Puncturing of soft gels with multi-tip needles

Resistance to insertion of a sharp object into a soft solid is known to depend upon the macroscopic geometry of the tool and the consequent fracture mechanisms involved. In this article, we have examined it by carrying out displacement controlled puncturing into soft, brittle polyacrylamide gels using hypodermic needles of different diameters. These experiments show that in contrast to a rigid, flat bottom punch and a punch with sharp tip, for these needles, puncturing of the gel does not occur continuously but intermittently with the fracture progressing alternately in the wedge opening and the shear fracture modes. Correspondingly, the force versus displacement plot too shows sharp increase in load and catastrophic falls, occurring at regular intervals. We have shown that these data can be used to estimate the toughness of the material in both these fracture modes. In addition, we have examined the effect of asymmetry in the needle geometry and importantly the co-operative effect of several closely spaced needles by designing multi-tip needles consisting of several tips of different diameters and relative orientations. Experiments with these needles show that compared to a conventional single tip needle of equivalent diameter, the multi-tip needles penetrate the solid at smaller load with a smaller energy cost for fracture. Importantly, fracture occurs near simultaneously in the two fracture modes, so that the oscillations in the puncturing load is arrested. The lateral fracture of the gel and the consequent excess damage too is minimized.