Journal of Colloid and Interface Science 315 (2007) 248–254 www.elsevier.com/locate/jcis Quartz resonator signatures under Newtonian liquid loading for initial instrument check Nam-Joon Cho a,b , J. Nelson D’Amour b , Johan Stalgren b , Wolfgang Knoll c , Kay Kanazawa b , Curtis W. Frank b,∗ a Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA b Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA c Max-Planck-Institut für Polymerforschung, Ackermannweg 10, Mainz 55128, Germany Received 10 March 2007; accepted 13 June 2007 Available online 13 August 2007 Abstract The quartz crystal microbalance (QCM) has been increasingly utilized in the monitoring of the deposition of thin macromolecular films. Studies in the deposition of polymers, biomaterials, and interfacial reactions under electrochemical environment are some of the conditions for the study of these material and deposition properties at a lipid interface. Numerous studies have shown the difficulties in configuring an experimental setup for the QCM such that the recorded data reflect only the behavior of the quartz crystal and its load, and not some artifact. Such artifacts for use in liquids include mounting stress, surface properties such as hydrophobicity, surface roughness coupling to loading liquids, influence of compressional waves, and even problems with the electronic circuitry including the neglect of the quartz capacitance and the hysteretic effects of electronic components. It is thought useful to obtain a simple test by which the user could make a quick initial assessment of the instrument’s performance. When a smooth quartz crystal resonator is immersed from air into a Newtonian liquid, the resonance and loss characteristics of the QCM are changed. A minimum of two experimental parameters is needed to characterize these changes. One of the changes is that of the resonant frequency. The second is characterized by either a change in the equivalent circuit resistance (R) or a change in the resonance dissipation (D). Two combinations of these observables, in terms of either f and R or f and D, which we define as Newtonian signatures of S 1 and S 2 , are calculated to have fixed values and to be independent of the harmonic and of the physical values of the Newtonian liquid. We have experimentally determined the values of S 1 and S 2 using three different QCM systems. These are the standard oscillator, the network analyzer, and the QCM dissipation instrument. To test the sensitivity of these signatures to surface roughness, which is potential experimental artifact, we determined the values of S 1 and S 2 for roughened crystals and found that these signatures do reflect that experimental condition. Moreover, these results were qualitatively in accord with the roughness scaling factor described by Martin.  2007 Elsevier Inc. All rights reserved. Keywords: Quartz crystal microbalance (QCM); Newtonian liquid; Newtonian signatures; Atomic force microscope (AFM) 1. Introduction The quartz crystal microbalance (QCM) is a sensitive tech- nique that responds to a variety of different physical loads in contact with its surface. Changes in the quartz crystal’s reso- nant frequency and energy loss (measured by dissipation or resistance changes) give insight as to the type and magnitude of loading. Given the flexibility of the QCM, an increasing * Corresponding author. E-mail address: curt.frank@stanford.edu (C.W. Frank). number of applications for the QCM involve investigations of adsorption/desorption processes, deposition/dissolution of sur- face films, electrochemical reactions, viscoelastic film proper- ties, and interfacial slip [1–13]. The fluids in most experimental situations are Newtonian, for which the elastic modulus is zero and the viscosity is frequency independent. A Newtonian liq- uid is described simply by its density, ρ L , and its viscosity, η L . Under this condition, the effects of the liquid can be treated as a constant offset, and the additional influences of further light loading by deposition, for example, can be treated inde- pendently [11,14]. However, researchers often find difficulty interpreting their observations of complex systems in terms of 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.020