery of a large irregular volume, so that the flow in the vicinity of the scanning cantilever is unpredictable. We have redesigned the cell by the addition of a new inlet port so that solution can be delivered through a precisely shaped and positioned stainless-steel duct directly to the front of the cantilever chip (Fig. 1). The liquid jet is aligned parallel to the front of the cantilever support chip and transverse to the cantilever. This gives a partly free field flow, not confined except by the sample below and the cantilever chip to one side, so the flow pattern is complex and is not amenable to an analytic solution. Such flow fields can however be solved using computational fluid-dynamics programs. We report here preliminary studies in which the flow field was calculated by two-dimensional simulations, together with experimental data to demonstrate the validity of this design. Initial studies of an inclined jet using the FLOTRAN 4 and subsequently FIDAP 5 programs showed that a satisfactory flow field would be set up. In particular, it was confirmed that all of the solution reaching the surface of the sample will be fresh solution from the jet with no mixture or entrainment of the surrounding bulk solution, which is an essential requirement for full characterisation. Initial tests of the cell using a gold calibration microgrid also confirmed that AFM images could be obtained under flow conditions without detectable deterioration of image quality up to the highest flow rates used. In the first study, the flow behaviour has been confirmed over a wide range of flow rates by determining the limiting current at a 1 3 1 mm platinum electrode which was placed in the sample position, Fig. 2. An aqueous solution of 5 mM potassium hexacyanoferrate(II ) with 1 M KCl was used, and the limiting current for the reversible one-electron oxidation to hexa- cyanoferrate(III ) determined for volume flow rates in the range 0.029 to 1.4 3 10 25 cm 3 s 21 , with a stainless-steel jet of internal dimensions 0.057 3 0.057 cm. The FIDAP simulation was based on a two-dimensional finite element model of a section of fluid 5.86 3 3.06 mm, including the cross-section of the jet tube and bounded below by the sample surface. The remaining boundaries represented the bulk fluid in the cell and permitted free flow. The model comprised 6564 elements of graded size, with a fine element mesh in regions of high gradients of velocity or of concentration, such as at the edges of the electrode. The simulation for a flow rate of 0.0014 cm 3 s 21 is shown in Fig. 2 as a streamline contour plot, Fig. 1 Top view of the Topometrix sample cell showing the new inlet duct Fig. 2 The two-dimensional cross-section of solution modelled with FIDAP, showing jet and electrode positions, with streamline contours for a flow rate of 0.0014 ml s 21 Chem. Commun., 1997 619 Published on 01 January 1997. Downloaded on 23/10/2014 17:53:48. View Article Online / Journal Homepage / Table of Contents for this issue