Simulations of dense suspensions containing arbitrary-shaped particles must deal with particle collision models. This paper deals with the wet collisional interactions between particles in a carrier fluid, in the context of immersed boundary treatment of fully resolved particles. When collisions of non-spherical particles occur, several parameters that govern collisional mechanics are challenging to obtain: these include collision point, normal and tangential forces, degree of overlapping between particles, the local relative velocity and the local curvatures at the collision point. These challenges are overcome in this paper by defining a field variable, i.e. a distance function that describes the particle on a regular Cartesian grid. A Simplified Spring Collision Method (SSCM) is proposed to model wet collision forces. The Stokes number at the collision point is computed from the particle's relative velocities and local curvatures and is used to calculate the restitution coefficient; damping is incorporated into the model by modifying the restitution coefficient when the rebound phase starts. The methods are validated against experiments and other benchmarks and shown to provide good results over a wide range of Stokes numbers. The capability of the method is also tested successfully for non-circular particle shapes. (C) 2018 Elsevier B.V. All rights reserved.