Interpretation of this behaviour is not fully understood, but it is likely to be due to the ionic interaction between the DNA and the buffer solution. A natural negative charge on the DNA fragments means that in a neutral environment, such as water, their preferred orientation would be for the molecules to lay on the gold layer with no preferred orientation. When the buffer solution is added there is an interaction with the positive ions in the solution, and the DNA fragments stand up leading to an increase in layer thickness.