Miniaturized fluid handling devices have recently attracted considerable interest in many areas of science1. Such microfluidic chips perform a variety of functions, ranging from analysis of biological macromolecules2,3 to catalysis of reactions and sensing in the gas phase4,5. To enable precise fluid handling, accurate knowledge of the flow properties within these devices is important. Due to low Reynolds numbers, laminar flow is usually assumed. However, either by design or unintentionally, the flow characteristic in small channels is often altered, for example by surface interactions, viscous and diffusional effects, or electrical potentials. Therefore, its prediction is not always straight-forward6-8. Currently, most microfluidic flow measurements rely on optical detection of markers9,10, requiring the injection of tracers and transparent devices. Here, we show profiles of microfluidic gas flow in capillaries and chip devices obtained by NMR in the remote detection modality11,12. Through the transient measurement of dispersion13, NMR is well adaptable for non-invasive, yet sensitive determination of the flow field and provides a novel and potentially more powerful tool to profile flow in capillaries and miniaturized flow devices.

A novel technique is proposed in which a nuclear magnetic resonance (NMR) spectrum or magnetic resonance image (MRI) is encoded and stored as spin polarization and is then moved to a different physical location to be detected. Remote detection allows the separate optimization of the encoding and detection steps, permitting the independent choice of experimental conditions, and excitation and detection methodologies. In the first experimental demonstration of this technique, we show that NMR signal can be amplified by taking diluted 129Xe from a porous sample placed inside a large encoding coil, and concentrating it into a smaller detection coil. In general, the study of NMR active molecules at low concentration that have low physical filling factor is facilitated by remote detection. In the second experiment, MRI information encoded in a very low field magnet (4-7mT) is transferred to a high field magnet (4.2 T) in order to be detected under optimized conditions. Furthermore, remote detection allows the utilization of ultra-sensitive optical or superconducting detection techniques, which broadens the horizon of NMR experimentation.

A time-of-flight imaging technique is introduced to visualize fluid flow and dispersion through porous media using NMR. As the fluid flows through a sample, the nuclear spin magnetization is modulated by RF pulses and magnetic field gradients to encode the spatial coordinates of the fluid. When the fluid leaves the sample, its magnetization is recorded by a second RF coil. This scheme not only facilitates a time-dependent imaging of fluid flow, it also allows a separate optimization of encoding and detection subsystems to enhance overall sensitivity. The technique is demonstrated by imaging gas flow through a porous rock.