The　capillary　barrier　effect　(CBE)　is　employed　in　a　large　number　of　geotechnical　applications　to　decrease　deep　percolation　or　increase　slope　stability.　However,　the　micro-scale　behaviour　of　CBE　is　rarely　investigated,　and　thus　hampers　the　scientific　design　of　capillary　barrier　systems.　This　study　uses　microfluidics　to　explore　the　micro-scale　behaviour　of　CBE.　Capillarity-driven　water　flow　processes　from　fine　to　coarse　porous　media　with　different　pore　topologies　and　sizes　were　performed　and　analysed.　The　experimental　results　demonstrate　that　the　basic　physics　of　CBE　is　the　preferential　water　movement　into　the　fine　porous　media　due　to　the　larger　capillarity.　The　effects　of　CBE　on　water　flow　processes　can　be　identified　as　delaying　the　occurrence　of　breakthrough　into　the　coarse　porous　media　and　increasing　the　water　storage　of　the　fine　porous　media.　The　CBE　can　impede　the　increase　of　the　normalised　length　and　decrease　the　normalised　width　of　the　water　front,　suggesting　that　the　two　normalised　parameters　are　potential　indicators　to　assess　the　performance　of　CBE　at　the　micro　scale.　CBE　can　be　formed　in　square　and　honeycomb　networks　with　the　ratio　of　coarse　to　fine　pore　throat　width　larger　than　2　center　dot　0　when　gravity　is　neglected,　and　its　performance　can　be　affected　by　pore　topology　and　size.