Finite Element Modified Method of Characteristics for Ion/Particle Transport in an IMS Micro-Chamber

Abstract

We consider advection of a cluster of ions/particles along a backward-facing-step-like geometry that mimics the longitudinal mid-section of the microscale chamber of an Ion Mobility Spectrometer (IMS). In order to study the cluster’s transport along the IMS chamber, we implement a Finite Element algorithm for the solution of the incompressible Navier-Stokes equations based on the Modified Method of Characteristics. The scheme combines a Lagrangian approach given by the Modified Method of Characteristics, which transforms the time dependent Navier-Stokes problem at every time step into a stationary Stokes problem, and an Eulerian perspective, which solves the Stokes problem by a Conjugate Gradient Algorithm (CGA). The Navier-Stokes equations are solved in the laminar regime using a mesh composed of Taylor-Hood triangular elements. The detection electrodes are simulated by means of outflow boundary conditions that provide estimates of the energy necessary to attract the cluster. Ions/particles trajectories are monitored to study their mobility within the flow and to optimize the electrodes’ location.