A mixing device for distributing a mixture of a first phase and a second phase of a first fluid generally along a longitudinal direction (z) in at least one passage of a heat exchanger, said mixing device comprising: at least one lateral channel configured for the flow of the first phase from at least one first inlet, a series of longitudinal channels extending along the longitudinal direction (z) and each configured for the flow of the second phase from a second inlet to a second outlet, said longitudinal channels succeeding one another along a lateral direction (y) orthogonal to the longitudinal direction (z), and at least one orifice fluidly connecting said lateral channel to at least one longitudinal channel so that the mixing device is configured to distribute a mixture of the first phase and the second phase through the second outlet of said longitudinal channel, said at least one longitudinal channel of the mixing device is divided, along the longitudinal direction (z), into an upstream portion and a downstream portion each having a length (L 3, L 4) measured in the longitudinal direction (z) and a width (D 3, D y) measured in the lateral direction (y), the downstream portion being arranged between the upstream portion and the second outlet, said downstream portion having, at any point of its length (L 4), a width (D y) greater than the width (D 3) of the upstream portion, the downstream portion opening out at the level of a downstream face of the mixing device, the second outlet being arranged at the level of the downstream face, characterized in that the downstream portion has, in a longitudinal cross-section in a plane parallel to the longitudinal direction (z) and to the lateral direction (y), an external profile forming an angle (θ), measured between the tangent to said external profile at the intersection point with the downstream face and the axis of symmetry (AA') of the longitudinal channel, comprised between 5 and 85°. at least one lateral channel configured for the flow of the first phase from at least one first inlet, a series of longitudinal channels extending along the longitudinal direction (z) and each configured for the flow of the second phase from a second inlet to a second outlet, said longitudinal channels succeeding one another along a lateral direction (y) orthogonal to the longitudinal direction (z), and at least one orifice fluidly connecting said lateral channel to at least one longitudinal channel so that the mixing device is configured to distribute a mixture of the first phase and the second phase through the second outlet of said longitudinal channel, said at least one longitudinal channel of the mixing device is divided, along the longitudinal direction (z), into an upstream portion and a downstream portion each having a length (L 3, L 4) measured in the longitudinal direction (z) and a width (D 3, D y) measured in the lateral direction (y), the downstream portion being arranged between the upstream portion and the second outlet, said downstream portion having, at any point of its length (L 4), a width (D y) greater than the width (D 3) of the upstream portion, the downstream portion opening out at the level of a downstream face of the mixing device, the second outlet being arranged at the level of the downstream face, characterized in that the downstream portion has, in a longitudinal cross-section in a plane parallel to the longitudinal direction (z) and to the lateral direction (y), an external profile forming an angle (θ), measured between the tangent to said external profile at the intersection point with the downstream face and the axis of symmetry (AA') of the longitudinal channel, comprised between 5 and 85°.