We study the collision of a DNA or other polyelectrolyte chain with a point obstacle in the presence of an electric field via computer simulation. We find a very strong dependence of the average collision time, ktcl, and the average distance traveled during a collision, kzcl, upon the impact parameter, b. Despite the complexities of the chain-post interaction, ktcl and kzcl follow universal curves over a large range of molecular weights and field strengths. This result provides analytic formulas for the chain’s mobility in an array of posts and yields insight into the effect of post spacing. Electrophoresis is one of the most widely used techniques for size-separating charged chains such as nucleic acids or synthetic polyelectrolytes. The separation is achieved by driving charged chains through a random array of obstacles with an electric field. Traditionally, the obstacles are fibers of a gel; but, more recently, lithographically etched arrays of silicon have been proposed and demonstrated as a novel electrophoretic medium [1 ‐ 3]. This medium has several advantages over traditional gel media; one of these is that nearly any array geometry can be fabricated. This freedom allows one to design an array geometry that provides a large variation of chain mobility with chain length, and, hence, enhances size separation. The crucial question is: What array geometry is optimal? The answer to this question requires a detailed knowledge of the interaction between a chain and an obstacle. Fluorescence imaging of single DNA chains during electrophoretic migration through gels [4‐9] and a post array [1 ‐ 3] demonstrates that this interaction is complex. The time and the downfield advance of the chain during its interaction with the post affect the overall mobility of the chain. In this Letter, we present a computer simulation study of the wide range of interactions experienced by a chain and post, allowing for glancing as well as head-on impacts. Apart from the application to electrophoresis, this is a fundamental problem in polymer science which is analogous to the Rutherford scattering problem in atomic physics and has application to chains in any convective flow which impinge upon stationary obstacles [10]. Our simulations demonstrate that the average collision time, ktcl, and the average downfield distance advanced during a collision, kzcl, can be scaled to fall on universal curves which are independent of the molecular weight and field strength. Furthermore, these quantities depend strongly on the type of impact. These universal formulas are used to construct a general expression for the chain mobility in an array of posts in the single post approximation. The polyelectrolyte chain is modeled as follows. Each chain has a degree of polymerization N, monomer size a, an effective charge per unit length l, and is surrounded by fluid of viscosity h. All lengths are measured in terms of the monomer size. An electric field of strength E is applied in the z direction, and the x direction is perpendicular to the field. The polyelectrolyte is free draining and, in the absence of any obstacles, drifts along the field direction with a speed y0 › ElyhA, where A is a numerical constant. As shown previously [8,11] there are two distinct regimes, strong and weak stretching, delineated by NE , where E ; ElykT is the dimensionless field strength. Here we consider only the high-field, strong-stretching regime E N ? 1 , which describes usual experimental conditions. In this high-field regime, diffusion can be ignored and the motion of each chain can be described by a deterministic model. Figure 1 represents events of the chain-post interaction. A chain impacts a post with impact parameter b, defined as the distance between the chain’s center of mass and the post, measured perpendicular to the field. At short times, portions of the chain extend downfield on either side of the post, forming a multiply hooked conformation. The time from initial impact to hooking is assumed to be t › z 0