We present a method to perform sample concentration within a lab-on-a-chip using a microfluidic structure which controls the liquid-gas interface through a micropillar array fabricated in polydimethylsiloxane between microfluidic channels. The microstructure confines the liquid flow and a thermal gradient is used to drive evaporation at the liquid-gas-interface. The evaporation occurs in-plane to the microfluidic device, allowing for precise control of the ambient environment. This method is demonstrated with a sample containing 1 mu m, 100 nm fluorescent beads and SYTO-9 labelled Escherichia coli bacteria. Over 100 s, the fluorescent beads and bacteria are concentrated by a factor of 10. (C) 2014 AIP Publishing LLC.