Condensed-Matter Physics & Materials Science Seminar

Local water capillary bridges play important role in micro-/nano- mechanical systems,&in dip-pen nanolitography.Water in sub-nanometer gaps defines fundamental physics of ion channels inside each cell membrane.Through atomic force microscope(AFM)friction measurements in air on hydrophilic samples we show how&why it is possible to tune water capillary bridges in a predictable way,&to validate the water gas-liquid phase diagram at nanoscopic lengths[1].We find as well that nucleation times of local water capillaries increase from 0.6ms up to 4.2ms when temp. decreases from 332K to 299K at 40% relative humidity(AFM tips w/curvature radius below 50nm)[2].These findings obey the Arrhenius law,&agree w/theoretical predictions. Molecular dynamics(MD)simulations for much smaller tips(curvature radius below 5nm)show similar Arrhenius behavior,but with several orders of magnitude smaller nucleation times indicating some diffusion barriers for water at the local scales[3,4].Furthermore,by analyzing AFM pull-off exper. in air we infer that stretched nanoscopic water bridges are in mechanical equilibrium with the external pull-off force acting at the contact,but not in thermodynamic equilibrium with the water vapor in air[5].Investigations of the forces encountered by an AFM tip approaching a solid surf. in purified water probe water confined in nano-gaps[3].For tip-sample distances between 0 +/-0.03nm to 2nm,we find oscillatory solvation forces for hydrophilic surfaces(mica,glass),&less pronounced oscillations for a hydrophobic surface(graphite).The MD simulations reveal layering&order in the confined water.The experiments&MD simulations show,as well,four orders of magnitude increase of water viscosity with respect to bulk&sharp decrease in the diffusion constant.[1]R. Szoszkiewicz,E.Riedo,APL 87,033105(2005);[2] R.Szoszkiewicz,E.Riedo,PRL 95,135502(2005);[3]T.-D.Li etal.,sub.toPRL(2006);[4]R.Szoszkiewicz et al.,in prep.;[5]L.Sirgh,R.Szoszkiewicz,E.Riedo,Langmuir22,1093(2006).