Collision detection not working unity

First, I know that this question has been asked a lot, but I cant find a solution, so mi problem is, Im making an educational game, and I have a vein and the blood flow (with many box colliders) and a single blood cell (also with a box collider) however i want the cell to destroy when it reaches the wall collider, but it doesn't it just stays there, here is the project!

http://tinypic.com/r/10706es/9

(cant upload images because of my reputation, sorry)

The collider where I want to destroy my cell is the pink collider, however when it touches it it just does nothing, here's my script

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class collision : MonoBehaviour {
    // Use this for initialization
    void Start () {
    }

    // Update is called once per frame
    void OnCollisionEnter(Collision col)
    {
        print("hihi");
        if (col.gameObject.tag == "Collider")
        {
            Destroy(gameObject);
        }
    }
}

Also, here is the AddForce script

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class AddForce : MonoBehaviour {

    public float thrust;
    public Rigidbody rb;
    private Vector3 up;
    private bool move;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
        up = new Vector3(0, 1, 0);
        move = false;
    }

    void FixedUpdate()
    {
        if (Input.GetKey("space"))
        {
            if (rb.velocity.magnitude < 5)
                rb.AddForce(up * thrust);
            move = true;
        }

        else
        {
            if (move == true)
                rb.velocity = new Vector3(0, -0.5F, 0);
        }


    }
}

thanks for your help guys! :D