I have modified the above simulation and make a new one.
Please click [eye]

translate strings in simulation to different language format before download
Full screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(1399.5kB):double click downloaded file to run it. (1 times by 1 users) , Download EJS source View EJS source
[/eye] to view the new version.
I hope it is what you want. (You need to click the - checkbox to make the particle being pulled down.
All the particles will have ghost particles a few times steps backward or forward (you need to change dt2 to see the difference)